TITLE

Synopsis 6: Subroutines

AUTHOR

Damian Conway <damian@conway.org> and Allison Randal <al@shadowed.net>

VERSION

Maintainer: Larry Wall <larry@wall.org>
Date: 21 Mar 2003
Last Modified: 26 Oct 2007
Number: 6
Version: 90

This document summarizes Apocalypse 6, which covers subroutines and the new type system.

Subroutines and other code objects

Subroutines (keyword: sub) are non-inheritable routines with parameter lists.

Methods (keyword: method) are inheritable routines which always have an associated object (known as their invocant) and belong to a particular kind or class.

Submethods (keyword: submethod) are non-inheritable methods, or subroutines masquerading as methods. They have an invocant and belong to a particular kind or class.

Regexes (keyword: regex) are methods (of a grammar) that perform pattern matching. Their associated block has a special syntax (see Synopsis 5). (We also use the term "regex" for anonymous patterns of the traditional form.)

Tokens (keyword: token) are regexes that perform low-level non-backtracking (by default) pattern matching.

Rules (keyword: rule) are regexes that perform non-backtracking (by default) pattern matching (and also enable rules to do whitespace dwimmery).

Macros (keyword: macro) are routines whose calls execute as soon as they are parsed (i.e. at compile-time). Macros may return another source code string or a parse-tree.

Routine modifiers

Multis (keyword: multi) are routines that can have multiple variants that share the same name, selected by arity, types, or some other constraints.

Prototypes (keyword: proto) specify the commonalities (such as parameter names, fixity, and associativity) shared by all multis of that name in the scope of the proto declaration. A proto also adds an implicit multi to all routines of the same short name within its scope, unless they have an explicit modifier. (This is particularly useful when adding to rule sets or when attempting to compose conflicting methods from roles.)

Only (keyword: only) routines do not share their short names with other routines. This is the default modifier for all routines, unless a proto of the same name was already in scope.

A modifier keyword may occur before the routine keyword in a named routine:

only sub foo {...}
proto sub foo {...}
multi sub foo {...}

only method bar {...}
proto method bar {...}
multi method bar {...}

If the routine keyword is omitted, it defaults to sub.

Modifier keywords cannot apply to anonymous routines.

Named subroutines

The general syntax for named subroutines is any of:

 my RETTYPE sub NAME ( PARAMS ) TRAITS {...}    # lexical only
our RETTYPE sub NAME ( PARAMS ) TRAITS {...}    # also package-scoped
            sub NAME ( PARAMS ) TRAITS {...}    # same as "our"

The return type may also be put inside the parentheses:

sub NAME (PARAMS --> RETTYPE) {...}

Unlike in Perl 5, named subroutines are considered expressions, so this is valid Perl 6:

my @subs = (sub foo { ... }, sub bar { ... });

Anonymous subroutines

The general syntax for anonymous subroutines is:

sub ( PARAMS ) TRAITS {...}

But one can also use a scope modifier to introduce the return type first:

 my RETTYPE sub ( PARAMS ) TRAITS {...}
our RETTYPE sub ( PARAMS ) TRAITS {...}

In this case there is no effective difference, since the distinction between my and our is only in the handling of the name, and in the case of an anonymous sub, there's isn't one.

Trait is the name for a compile-time (is) property. See "Properties and traits".

Perl5ish subroutine declarations

You can declare a sub without parameter list, as in Perl 5:

sub foo {...}

Arguments implicitly come in via the @_ array, but they are readonly aliases to actual arguments:

sub say { print qq{"@_[]"\n}; }   # args appear in @_

sub cap { $_ = uc $_ for @_ }   # Error: elements of @_ are read-only

If you need to modify the elements of @_, declare the array explicitly with the is rw trait:

sub swap (*@_ is rw) { @_[0,1] = @_[1,0] }

Blocks

Raw blocks are also executable code structures in Perl 6.

Every block defines an object of type Code, which may either be executed immediately or passed on as a Code object. How a block is parsed is context dependent.

A bare block where an operator is expected terminates the current expression and will presumably be parsed as a block by the current statement-level construct, such as an if or while. (If no statement construct is looking for a block there, it's a syntax error.) This form of bare block requires leading whitespace because a bare block where a postfix is expected is treated as a hash subscript.

A bare block where a term is expected merely produces a Code object. If the term bare block occurs in a list, it is considered the final element of that list unless followed immediately by a comma or colon (intervening \h* or "unspace" is allowed).

"Pointy blocks"

Semantically the arrow operator -> is almost a synonym for the sub keyword as used to declare an anonymous subroutine, insofar as it allows you to declare a signature for a block of code. However, the parameter list of a pointy block does not require parentheses, and a pointy block may not be given traits. In most respects, though, a pointy block is treated more like a bare block than like an official subroutine. Syntactically, a pointy block may be used anywhere a bare block could be used:

my $sq = -> $val { $val**2 };
say $sq(10); # 100

my @list = 1..3;
for @list -> $elem {
    say $elem; # prints "1\n2\n3\n"
}

It also behaves like a block with respect to control exceptions. If you return from within a pointy block, the block is transparent to the return; it will return from the innermost enclosing sub or method, not from the block itself. It is referenced by &?BLOCK, not &?ROUTINE.

A normal pointy block's parameters default to readonly, just like parameters to a normal sub declaration. However, the double-pointy variant defaults parameters to rw:

for @list <-> $elem {
    $elem++;
}

This form applies rw to all the arguments:

for @kv <-> $key, $value {
    $key ~= ".jpg";
    $value *= 2 if $key ~~ :e;
}

Stub declarations

To predeclare a subroutine without actually defining it, use a "stub block":

sub foo {...}     # Yes, those three dots are part of the actual syntax

The old Perl 5 form:

sub foo;

is a compile-time error in Perl 6 (because it would imply that the body of the subroutine extends from that statement to the end of the file, as class and module declarations do). The only allowed use of the semicolon form is to declare a MAIN sub--see "Declaring a MAIN subroutine" below.

Redefining a stub subroutine does not produce an error, but redefining an already-defined subroutine does. If you wish to redefine a defined sub, you must explicitly use the "is instead" trait.

The ... is the "yadayadayada" operator, which is executable but returns a failure. You can also use ??? to produce a warning, or !!! to always die. These also officially define stub blocks if used as the only expression in the block.

It has been argued that ... as literal syntax is confusing when you might also want to use it for metasyntax within a document. Generally this is not an issue in context; it's never an issue in the program itself, and the few places where it could be an issue in the documentation, a comment will serve to clarify the intent, as above. The rest of the time, it doesn't really matter whether the reader takes ... as literal or not, since the purpose of ... is to indicate that something is missing whichever way you take it.

Globally scoped subroutines

Subroutines and variables can be declared in the global namespace, and are thereafter visible everywhere in a program.

Global subroutines and variables are normally referred to by prefixing their identifiers with * (short for "GLOBAL::"). The * is required on the declaration unless the GLOBAL namespace can be inferred some other way, but the * may be omitted on use if the reference is unambiguous:

$*next_id = 0;
sub *saith($text)  { print "Yea verily, $text" }

module A {
    my $next_id = 2;    # hides any global or package $next_id
    saith($next_id);    # print the lexical $next_id;
    saith($*next_id);   # print the global $next_id;
}

module B {
    saith($next_id);    # Unambiguously the global $next_id
}

However, under stricture (the default for most code), the * is required on variable references. It's never required on sub calls, and in fact, the syntax

$x = *saith($y);

is illegal, because a * where a term is expected is always parsed as the "whatever" token. If you really want to use a *, you must also use the sigil along with the twigil:

$x = &*saith($y);

Only the name is installed into the GLOBAL package by *. To define subs completely within the scope of the GLOBAL namespace you should use "package GLOBAL {...}" around the declaration.

Lvalue subroutines

Lvalue subroutines return a "proxy" object that can be assigned to. It's known as a proxy because the object usually represents the purpose or outcome of the subroutine call.

Subroutines are specified as being lvalue using the is rw trait.

An lvalue subroutine may return a variable:

my $lastval;
sub lastval () is rw { return $lastval }

or the result of some nested call to an lvalue subroutine:

sub prevval () is rw { return lastval() }

or a specially tied proxy object, with suitably programmed FETCH and STORE methods:

sub checklastval ($passwd) is rw {
    return new Proxy:
            FETCH => method {
                        return lastval();
                     },
            STORE => method ($val) {
                        die unless check($passwd);
                        lastval() = $val;
                     };
}

Other methods may be defined for specialized purposes such as temporizing the value of the proxy.

Operator overloading

Operators are just subroutines with special names and scoping. An operator name consists of a grammatical category name followed by a single colon followed by an operator name specified as if it were a hash subscript (but evaluated at compile time). So any of these indicates the same binary addition operator:

infix:<+>
infix:«+»
infix:<<+>>
infix:{'+'}
infix:{"+"}

Use the & sigil just as you would on ordinary subs.

Unary operators are defined as prefix or postfix:

sub prefix:<OPNAME>  ($operand) {...}
sub postfix:<OPNAME> ($operand) {...}

Binary operators are defined as infix:

sub infix:<OPNAME> ($leftop, $rightop) {...}

Bracketing operators are defined as circumfix where a term is expected or postcircumfix where a postfix is expected. A two-element slice containing the leading and trailing delimiters is the name of the operator.

sub circumfix:<LEFTDELIM RIGHTDELIM> ($contents) {...}
sub circumfix:{'LEFTDELIM','RIGHTDELIM'} ($contents) {...}

Contrary to Apocalypse 6, there is no longer any rule about splitting an even number of characters. You must use a two-element slice. Such names are canonicalized to a single form within the symbol table, so you must use the canonical name if you wish to subscript the symbol table directly (as in PKG::{'infix:<+>'}). Otherwise any form will do. (Symbolic references do not count as direct subscripts since they go through a parsing process.) The canonical form always uses angle brackets and a single space between slice elements. The elements are not escaped, so PKG::circumfix:{'<','>'} is canonicalized to PKG::{'circumfix:<< >>'}, and decanonicalizing always involves stripping the outer angles and splitting on space, if any. This works because a hash key knows how long it is, so there's no ambiguity about where the final angle is. And space works because operators are not allowed to contain spaces.

Operator names can be any sequence of non-whitespace characters including Unicode characters. For example:

sub infix:<(c)> ($text, $owner) { return $text but Copyright($owner) }
method prefix:<±> (Num $x --> Num) { return +$x | -$x }
multi sub postfix:<!> (Int $n) { $n < 2 ?? 1 !! $n*($n-1)! }
macro circumfix:«<!-- -->» ($text) is parsed / .*? / { "" }

my $document = $text (c) $me;

my $tolerance = ±7!;

<!-- This is now a comment -->

Whitespace may never be part of the name (except as separator within a <...> or «...» slice subscript, as in the example above).

A null operator name does not define a null or whitespace operator, but a default matching subrule for that syntactic category, which is useful when there is no fixed string that can be recognized, such as tokens beginning with digits. Such an operator must supply an is parsed trait. The Perl grammar uses a default subrule for the :1st, :2nd, :3rd, etc. regex modifiers, something like this:

sub regex_mod_external:<> ($x) is parsed(token { \d+[st|nd|rd|th] }) {...}

Such default rules are attempted in the order declared. (They always follow any rules with a known prefix, by the longest-token-first rule.)

Although the name of an operator can be installed into any package or lexical namespace, the syntactic effects of an operator declaration are always lexically scoped. Operators other than the standard ones should not be installed into the * namespace. Always use exportation to make non-standard syntax available to other scopes.

Parameters and arguments

Perl 6 subroutines may be declared with parameter lists.

By default, all parameters are readonly aliases to their corresponding arguments--the parameter is just another name for the original argument, but the argument can't be modified through it. This is vacuously true for value arguments, since they may not be modified in any case. However, the default forces any container argument to also be treated as an immutable value. This extends down only one level; an immutable container may always return an element that is mutable if it so chooses. (For this purpose a scalar variable is not considered a container of its singular object, though, so the top-level object within a scalar variable is considered immutable by default. Perl 6 does not have references in the same sense that Perl 5 does.)

To allow modification, use the is rw trait. This requires a mutable object or container as an argument (or some kind of protoobject that can be converted to a mutable object, such as might be returned by an array or hash that knows how to autovivify new elements). Otherwise the signature fails to bind, and this candidate routine cannot be considered for servicing this particular call. (Other multi candidates, if any, may succeed if the don't require rw for this parameter.) In any case, failure to bind does not by itself cause an exception to be thrown; that is completely up to the dispatcher.

To pass-by-copy, use the is copy trait. An object container will be cloned whether or not the original is mutable, while an (immutable) value will be copied into a suitably mutable container. The parameter may bind to any argument that meets the other typological constraints of the parameter.

If you have a readonly parameter $ro, it may never be passed on to a rw parameter of a subcall, whether or not $ro is currently bound to a mutable object. It may only be rebound to readonly or copy parameters. It may also be rebound to a ref parameter (see "is ref" below), but modification will fail as in the case where an immutable value is bound to a ref parameter.

Aliases of $ro are also readonly, whether generated explicitly with := or implicitly within a Capture object (which are themselves immutable).

Also, $ro may not be returned from an lvalue subroutine or method.

Parameters may be required or optional. They may be passed by position, or by name. Individual parameters may confer a scalar or list context on their corresponding arguments, but unlike in Perl 5, this is decided lazily at parameter binding time.

Arguments destined for required positional parameters must come before those bound to optional positional parameters. Arguments destined for named parameters may come before and/or after the positional parameters. (To avoid confusion it is highly recommended that all positional parameters be kept contiguous in the call syntax, but this is not enforced, and custom arg list processors are certainly possible on those arguments that are bound to a final slurpy or arglist variable.)

Named arguments

Named arguments are recognized syntactically at the "comma" level. Since parameters are identified using identifiers, the recognized syntaxes are those where the identifier in question is obvious. You may use either the adverbial form, :name($value), or the autoquoted arrow form, name => $value. These must occur at the top "comma" level, and no other forms are taken as named pairs by default. Pairs intended as positional arguments rather than named arguments may be indicated by extra parens or by explicitly quoting the key to suppress autoquoting:

doit :when<now>,1,2,3;      # always a named arg
doit (:when<now>),1,2,3;    # always a positional arg

doit when => 'now',1,2,3;   # always a named arg
doit (when => 'now'),1,2,3; # always a positional arg
doit 'when' => 'now',1,2,3; # always a positional arg

Only bare keys with valid identifier names are recognized as named arguments:

doit when => 'now';         # always a named arg
doit 'when' => 'now';       # always a positional arg
doit 123  => 'now';         # always a positional arg
doit :123<now>;             # always a positional arg

Going the other way, pairs intended as named arguments that don't look like pairs must be introduced with the | prefix operator:

$pair = :when<now>;
doit $pair,1,2,3;                # always a positional arg
doit |$pair,1,2,3;               # always a named arg
doit |get_pair(),1,2,3;          # always a named arg
doit |('when' => 'now'),1,2,3;   # always a named arg

Note the parens are necessary on the last one due to precedence.

Likewise, if you wish to pass a hash and have its entries treated as named arguments, you must dereference it with a |:

%pairs = (:when<now>, :what<any>);
doit %pairs,1,2,3;               # always a positional arg
doit |%pairs,1,2,3;              # always named args
doit |%(get_pair()),1,2,3;       # always a named arg
doit |%('when' => 'now'),1,2,3;  # always a named arg

Variables with a : prefix in rvalue context autogenerate pairs, so you can also say this:

$when = 'now';
doit $when,1,2,3;   # always a positional arg of 'now'
doit :$when,1,2,3;  # always a named arg of :when<now>

In other words :$when is shorthand for :when($when). This works for any sigil:

:$what      :what($what)
:@what      :what(@what)
:%what      :what(%what)
:&what      :what(&what)

Ordinary hash notation will just pass the value of the hash entry as a positional argument regardless of whether it is a pair or not. To pass both key and value out of hash as a positional pair, use :p instead:

doit %hash<a>:p,1,2,3;
doit %hash{'b'}:p,1,2,3;

The :p stands for "pairs", not "positional"--the :p adverb may be placed on any Hash access to make it mean "pairs" instead of "values". If you want the pair (or pairs) to be interpreted a named argument, you may do so by prefixing with the prefix:<|> operator:

doit |%hash<a>:p,1,2,3;
doit |%hash{'b'}:p,1,2,3;

Pair constructors are recognized syntactically at the call level and put into the named slot of the Capture structure. Hence they may be bound to positionals only by name, not as ordinary positional Pair objects. Leftover named arguments can be slurped into a slurpy hash.

Because named and positional arguments can be freely mixed, the programmer always needs to disambiguate pairs literals from named arguments with parentheses or quotes:

# Named argument "a"
push @array, 1, 2, :a<b>;

# Pair object (a=>'b')
push @array, 1, 2, (:a<b>);
push @array, 1, 2, 'a' => 'b';

Perl 6 allows multiple same-named arguments, and records the relative order of arguments with the same name. When there are more than one argument, the @ sigil in the parameter list causes the arguments to be concatenated:

sub fun (Int @x) { ... }
fun( x => 1, x => 2 );              # @x := (1, 2)
fun( x => (1, 2), x => (3, 4) );    # @x := (1, 2, 3, 4)

Other sigils bind only to the last argument with that name:

sub fun (Int $x) { ... }
f( x => 1, x => 2 );                # $x := 2
fun( x => (1, 2), x => (3, 4) );    # $x := (3, 4)

This means a hash holding default values must come before known named parameters, similar to how hash constructors work:

# Allow "x" and "y" in %defaults to be overridden
f( |%defaults, x => 1, y => 2 );

Invocant parameters

A method invocant may be specified as the first parameter in the parameter list, with a colon (rather than a comma) immediately after it:

method get_name ($self:) {...}
method set_name ($_: $newname) {...}

The corresponding argument (the invocant) is evaluated in scalar context and is passed as the left operand of the method call operator:

print $obj.get_name();
$obj.set_name("Sam");

For the purpose of matching positional arguments against invocant parameters, the invocant argument passed via the method call syntax is considered the first positional argument when failover happens from single dispatch to multiple dispatch:

handle_event($w, $e, $m);   # calls the multi sub
$w.handle_event($e, $m);    # ditto, but only if there is no
                            # suitable $w.handle_event method

Invocants may also be passed using the indirect object syntax, with a colon after them. The colon is just a special form of the comma, and has the same precedence:

set_name $obj: "Sam";   # try $obj.set_name("Sam") first, then
                        # fall-back to set_name($obj, "Sam")
$obj.set_name("Sam");   # same as the above

An invocant is the topic of the corresponding method if that formal parameter is declared with the name $_. A method's invocant always has the alias self. Other styles of self can be declared with the self pragma.

Longname parameters

A routine marked with multi can mark part of its parameters to be considered in the multi dispatch. These are called longnames; see S12 for more about the semantics of multiple dispatch.

You can choose part of a multi's parameters to be its longname, by putting a double semicolon after the last one:

multi sub handle_event ($window, $event;; $mode) {...}
multi method set_name ($self: $name;; $nick) {...}

A parameter list may have at most one double semicolon; parameters after it are never considered for multiple dispatch (except of course that they can still "veto" if their number or types mismatch).

[Conjecture: It might be possible for a routine to advertise multiple long names, delimited by single semicolons. See S12 for details.]

If the parameter list for a multi contains no semicolons to delimit the list of important parameters, then all positional parameters are considered important. If it's a multi method or multi submethod, an additional implicit unnamed self invocant is added to the signature list unless the first parameter is explicitly marked with a colon.

Required parameters

Required parameters are specified at the start of a subroutine's parameter list:

sub numcmp ($x, $y) { return $x <=> $y }

Required parameters may optionally be declared with a trailing !, though that's already the default for positional parameters:

sub numcmp ($x!, $y!) { return $x <=> $y }

The corresponding arguments are evaluated in scalar context and may be passed positionally or by name. To pass an argument by name, specify it as a pair: parameter_name => argument_value.

$comparison = numcmp(2,7);
$comparison = numcmp(x=>2, y=>7);
$comparison = numcmp(y=>7, x=>2);

Pairs may also be passed in adverbial pair notation:

$comparison = numcmp(:x(2), :y(7));
$comparison = numcmp(:y(7), :x(2));

Passing the wrong number of required arguments to a normal subroutine is a fatal error. Passing a named argument that cannot be bound to a normal subroutine is also a fatal error. (Methods are different.)

The number of required parameters a subroutine has can be determined by calling its .arity method:

$args_required = &foo.arity;

Optional parameters

Optional positional parameters are specified after all the required parameters and each is marked with a ? after the parameter:

sub my_substr ($str, $from?, $len?) {...}

Alternately, optional fields may be marked by supplying a default value. The = sign introduces a default value:

sub my_substr ($str, $from = 0, $len = Inf) {...}

Default values can be calculated at run-time. They may even use the values of preceding parameters:

sub xml_tag ($tag, $endtag = matching_tag($tag) ) {...}

Arguments that correspond to optional parameters are evaluated in scalar context. They can be omitted, passed positionally, or passed by name:

my_substr("foobar");            # $from is 0, $len is infinite
my_substr("foobar",1);          # $from is 1, $len is infinite
my_substr("foobar",1,3);        # $from is 1, $len is 3
my_substr("foobar",len=>3);     # $from is 0, $len is 3

Missing optional arguments default to their default values, or to an undefined value if they have no default. (A supplied argument that is undefined is not considered to be missing, and hence does not trigger the default. Use //= within the body for that.)

(Conjectural: Within the body you may also use exists on the parameter name to determine whether it was passed. Maybe this will have to be restricted to the ? form, unless we're willing to admit that a parameter could be simultaneously defined and non-existent.)

Named parameters

Named-only parameters follow any required or optional parameters in the signature. They are marked by a prefix ::

sub formalize($text, :$case, :$justify) {...}

This is actually shorthand for:

sub formalize($text, :case($case), :justify($justify)) {...}

If the longhand form is used, the label name and variable name can be different:

sub formalize($text, :case($required_case), :justify($justification)) {...}

so that you can use more descriptive internal parameter names without imposing inconveniently long external labels on named arguments. Multiple name wrappings may be given; this allows you to give both a short and a long external name:

sub globalize (:g(:global($gl))) {...}

Or equivalently:

sub globalize (:g(:$global)) {...}

Arguments that correspond to named parameters are evaluated in scalar context. They can only be passed by name, so it doesn't matter what order you pass them in:

$formal = formalize($title, case=>'upper');
$formal = formalize($title, justify=>'left');
$formal = formalize($title, :justify<right>, :case<title>);

See S02 for the correspondence between adverbial form and arrow notation.

While named and position arguments may be intermixed, it is suggested that you keep all the positionals in one place for clarity unless you have a good reason not to. This is likely bad style:

$formal = formalize(:justify<right>, $title, :case<title>, $date);

Named parameters are optional unless marked with a following !. Default values for optional named parameters are defined in the same way as for positional parameters, but may depend only on existing values, including the values of parameters that have already been bound. Named optional parameters default to undef if they have no default. Named required parameters fail unless an argument pair of that name is supplied.

Bindings happen in declaration order, not call order, so any default may reliably depend on formal parameters to its left in the signature. In other words, if the first parameter is $a, it will bind to a :a() argument in preference to the first positional argument. It might seem that performance of binding would suffer by requiring a named lookup before a positional lookup, but the compiler is able to guarantee that subs with known fixed signatures (both onlys and multis with protos) translate named arguments to positional in the first N positions. Also, purely positional calls may obviously omit any named lookups, as may bindings that have already used up all the named arguments. The compiler is also free to intuit proto signatures for a given sub or method name as long as the candidate list is stable..

List parameters

List parameters capture a variable length list of data. They're used in subroutines like print, where the number of arguments needs to be flexible. They're also called "variadic parameters", because they take a variable number of arguments. But generally we call them "slurpy" parameters because they slurp up arguments.

Slurpy parameters follow any required or optional parameters. They are marked by a * before the parameter:

sub duplicate($n, *%flag, *@data) {...}

Named arguments are bound to the slurpy hash (*%flag in the above example). Such arguments are evaluated in scalar context. Any remaining variadic arguments at the end of the argument list are bound to the slurpy array (*@data above) and are evaluated in list context.

For example:

duplicate(3, reverse => 1, collate => 0, 2, 3, 5, 7, 11, 14);
duplicate(3, :reverse, :!collate, 2, 3, 5, 7, 11, 14);  # same

# The @data parameter receives [2, 3, 5, 7, 11, 14]
# The %flag parameter receives { reverse => 1, collate => 0 }

Slurpy scalar parameters capture what would otherwise be the first elements of the variadic array:

sub head(*$head, *@tail)         { return $head }
sub neck(*$head, *$neck, *@tail) { return $neck }
sub tail(*$head, *@tail)         { return @tail }

head(1, 2, 3, 4, 5);        # $head parameter receives 1
                            # @tail parameter receives [2, 3, 4, 5]

neck(1, 2, 3, 4, 5);        # $head parameter receives 1
                            # $neck parameter receives 2
                            # @tail parameter receives [3, 4, 5]

Slurpy scalars still impose list context on their arguments.

Slurpy parameters are treated lazily -- the list is only flattened into an array when individual elements are actually accessed:

@fromtwo = tail(1..Inf);        # @fromtwo contains a lazy [2..Inf]

You can't bind to the name of a slurpy parameter: the name is just there so you can refer to it within the body.

sub foo(*%flag, *@data) {...}

foo(:flag{ a => 1 }, :data[ 1, 2, 3 ]);
    # %flag has elements (flag => (a => 1)) and (data => [1,2,3])
    # @data has nothing

Slurpy block

It's also possible to declare a slurpy block: *&block. It slurps up any nameless block, specified by {...}, at either the current positional location or the end of the syntactic list. Put it first if you want the option of putting a block either first or last in the arguments. Put it last if you want to force it to come in as the last argument.

Argument list binding

The underlying Capture object may be bound to a single scalar parameter marked with a |.

sub bar ($a,$b,$c,:$mice) { say $mice }
sub foo (|$args) { say $args.perl; &bar.callwith(|$args); }

This prints:

foo 1,2,3,:mice<blind>;     # says "\(1,2,3,:mice<blind>)" then "blind"

As demonstrated above, the capture may be interpolated into another call's arguments. (The | prefix is described in the next section.) Use of callwith allows the routine to be called without introducing an official CALLER frame. For more see "Wrapping" below.

It is allowed to rebind the parameters within the signature, but only as a subsignature of the capture argument:

sub compare (|$args (Num $x, Num $y --> Bool)) { ... }

For all normal declarative purposes (invocants and multiple dispatch types, for instance), the inner signature is treated as the entire signature:

method addto (|$args ($self: @x)) { trace($args); $self += [+] @x }

The inner signature is not required for non-multies since there can only be one candidate, but for multiple dispatch the inner signature is required at least for its types, or the declaration would not know what signature to match against.

multi foo (|$args (Int, Bool?, *@, *%)) { reallyintfoo($args) }
multi foo (|$args (Str, Bool?, *@, *%)) { reallystrfoo($args) }

Flattening argument lists

The unary | operator casts its argument to a Capture object, then splices that capture into the argument list it occurs in. To get the same effect on multiple arguments you can use the |« hyperoperator.

Pair and Hash become named arguments:

|(x=>1);          # Pair, becomes \(x=>1)
|{x=>1, y=>2};    # Hash, becomes \(x=>1, y=>2)

List (also Seq, Range, etc.) are simply turned into positional arguments:

|(1,2,3);         # Seq, becomes \(1,2,3)
|(1..3);          # Range, becomes \(1,2,3)
|(1..2, 3);       # List, becomes \(1,2,3)
|([x=>1, x=>2]);  # List (from an Array), becomes \((x=>1), (x=>2))

For example:

sub foo($x, $y, $z) {...}    # expects three scalars
@onetothree = 1..3;          # array stores three scalars

foo(1,2,3);                  # okay:  three args found
foo(@onetothree);            # error: only one arg
foo(|@onetothree);           # okay:  @onetothree flattened to three args

The | operator flattens lazily -- the array is flattened only if flattening is actually required within the subroutine. To flatten before the list is even passed into the subroutine, use the eager list operator:

foo(|eager @onetothree);          # array flattened before &foo called

Multidimensional argument list binding

Some functions take more than one list of positional and/or named arguments, that they wish not to be flattened into one list. For instance, zip() wants to iterate several lists in parallel, while array and hash subscripts want to process a multidimensional slice. The set of underlying argument lists may be bound to a single array parameter declared with a double @@ sigil:

sub foo (*@@slice) { ... }

Note that this is different from

sub foo (\$slice) { ... }

insofar as \$slice is bound to a single argument-list object that makes no commitment to processing its structure (and maybe doesn't even know its own structure yet), while *@@slice has to create an array that binds the incoming dimensional lists to the array's dimensions, and make that commitment visible to the rest of the scope via the sigil so that constructs expecting multidimensional lists know that multidimensionality is the intention.

It is allowed to specify a return type:

sub foo (*@@slice --> Num) { ... }

The invocant does not participate in multi-dimensional argument lists, so self is not present in the @@slice below:

method foo (*@@slice) { ... }

The @@ sigil is just a variant of the @ sigil, so @@slice and @slice are really the same array. In particular, @@_ is really the good old @_ array viewed as multidimensional.

Zero-dimensional argument list

If you call a function without parens and supply no arguments, the argument list becomes a zero-dimensional slice. It differs from \() in several ways:

sub foo (*@@slice) {...}
foo;        # +@@slice == 0
foo();      # +@@slice == 1

sub bar (\$args = \(1,2,3)) {...}
bar;        # $args === \(1,2,3)
bar();      # $args === \()

Feed operators

The variadic list of a subroutine call can be passed in separately from the normal argument list, by using either of the feed operators: <== or ==>. Syntactically, feed operators expect to find a statement on either end. Any statement can occur on the source end; however not all statements are suitable for use on the sink end of a feed.

Each operator expects to find a call to a variadic receiver on its "sharp" end, and a list of values on its "blunt" end:

grep { $_ % 2 } <== @data;

@data ==> grep { $_ % 2 };

It binds the (potentially lazy) list from the blunt end to the slurpy parameter(s) of the receiver on the sharp end. In the case of a receiver that is a variadic function, the feed is received as part of its slurpy list. So both of the calls above are equivalent to:

grep { $_ % 2 }, @data;

Note that all such feeds (and indeed all lazy argument lists) supply an implicit promise that the code producing the lists may execute in parallel with the code receiving the lists. (Feeds, hyperops, and junctions all have this promise of parallelizability in common, but differ in interface. Code which violates these promises is erroneous, and will produce undefined results when parallelized.)

However, feeds go a bit further than ordinary lazy lists in enforcing the parallel discipline: they explicitly treat the blunt end as a cloned closure that starts a subthread (presumably cooperative). The only variables shared by the inner scope with the outer scope are those lexical variables declared in the outer scope that are visible at the time the closure is cloned and the subthread spawned. Use of such shared variables will automatically be subject to transactional protection (and associated overhead). Package variables are not cloned unless predeclared as lexical names with our. Variables declared within the blunt end are not visible outside, and in fact it is illegal to declare a lexical on the blunt end that is not enclosed in curlies somehow.

Because feeds are defined as lazy pipes, a chain of feeds may not begin and end with the same array without some kind of eager sequence point. That is, this isn't guaranteed to work:

@data <== grep { $_ % 2 } <== @data;

either of these do:

@data <== grep { $_ % 2 } <== eager @data;
@data <== eager grep { $_ % 2 } <== @data;

Conjecture: if the cloning process eagerly duplicates @data, it could be forced to work. Not clear if this is desirable, since ordinary clones just clone the container, not the value.

Leftward feeds are a convenient way of explicitly indicating the typical right-to-left flow of data through a chain of operations:

@oddsquares = map { $_**2 }, sort grep { $_ % 2 }, @nums;

# perhaps more clearly written as...

@oddsquares = do {
    map { $_**2 } <== sort <== grep { $_ % 2 } <== @nums;
}

Rightward feeds are a convenient way of reversing the normal data flow in a chain of operations, to make it read left-to-right:

@oddsquares = do {
    @nums ==> grep { $_ % 2 } ==> sort ==> map { $_**2 };
}

Note that something like the do is necessary because feeds operate at the statement level. Parens would also work, since a statement is expected inside:

@oddsquares = (
    @nums ==> grep { $_ % 2 } ==> sort ==> map { $_**2 };
);

But as described below, you can also just write:

@nums ==> grep { $_ % 2 } ==> sort ==> map { $_**2 } ==> @oddsquares;

If the operand on the sharp end of a feed is not a call to a variadic operation, it must be something else that can be interpreted as a list receiver, or a scalar expression that can be evaluated to produce an object that does the KitchenSink role, such as an IO object. Such an object provides .clear and .push methods that will be called as appropriate to send data. (Note that an IO object used as a sink will force eager evaluation on its pipeline, so the next statement is guaranteed not to run till the file is closed. In contrast, an Array object used as a sink turns into a lazy array.)

Any non-variadic object (such as an Array or IO object) used as a filter between two feeds is treated specially as a tap that merely captures data en passant. You can safely install such a tap in an extended pipeline without changing the semantics. An IO object used as a tap does not force eager evaluation since the eagerness is controlled instead by the downstream feed.

Any prefix list operator is considered a variadic operation, so ordinarily a list operator adds any feed input to the end of its list. But sometimes you want to interpolate elsewhere, so any contextualizer with * as an argument may be used to indicate the target of a feed without the use of a temporary array:

foo() ==> say @(*), " is what I meant";
bar() ==> @@(*).baz();

Likewise, an Array used as a tap may be distinguished from an Array used as a translation function:

numbers() ==> @array ==> bar()          # tap
numbers() ==> @array[@(*)] ==> bar()    # translation

Feeding into the * "whatever" term sets the source for the next sink. To append multiple sources to the next sink, double the angle:

0..*       ==>  *;
'a'..*     ==>> *;
pidigits() ==>> *;

# outputs "(0, 'a', 3)\n"...
for zip(@@(*)) { .perl.say }

You may use a variable (or variable declaration) as a receiver, in which case the list value is bound as the "todo" of the variable. (The append form binds addition todos to the receiver's todo list.) Do not think of it as an assignment, nor as an ordinary binding. Think of it as iterator creation. In the case of a scalar variable, that variable contains the newly created iterator itself. In the case of an array, the new iterator is installed as the method for extending the array. As with assignment, the old todo list is clobbered; use the append form to avoid that and get push semantics.

In general you can simply think of a receiver array as representing the results of the chain, so you can equivalently write any of:

my @oddsquares <== map { $_**2 } <== sort <== grep { $_ % 2 } <== @nums;

my @oddsquares
    <== map { $_**2 }
    <== sort
    <== grep { $_ % 2 }
    <== @nums;

@nums ==> grep { $_ % 2 } ==> sort ==> map { $_**2 } ==> my @oddsquares;

@nums
==> grep { $_ % 2 }
==> sort
==> map { $_**2 }
==> my @oddsquares;

Since the feed iterator is bound into the final variable, the variable can be just as lazy as the feed that is producing the values.

When feeds are bound to arrays with "push" semantics, you can have a receiver for multiple feeds:

my @foo;
0..2       ==>  @foo;
'a'..'c'   ==>> @foo;
say @foo;   # 0,1,2,'a','b','c'

Note how the feeds are concatenated in @foo so that @foo is a list of 6 elements. This is the default behavior. However, sometimes you want to capture the outputs as a list of two iterators, namely the two iterators that represent the two input feeds. You can get at those two iterators by using the name @@foo instead, where the "slice" sigil marks a multidimensional array, that is, an array of lists, each of which may be treated independently.

0..*       ==>  @@foo;
'a'..*     ==>> @@foo;
pidigits() ==>> @@foo;

for zip(@@foo) { .say }

    [0,'a',3]
    [1,'b',1]
    [2,'c',4]
    [3,'d',1]
    [4,'e',5]
    [5,'f',9]
    ...

Here @@foo is an array of three iterators, so

zip(@@foo)

is equivalent to

zip(@@foo[0]; @@foo[1]; @@foo[2])

A semicolon inside brackets is equivalent to stacked feeds. The code above could be rewritten as:

(0..*; 'a'..*; pidigits()) ==> my @@foo;
for @@foo.zip { .say }

which is in turn equivalent to

for zip(0..*; 'a'..*; pidigits()) { .say }

A named receiver array is useful when you wish to feed into an expression that is not an ordinary list operator, and you wish to be clear where the feed's destination is supposed to be:

picklist() ==> my @baz;
my @foo = @bar[@baz];

Various contexts may or may not be expecting multi-dimensional slices or feeds. By default, ordinary arrays are flattened, that is, they have "list" semantics. If you say

(0..2; 'a'..'c') ==> my @tmp;
for @tmp { .say }

then you get 0,1,2,'a','b','c'. If you have a multidim array, you can ask for list semantics explicitly with list():

(0..2; 'a'..'c') ==> my @@tmp;
for @@tmp.list { .say }

As we saw earlier, "zip" produces an interleaved result by taking one element from each list in turn, so

(0..2; 'a'..'c') ==> my @@tmp;
for @@tmp.zip { .say }

produces 0,'a',1,'b',2,'c'.

If you want the result as a list of subarrays, then you need to put the zip into a "chunky" context instead:

(0..2; 'a'..'c') ==> my @@tmp;
for @@tmp.zip.@@() { .say }

This produces [0,'a'],[1,'b'],[2,'c']. But usually you want the flat form so you can just bind it directly to a signature:

for @@tmp.zip -> $i, $a { say "$i: $a" }

Otherwise you'd have to say this:

for @@tmp.zip.@@() -> [$i, $a] { say "$i: $a" }

In list context the @@foo notation is really a shorthand for [;](@@foo). In particular, you can use @@foo to interpolate a multidimensional slice in an array or hash subscript.

If @@foo is currently empty, then for zip(@@foo) {...} acts on a zero-dimensional slice (i.e. for (zip) {...}), and outputs nothing at all.

Note that with the current definition, the order of feeds is preserved left to right in general regardless of the position of the receiver.

So

('a'..*; 0..*) ==> *;
 for zip(@@() <== @foo) -> $a, $i, $x { ... }

is the same as

'a'..* ==> *;
 0..*  ==> *;
 for zip(@@ <== @foo) -> $a, $i, $x { ... }

which is the same as

for zip('a'..*; 0..*; @foo) -> $a, $i, $x { ... }

Also note that these come out to be identical for ordinary arrays:

@foo.zip
@foo.cat

The @@($foo) coercer can be used to pull a multidim out of some object that contains one, such as a Capture or Match object. Like @(), @@() defaults to @@($/), and returns a multidimensional view of any match that repeatedly applies itself with :g and the like. In contrast, @() would flatten those into one list.

Closure parameters

Parameters declared with the & sigil take blocks, closures, or subroutines as their arguments. Closure parameters can be required, optional, named, or slurpy.

sub limited_grep (Int $count, &block, *@list) {...}

# and later...

@first_three = limited_grep 3, {$_<10}, @data;

(The comma is required after the closure.)

Within the subroutine, the closure parameter can be used like any other lexically scoped subroutine:

sub limited_grep (Int $count, &block, *@list) {
    ...
    if block($nextelem) {...}
    ...
}

The closure parameter can have its own signature in a type specification written with :(...):

sub limited_Dog_grep ($count, &block:(Dog), Dog *@list) {...}

and even a return type:

sub limited_Dog_grep ($count, &block:(Dog --> Bool), Dog *@list) {...}

When an argument is passed to a closure parameter that has this kind of signature, the argument must be a Code object with a compatible parameter list and return type.

Type parameters

Unlike normal parameters, type parameters often come in piggybacked on the actual value as "kind", and you'd like a way to capture both the value and its kind at once. (A "kind" is a class or type that an object is allowed to be. An object is not officially allowed to take on a constrained or contravariant type.) A type variable can be used anywhere a type name can, but instead of asserting that the value must conform to a particular type, it captures the actual "kind" of the object and also declares a package/type name by which you can refer to that kind later in the signature or body. For instance, if you wanted to match any two Dogs as long as they were of the same kind, you can say:

sub matchedset (Dog ::T $fido, T $spot) {...}

(Note that ::T is not required to contain Dog, only a type that is compatible with Dog.)

The :: sigil is short for "subset" in much the same way that & is short for "sub". Just as & can be used to name any kind of code, so too :: can be used to name any kind of type. Both of them insert a bare identifier into the symbol table, though they fill different syntactic spots.

Note that it is not required to capture the object associated with the class unless you want it. The sub above could be written as

sub matchedset (Dog ::T, T) {...}

if we're not interested in $fido or $spot. Or just

sub matchedset (::T, T) {...}

if we don't care about anything but the matching.

Unpacking array parameters

Instead of specifying an array parameter as an array:

sub quicksort (@data, $reverse?, $inplace?) {
    my $pivot := shift @data;
    ...
}

it may be broken up into components in the signature, by specifying the parameter as if it were an anonymous array of parameters:

sub quicksort ([$pivot, *@data], $reverse?, $inplace?) {
    ...
}

This subroutine still expects an array as its first argument, just like the first version.

Unpacking a single list argument

To match the first element of the slurpy list, use a "slurpy" scalar:

sub quicksort (:$reverse, :$inplace, *$pivot, *@data)

Unpacking hash parameters

Likewise, a hash argument can be mapped to a hash of parameters, specified as named parameters within curlies. Instead of saying:

sub register (%guest_data, $room_num) {
    my $name := delete %guest_data<name>;
    my $addr := delete %guest_data<addr>;
    ...
}

you can get the same effect with:

sub register ({:$name, :$addr, *%guest_data}, $room_num) {
    ...
}

Unpacking tree node parameters

You can unpack tree nodes in various dwimmy ways by enclosing the bindings of child nodes and attributes in parentheses following the declaration of the node itself:

sub traverse ( BinTree $top ( $left, $right ) ) {
    traverse($left);
    traverse($right);
}

In this, $left and $right are automatically bound to the left and right nodes of the tree. If $top is an ordinary object, it binds the $top.left and $top.right attributes. If it's a hash, it binds $top<left> and $top<right>. If BinTree is a signature type and $top is a List (argument list) object, the child types of the signature are applied to the actual arguments in the argument list object. (Signature types have the benefit that you can view them inside-out as constructors with positional arguments, such that the transformations can be reversible.)

However, the full power of signatures can be applied to pattern match just about any argument or set of arguments, even though in some cases the reverse transformation is not derivable. For instance, to bind to an array of children named .kids or .<kids>, use something like:

multi traverse ( NAry $top ( :kids [$eldest, *@siblings] ) ) {
    traverse($eldest);
    traverse(:kids(@siblings));  # (binds @siblings to $top)
}
multi traverse ( $leaf ) {...}

The second candidate is called only if the parameter cannot be bound to both $top and to the "kids" parsing subparameter.

Likewise, to bind to a hash element of the node and then bind to keys in that hash by name:

sub traverse ( AttrNode $top ( :%attr{ :$vocalic, :$tense } ) ) {
    say "Has {+%attr} attributes, of which";
    say "vocalic = $vocalic";
    say "tense = $tense";
}

You may omit the top variable if you prefix the parentheses with a colon to indicate a signature. Otherwise you must at least put the sigil of the variable, or we can't correctly differentiate:

my Dog ($fido, $spot)   := twodogs();       # list of two dogs
my Dog $ ($fido, $spot) := twodogs();       # one twodog object
my Dog :($fido, $spot)  := twodogs();       # one twodog object

Sub signatures can be matched directly within regexes by using :(...) notation.

push @a, "foo";
push @a, \(1,2,3);
push @a, "bar";
...
my ($i, $j, $k);
@a ~~ rx/
        <,>                         # match initial elem boundary
        :(Int $i,Int $j,Int? $k)    # match lists with 2 or 3 ints
        <,>                         # match final elem boundary
      /;
say "i = $<i>";
say "j = $<j>";
say "k = $<k>" if defined $<k>;

If you want a parameter bound into $/, you have to say $<i> within the signature. Otherwise it will try to bind an external $i instead, and fail if no such variable is declared.

Note that unlike a sub declaration, a regex-embedded signature has no associated "returns" syntactic slot, so you have to use --> within the signature to specify the of type of the signature, or match as an arglist:

:(Num, Num --> Coord)
:(\Coord(Num, Num))

A consequence of the latter form is that you can match the type of an object with :(\Dog) without actually breaking it into its components. Note, however, that it's not equivalent to say

:(--> Dog)

which would be equivalent to

:(\Dog())

that is, match a nullary function of type Dog. Nor is it equivalent to

:(Dog)

which would be equivalent to

:(\Any(Dog))

and match a function taking a single parameter of type Dog.

Note also that bare \(1,2,3) is never legal in a regex since the first (escaped) paren would try to match literally.

Attributive parameters

If a submethod's parameter is declared with a . or ! after the sigil (like an attribute):

submethod initialize($.name, $!age) {}

then the argument is assigned directly to the object's attribute of the same name. This avoids the frequent need to write code like:

submethod initialize($name, $age) {
    $.name = $name;
    $!age  = $age;
}

To rename an attribute parameter you can use the explicit pair form:

submethod initialize(:moniker($.name), :youth($!age)) {}

The :$name shortcut may be combined with the $.name shortcut, but the twigil is ignored for the parameter name, so

submethod initialize(:$.name, :$!age) {}

is the same as:

submethod initialize(:name($.name), :age($!age)) {}

Note that $!age actually refers to the private "has" variable that can be referred to as either $age or $!age.

Placeholder variables

Even though every bare block is a closure, bare blocks can't have explicit parameter lists. Instead, they use "placeholder" variables, marked by a caret (^) after their sigils.

Using placeholders in a block defines an implicit parameter list. The signature is the list of distinct placeholder names, sorted in Unicode order. So:

{ $^y < $^z && $^x != 2 }

is a shorthand for:

-> $x,$y,$z { $y < $z && $x != 2 }

Note that placeholder variables syntactically cannot have type constraints. Also, it is illegal to use placeholder variables in a block that already has a signature, because the autogenerated signature would conflict with that. Placeholder names consisting of a single uppercase letter are disallowed, not because we're mean, but because it helps us catch references to obsolete Perl 5 variables such as $^O.

Properties and traits

Compile-time properties are called "traits". The is NAME (DATA) syntax defines traits on containers and subroutines, as part of their declaration:

constant $pi is Approximated = 3;   # variable $pi has Approximated trait

my $key is Persistent(:file<.key>);

sub fib is cached {...}

The will NAME BLOCK syntax is a synonym for is NAME (BLOCK):

my $fh will undo { close $fh };    # Same as: my $fh is undo({ close $fh });

The but NAME (DATA) syntax specifies run-time properties on values:

constant $pi = 3 but Inexact;       # value 3 has Inexact property

sub system {
    ...
    return $error but False if $error;
    return 0 but True;
}

Properties are predeclared as roles and implemented as mixins--see S12.

Subroutine traits

These traits may be declared on the subroutine as a whole (individual parameters take other traits). Trait syntax depends on the particular auxiliary you use, but for is, the subsequent syntax is identical to adverbial syntax, except that that colon may be omitted or doubled depending on the degree of ambiguity desired:

is ::Foo[...]       # definitely a parameterized typename
is :Foo[...]        # definitely a pair with a list
is Foo[...]         # depends on whether Foo is predeclared as type

is signature

The signature of a subroutine. Normally declared implicitly, by providing a parameter list and/or return type.

returns/is returns

The inner type constraint that a routine imposes on its return value.

of/is of

The of type that is the official return type of the routine. Or you can think of "of" as outer/formal. If there is no inner type, the outer type also serves as the inner type to constrain the return value.

will do

The block of code executed when the subroutine is called. Normally declared implicitly, by providing a block after the subroutine's signature definition.

is rw

Marks a subroutine as returning an lvalue.

is parsed

Specifies the subrule by which a macro call is parsed. The parse always starts after the macro's initial token. If the operator has two parts (circumfix or postcircumfix), the final token is also automatically matched, and should not be matched by the supplied regex.

is reparsed

Also specifies the subrule by which a macro call is parsed, but restarts the parse before the macro's initial token, usually because you want to parse using an existing rule that expects to traverse the initial token. If the operator has two parts (circumfix or postcircumfix), the final token must also be explicitly matched by the supplied regex.

is cached

Marks a subroutine as being memoized, or at least memoizable. In the abstract, this cache is just a hash where incoming argument Captures are mapped to return values. If the Capture is found in the hash, the return value need not be recalculated. If you use this trait, the compiler will assume two things:

• A given Capture would always calculate the same return value. That is, there is no state hidden within the dynamic scope of the call.

• The cache lookup is likely to be more efficient than recalculating the value in at least some cases, because either most uncached calls would be slower (and reduce throughput), or you're trying to avoid a significant number of pathological cases that are unacceptably slow (and increase latency).

This trait is a suggestion to the compiler that caching is okay. The compiler is free to choose any kind of caching algorithm (including non-expiring, random, lru, pseudo-lru, or adaptive algoritms, or even no caching algorithm at all). The run-time system is free to choose any kind of maximum cache size depending on the availability of memory and trends in usage patterns. You may suggest a particular cache size by passing a numeric argument (representing the maximum number of unique Capture values allowed), and some of the possible algorithms may pay attention to it. You may also pass * for the size to request a non-expiring cache (complete memoization). The compiler is free to ignore this too.

The intent of this trait is to specify performance hints without mandating any exact behavior. Proper use of this trait should not change semantics of the program; it functions as a kind of "pragma". This trait will not be extended to reinvent other existing ways of achieving the same effect. To gain more control, write your own trait handler to allow the use of a more specific trait, such as "is lru(42)". Alternately, just use a state hash keyed on the sub's argument capture to write your own memoization with complete control from within the subroutine itself, or from within a wrapper around your subroutine.

is inline

Suggests to the compiler that the subroutine is a candidate for optimization via inlining. Basically promises that nobody is going to try to wrap this subroutine (or that if they do, you don't care).

is tighter/is looser/is equiv

Specifies the precedence of an operator relative to an existing operator. tighter and looser operators default to being left associative.

equiv on the other hand also clones other traits, so it specifies the default associativity to be the same as the operator to which the new operator is equivalent. The following are the default equivalents for various syntactic categories if neither equiv nor assoc is specified. (Many of these have no need of precedence or associativity because they are parsed specially. Nevertheless, equiv may be useful for cloning other traits of these operators.)

category:<prefix>
circumfix:<( )>
dotty:<.>
infix:<+>
infix_circumfix_meta_operator:{'»','«'}
infix_postfix_meta_operator:<=>
infix_prefix_meta_operator:<!>
package_declarator:<class>
postcircumfix:<( )>
postfix:<++>
postfix_prefix_meta_operator:{'»'}
prefix:<++>
prefix_circumfix_meta_operator:{'[',']'} 
prefix_postfix_meta_operator:{'«'}
q_backslash:<\\>
qq_backslash:<n>
quote_mod:<c>
quote:<q>
regex_assertion:<?>
regex_backslash:<w>
regex_metachar:<.>
regex_mod_internal:<i>
routine_declarator:<sub>
scope_declarator:<my>
sigil:<$>
special_variable:<$!>
statement_control:<if>
statement_mod_cond:<if>
statement_mod_loop:<while>
statement_prefix:<do>
term:<*>
trait_auxiliary:<is>
trait_verb:<of>
twigil:<?>
type_declarator:<subset>
version:<v>

The existing operator may be specified either as a function object or as a string argument equivalent to the one that would be used in the complete function name. In string form the syntactic category will be assumed to be the same as the new declaration. Therefore these all have the same effect:

sub postfix:<!> ($x) is equiv(&postfix:<++>) {...}
sub postfix:<!> ($x) is equiv<++> {...}
sub postfix:<!> ($x) {...}      # since equiv<++> is the default

Prefix operators that are identifiers are handled specially. Both of

sub foo ($) {...}
sub prefix:<foo> ($) {...}

default to named unary precedence despite declaring a prefix operator. Likewise postfix operators that look like method calls are forced to default to the precedence of method calls. Any prefix operator that requires multiple arguments defaults to listop precedence, even if it is not an identifier.

is assoc

Specifies the associativity of an operator explicitly. Valid values are:

Tag         Examples        Meaning of $a op $b op $c       Default equiv
===         ========        =========================       =============
left        + - * / x       ($a op $b) op $c                +
right       ** =            $a op ($b op $c)                **
non         cmp <=> ..      ILLEGAL                         cmp
chain       == eq ~~        ($a op $b) and ($b op $c)       eqv
list        | & ^ Z         op($a; $b; $c)                  |

Note that operators "equiv" to relationals are automatically considered chaining operators. When creating a new precedence level, the chaining is determined by the presence or absence of "is assoc<chain>", and other operators defined at that level are required to be the same.

Specifying an assoc without an explicit equiv substitutes a default equiv consistent with the associativity, as shown in the final column above.

PRE/POST

Mark blocks that are to be unconditionally executed before/after the subroutine's do block. These blocks must return a true value, otherwise an exception is thrown.

When applied to a method, the semantics provide support for the "Design by Contract" style of OO programming: a precondition of a particular method is met if all the PRE blocks associated with that method return true. Otherwise, the precondition is met if all of the parent classes' preconditions are met (which may include the preconditions of their parent classes if they fail, and so on recursively.)

In contrast, a method's postcondition is met if all the method's POST blocks return true and all its parents' postconditions are also met recursively.

POST blocks (and "will post" block traits) declared within a PRE or ENTER block are automatically hoisted outward to be called at the same time as other POST blocks. This conveniently gives "circum" semantics by virtue of wrapping the post lexical scope within the pre lexical scope.

method push ($new_item) {
    ENTER {
        my $old_height = self.height;
        POST { self.height == $old_height + 1 }
    }

    $new_item ==> push @.items;
}

method pop () {
    ENTER {
        my $old_height = self.height;
        POST { self.height == $old_height - 1 }
    }

    return pop @.items;
}

[Conjecture: class and module invariants can similarly be supplied by embedding POST/post declarations in a FOREIGN block that only runs when any routine of this module is called from "outside" the current module or type, however that's defined. The FOREIGN block itself could perhaps refine the concept of what is foreign, much like an exception handler.]

ENTER/LEAVE/KEEP/UNDO/etc.

These supply closures that are to be conditionally executed before or after the subroutine's do block (only if used at the outermost level within the subroutine; technically, these are block traits on the do block, not subroutine traits). These blocks are generally used only for their side effects, since most return values will be ignored.

Parameter traits

The following traits can be applied to many types of parameters.

is readonly

Specifies that the parameter cannot be modified (e.g. assigned to, incremented). It is the default for parameters. On arguments which are already immutable values it is a no-op at run time; on mutable containers it may need to create an immutable alias to the mutable object if the constraint cannot be enforced entirely at compile time. Binding to a readonly parameter never triggers autovivification.

is rw

Specifies that the parameter can be modified (assigned to, incremented, etc). Requires that the corresponding argument is an lvalue or can be converted to one.

When applied to a variadic parameter, the rw trait applies to each element of the list:

sub incr (*@vars is rw) { $_++ for @vars }

(The variadic array as a whole is always modifiable, but such modifications have no effect on the original argument list.)

is ref

Specifies that the parameter is passed by reference. Unlike is rw, the corresponding argument must already be a suitable lvalue. No attempt at coercion or autovivification is made, so unsuitable values throw an exception if you try to modify them within the body of the routine.

is copy

Specifies that the parameter receives a distinct, read-writable copy of the original argument. This is commonly known as "pass-by-value".

sub reprint ($text, $count is copy) {
    print $text while $count-- > 0;
}

Binding to a copy parameter never triggers autovivification.

is context(ACCESS)

Specifies that the parameter is to be treated as an "environmental" variable, that is, a lexical that is accessible from the dynamic scope (see S02). If ACCESS is omitted, defaults to readonly in any portions of the dynamic scope outside the current lexical scope.

Advanced subroutine features

The return function

The return function notionally throws a control exception that is caught by the current lexically enclosing Routine to force a return through the control logic code of any intermediate block constructs. (That is, it must unwind the stack of dynamic scopes to the proper lexical scope belonging to this routine.) With normal blocks (those that are autoexecuted in place because they're known to the compiler) this unwinding can likely be optimized away to a "goto". All Routine declarations have an explicit declarator such as sub or method; bare blocks and "pointy" blocks are never considered to be routines in that sense. To return from a block, use leave instead--see below.

The return function preserves its argument list as a Capture object, and responds to the left-hand Signature in a binding. This allows named return values if the caller expects one:

sub f () { return :x<1> }
sub g ($x) { print $x }

my $x := |(f);  # binds 1 to $x, via a named argument
g(|(f));        # prints 1, via a named argument

To return a literal Pair object, always put it in an additional set of parentheses:

return( (:x<1>), (:y<2>) ); # two positional Pair objects

Note that the postfix parentheses on the function call don't count as being "additional". However, as with any function, whitespace after the return keyword prevents that interpretation and turns it instead into a list operator:

return :x<1>, :y<2>; # two named arguments (if caller uses |)
return ( :x<1>, :y<2> ); # two positional Pair objects

If the function ends with an expression without an explicit return, that expression is also taken to be a Capture, just as if the expression were the argument to a return list operator (with whitespace):

sub f { :x<1> } # named-argument binding (if caller uses |)
sub f { (:x<1>) } # always just one positional Pair object 

On the caller's end, the Capture is interpolated into any new argument list much like an array would be, that is, as a scalar in scalar context, and as a list in list context. This is the default behavior, but the caller may use prefix:<|> to inline the returned values as part of the new argument list. The caller may also bind the returned Capture directly.

If any function called as part of a return list asks what its context is, it will be told it was called in list context regardless of the eventual binding of the returned Capture. (This is quite different from Perl 5, where a return statement always propagates its caller's context to its own argument(s).) If that is not the desired behavior you must coerce the call to an appropriate context, (or declare the return type of the function to perform such a coercion). In any event, such a function is called only once at the time the Capture object is generated, not when it is later bound (which could happen more than once).

The context and caller functions

The context function takes a list of matchers and interprets them as a navigation path from the current context to a location in the dynamic scope, either the current context itself or some context from which the current context was called. It returns an object that describes that particular dynamic scope, or a false value if there is no such scope. Numeric arguments are interpreted as number of contexts to skip, while non-numeric arguments scan outward for a context matching the argument as a smartmatch.

The current context is accessed with a null argument list.

say " file ", context().file,
    " line ", context().line;

which is equivalent to:

say " file ", CONTEXT::<$?FILE>,
    " line ", CONTEXT::<$?LINE>;

The immediate caller of this context is accessed by skipping one level:

say " file ", context(1).file,
    " line ", context(1).line;

You might think that that must be the current function's caller, but that's not necessarily so. This might return an outer block in our own routine, or even some function elsewhere that implements a control operator on behalf of our block. To get outside your current routine, see caller below.

The context function may be given arguments telling it which higher scope to look for. Each argument is processed in order, left to right. Note that Any and 0 are no-ops:

$ctx = context();        # currently running context for &?BLOCK
$ctx = context(Any);     # currently running context for &?BLOCK
$ctx = context(Any,Any); # currently running context for &?BLOCK
$ctx = context(1);       # my context's context
$ctx = context(2);       # my context's context's context
$ctx = context(3);       # my context's context's context's context
$ctx = context(1,0,1,1); # my context's context's context's context
$ctx = context($i);      # $i'th context

Note also that negative numbers are allowed as long as you stay within the existing context stack:

$ctx = context(4,-1);    # my context's context's context's context

Repeating any smartmatch just matches the same context again unless you intersperse a 1 to skip the current level:

$ctx = context(Method);              # nearest context that is method
$ctx = context(Method,Method);       # nearest context that is method
$ctx = context(Method,1,Method);     # 2nd nearest method context
$ctx = context(Method,1,Method,1)    # caller of that 2nd nearest method
$ctx = context(1,Block);             # nearest outer context that is block
$ctx = context(Sub,1,Sub,1,Sub);     # 3rd nearest sub context
$ctx = context({ .labels.any eq 'Foo' }); # nearest context labeled 'Foo'

Note that this last potentially differs from the answer returned by

Foo.context

which returns the context of the innermost Foo block in the lexical scope rather than the dynamic scope. A context also responds to the .context method, so a given context may be used as the basis for further navigation:

$ctx = context(Method,1,Method);
$ctx = context(Method).context(1).context(Method);     # same

You must supply args to get anywhere else, since .context is the identity operator when called on something that is already a Context:

$ctx = context;
$ctx = context.context.context.context;                # same

The caller function is special-cased to go outward just far enough to escape from the current routine scope, after first ignoring any inner blocks that are embedded, or are otherwise pretending to be "inline":

&caller ::= &context.assuming({ !.inline }, 1);

Note that this is usually the same as context(&?ROUTINE,1), but not always. A call to a returned closure might not even have &?ROUTINE in its dynamic scope anymore, but it still has a caller.

So to find where the current routine was called you can say:

say " file ", caller.file,
    " line ", caller.line;

which is equivalent to:

say " file ", CALLER::<$?FILE>,
    " line ", CALLER::<$?LINE>;

Additional arguments to caller are treated as navigational from the calling context. One context out from your current routine is not guaranteed to be a Routine context. You must say caller(Routine) to get to the next-most-inner routine.

Note that caller(Routine).line is not necessarily going to give you the line number that your current routine was called from; you're rather likely to get the line number of the topmost block that is executing within that outer routine, where that block contains the call to your routine.

For either context or caller, the returned context object supports at least the following methods:

.context
.caller
.leave
.want
.inline
.package
.file
.line
.my
.hints

The .context and .caller methods work the same as the functions except that they are relative to the context supplied as invocant. The .leave method can force an immediate return from the specified context. The .want method returns known smart-matchable characteristics of the specified context.

The .inline method says whether this block was entered implicitly by some surrounding control structure. Any time you invoke a block or routine explicitly with .() this is false. However, it is defined to be true for any block entered using dispatcher-level primitives such as .callwith, .callsame, .nextwith, or .nextsame.

The .my method provides access to the lexical namespace in effect at the given dynamic context's current position. It may be used to look up ordinary lexical variables in that lexical scope. It must not be used to change any lexical variable that is not marked as context<rw>.

The .hints method gives access to a snapshot of compiler symbols in effect at the point of the call when the call was originally compiled. (For instance, caller.hints('&?ROUTINE') will give you the caller's routine object.) Such values are always read-only, though in the case of some (like the caller's routine above) may return a fixed object that is nevertheless mutable.

The want function

The want function returns a Signature object that contains information about the context in which the current block, closure, or subroutine was called. The want function is really just short for caller.want. (Note that this is what your current routine's caller wants from your routine, not necessarily the same as context.want when you are embedded in a block within a subroutine. Use context.want if that's what you want.)

As with normal function signatures, you can test the result of want with a smart match (~~) or a when:

given want {
     when :($)       {...}   # called in scalar context
     when :(*@)      {...}   # called in list context
     when :($ is rw) {...}   # expected to return an lvalue
     when :($,$)     {...}   # expected to return two values
     ...
 }

Or use its shorthand methods to reduce line noise:

if    want.item      {...}  # called in non-lvalue scalar context
elsif want.list      {...}  # called in list context
elsif want.void      {...}  # called in void context
elsif want.rw        {...}  # expected to return an lvalue

The .arity and .count methods also work here:

if want.arity > 2    {...}  # must return more than two values
if want.count > 2    {...}  # can return more than two values

Their difference is that .arity considers only mandatory parts, while .count considers also optional ones, including *$:

($x, $y) = f();     # Within &f, want === :(*$?, *$?, *@)
                    #            want.arity === 0
                    #            want.count === 2

The leave function

As mentioned above, a return call causes the innermost surrounding subroutine, method, rule, token, regex (as a keyword) or macro to return. Only declarations with an explicit declarator keyword (sub, submethod, method, macro, regex, token, and rule) may be returned from. Statement prefixes such a do and try do not fall into that category. You cannot use return to escape directly into the surrounding context from loops, bare blocks, pointy blocks, or quotelike operators such as rx//; a return within one of those constructs will continue searching outward for a "proper" routine to return from. Nor may you return from property blocks such as BEGIN or CATCH (though blocks executing within the lexical and dynamic scope of a routine can of course return from that outer routine, which means you can always return from a CATCH or a FIRST, but never from a BEGIN or INIT.)

To return from blocks that aren't routines, the leave method is used instead. (It can be taken to mean either "go away from" or "bequeath to your successor" as appropriate.) The object specifies the scope to exit, and the method's arguments specify the return value. If the object is omitted (by use of the function or listop forms), the innermost block is exited. Otherwise you must use something like context or &?BLOCK or a contextual variable to specify the scope you want to exit. A label (such as a loop label) previously seen in the lexical scope also works as a kind of singleton context object: it names a statement that is serving both as an outer lexical scope and as a context in the current dynamic scope.

As with return, the arguments are taken to be a Capture holding the return values.

leave;                      # return from innermost block of any kind
context(Method).leave;      # return from innermost calling method
&?ROUTINE.leave(1,2,3);     # Return from current sub. Same as: return 1,2,3
&?ROUTINE.leave <== 1,2,3;  # same thing, force return as feed
OUTER.leave;                # Return from OUTER label in lexical scope
&foo.leave: 1,2,3;          # Return from innermost surrounding call to &foo

Note that these are equivalent in terms of control flow:

COUNT.leave;
last COUNT;

However, the first form explicitly sets the return value for the entire loop, while the second implicitly returns all the previous successful loop iteration values as a list comprehension. (It may, in fact, be too late to set a return value for the loop if it is being evaluated lazily!) A leave from the inner loop block, however, merely specifies the return value for that iteration:

for 1..10 { leave $_ * 2 }   # 2..20

Note that this:

leave COUNT;

will always be taken as the function, not the method, so it returns the COUNT object from the innermost block. The indirect object form of the method always requires a colon:

leave COUNT: ;

Temporization

The temp macro temporarily replaces the value of an existing variable, subroutine, context of a function call, or other object in a given scope:

{
   temp $*foo = 'foo';      # Temporarily replace global $foo
   temp &bar := sub {...};  # Temporarily replace sub &bar
   ...
} # Old values of $*foo and &bar reinstated at this point

temp invokes its argument's .TEMP method. The method is expected to return a Code object that can later restore the current value of the object. At the end of the lexical scope in which the temp was applied, the subroutine returned by the .TEMP method is executed.

The default .TEMP method for variables simply creates a closure that assigns the variable's pre-temp value back to the variable.

New kinds of temporization can be created by writing storage classes with their own .TEMP methods:

class LoudArray is Array {
    method TEMP {
        print "Replacing $.WHICH() at {caller.location}\n";
        my $restorer = $.SUPER::TEMP();
        return { 
            print "Restoring $.WHICH() at {caller.location}\n";
            $restorer();
        };
    }
}

You can also modify the behaviour of temporized code structures, by giving them a TEMP block. As with .TEMP methods, this block is expected to return a closure, which will be executed at the end of the temporizing scope to restore the subroutine to its pre-temp state:

my $next = 0;
sub next {
    my $curr = $next++;
    TEMP {{ $next = $curr }}  # TEMP block returns the closure { $next = $curr }
    return $curr;
}

# and later...

say next();     # prints 0; $next == 1
say next();     # prints 1; $next == 2
say next();     # prints 2; $next == 3
if ($hiccough) {
    say temp next();  # prints 3; closes $curr at 3; $next == 4
    say next();       # prints 4; $next == 5
    say next();       # prints 5; $next == 6
}                     # $next = 3
say next();     # prints 3; $next == 4
say next();     # prints 4; $next == 5

Note that temp must be a macro rather than a function because the temporization must be arranged before the function causes any state changes, and if it were a normal argument to a normal function, the state change would be happen before temp got control.

Hypothetical variables use the same mechanism, except that the restoring closure is called only on failure.

Note that contextual variables may be a better solution than temporized globals in the face of multithreading.

Wrapping

Every Routine object has a .wrap method. This method expects a single Code argument. Within the code, the special callsame, callwith, nextsame and nextwith functions will invoke the original routine, but do not introduce an official CALLER frame:

sub thermo ($t) {...}   # set temperature in Celsius, returns old value

# Add a wrapper to convert from Fahrenheit...
$handle = &thermo.wrap( { callwith( ($^t-32)/1.8 ) } );

The callwith function lets you pass your own arguments to the wrapped function. The callsame function takes no argument; it implicitly passes the original argument list through unchanged.

The call to .wrap replaces the original Routine with the Code argument, and arranges that any call to callsame, callwith, nextsame or nextwith invokes the previous version of the routine. In other words, the call to .wrap has more or less the same effect as:

&old_thermo := &thermo;
&thermo := sub ($t) { old_thermo( ($t-32)/1.8 ) }

except that &thermo is mutated in-place, so &thermo.WHICH stays the same after the .wrap.

The call to .wrap returns a unique handle that can later be passed to the .unwrap method, to undo the wrapping:

&thermo.unwrap($handle);

This does not affect any other wrappings placed to the routine.

A wrapping can also be restricted to a particular dynamic scope with temporization:

# Add a wrapper to convert from Kelvin
# wrapper self-unwraps at end of current scope
temp &thermo.wrap( { callwith($^t + 273.16) } );

The entire argument list may be captured by binding to a Capture parameter. It can then be passed to callwith using that name:

# Double the return value for &thermo
&thermo.wrap( -> |$args { callwith(|$args) * 2 } );

In this case only the return value is changed.

The wrapper is not required to call the original routine; it can call another Code object by passing the Capture to its callwith method:

# Transparently redirect all calls to &thermo to &other_thermo
&thermo.wrap( sub (|$args) { &other_thermo.callwith(|$args) } );

or more briefly:

&thermo.wrap( { &other_thermo.callsame } );

Since the method versions of callsame, callwith, nextsame, and nextwith specify an explicit destination, their semantics do not change outside of wrappers. However, the corresponding functions have no explicit destination, so instead they implicitly call the next-most-likely method or multi-sub; see S12 for details.

As with any return value, you may capture the returned Capture of call by binding:

my |$retval := callwith(|$args);
... # postprocessing
return |$retval;

Alternately, you may prevent any return at all by using the variants nextsame and nextwith. Arguments are passed just as with callsame and callwith, but a tail call is explicitly enforced; any code following the call will be unreached, as if a return had been executed there before calling into the destination routine.

Within an ordinary method dispatch these functions treat the rest of the dispatcher's candidate list as the wrapped function, which generally works out to calling the same method in one of our parent (or older sibling) classes. Likewise within a multiple dispatch the current routine may defer to candidates further down the candidate list. Although not necessarily related by a class hierarchy, such later candidates are considered more generic and hence likelier to be able to handle various unforeseen conditions (perhaps).

The &?ROUTINE object

&?ROUTINE is always an alias for the lexically innermost Routine (which may be a Sub, Method, or Submethod), so you can specify tail-recursion on an anonymous sub:

my $anonfactorial = sub (Int $n) {
                        return 1 if $n<2;
                        return $n * &?ROUTINE($n-1);
                    };

You can get the current routine name by calling &?ROUTINE.name. (The outermost routine at a file-scoped compilation unit is always named &MAIN in the file's package.)

Note that &?ROUTINE refers to the current single sub, even if it is declared "multi". To redispatch to the entire suite under a given short name, just use the named form, since there are no anonymous multis.

The &?BLOCK object

&?BLOCK is always an alias for the current block, so you can specify tail-recursion on an anonymous block:

my $anonfactorial = -> Int $n { $n < 2
                                    ?? 1
                                    !! $n * &?BLOCK($n-1)
                              };

&?BLOCK.labels contains a list of all labels of the current block. This is typically matched by saying

if &?BLOCK.labels.any eq 'Foo' {...}

If the innermost lexical block happens to be the main block of a Routine, then &?BLOCK just returns the Block object, not the Routine object that contains it.

[Note: to refer to any $? or &? variable at the time the sub or block is being compiled, use the COMPILING:: pseudopackage.]

Currying

Every Code object has a .assuming method. This method does a partial binding of a set of arguments to a signature and returns a new function that takes only the remaining arguments.

&textfrom := &substr.assuming(str=>$text, len=>Inf);

or equivalently:

&textfrom := &substr.assuming(:str($text) :len(Inf));

or even:

&textfrom := &substr.assuming:str($text):len(Inf);

It returns a Code object that implements the same behaviour as the original subroutine, but has the values passed to .assuming already bound to the corresponding parameters:

$all  = textfrom(0);   # same as: $all  = substr($text,0,Inf);
$some = textfrom(50);  # same as: $some = substr($text,50,Inf);
$last = textfrom(-1);  # same as: $last = substr($text,-1,Inf);

The result of a use statement is a (compile-time) object that also has a .assuming method, allowing the user to bind parameters in all the module's subroutines/methods/etc. simultaneously:

(use IO::Logging).assuming(logfile => ".log");

This special form should generally be restricted to named parameters.

To curry a particular multi variant, it may be necessary to specify the type for one or more of its parameters:

&woof ::= &bark:(Dog).assuming :pitch<low>;
&pine ::= &bark:(Tree).assuming :pitch<yes>;

Macros

Macros are functions or operators that are called by the compiler as soon as their arguments are parsed (if not sooner). The syntactic effect of a macro declaration or importation is always lexically scoped, even if the name of the macro is visible elsewhere. As with ordinary operators, macros may be classified by their grammatical category. For a given grammatical category, a default parsing rule or set of rules is used, but those rules that have not yet been "used" by the time the macro keyword or token is seen can be replaced by use of "is parsed" trait. (This means, for instance, that an infix operator can change the parse rules for its right operand but not its left operand.)

In the absence of a signature to the contrary, a macro is called as if it were a method on the current match object returned from the grammar rule being reduced; that is, all the current parse information is available by treating self as if it were a $/ object. [Conjecture: alternate representations may be available if arguments are declared with particular AST types.]

Macros may return either a string to be reparsed, or a syntax tree that needs no further parsing. The textual form is handy, but the syntax tree form is generally preferred because it allows the parser and debugger to give better error messages. Textual substitution on the other hand tends to yield error messages that are opaque to the user. Syntax trees are also better in general because they are reversible, so things like syntax highlighters can get back to the original language and know which parts of the derived program come from which parts of the user's view of the program. Nevertheless, it's difficult to return a syntax tree for an unbalanced construct, and in such cases a textual macro may be a clearer expression of the evil thing you're trying to do.

If you call a macro at runtime, the result of the macro is automatically evaluated again, so the two calls below print the same thing:

macro f { '1 + 1' }
say f();    # compile-time call to &f
say &f();   # runtime call to &f

Quasiquoting

In aid of returning syntax tree, Perl provides a "quasiquoting" mechanism using the quote q:code, followed by a block intended to represent an AST:

return q:code { say "foo" };

Modifiers to the :code adverb can modify the operation:

:ast(MyAst)         # Default :ast(AST)
:lang(Ruby)         # Default :lang($?PARSER)
:unquote<[: :]>     # Default "triple rule"

Within a quasiquote, variable and function names resolve according to the lexical scope of the macro definition. Unrecognized symbols raise errors when the macro is being compiled, not when it's being used.

To make a symbol resolve to the (partially compiled) scope of the macro call, use the COMPILING:: pseudo-package:

macro moose () { q:code { $COMPILING::x } }

moose(); # macro-call-time error
my $x;
moose(); # resolves to 'my $x'

If you want to mention symbols from the scope of the macro call, use the import syntax as modifiers to :code:

:COMPILING<$x>      # $x always refers to $x in caller's scope
:COMPILING          # All free variables fallback to caller's scope

If those symbols do not exist in the scope of the compiling scope, a compile-time exception is thrown at macro call time.

Similarly, in the macro body you may either refer to the $x declared in the scope of the macro call as $COMPILING::x, or bind to them explicitly:

my $x := $COMPILING::x;

You may also use an import list to bind multiple symbols into the macro's lexical scope:

require COMPILING <$x $y $z>;

Note that you need to use the run-time := and require forms, not ::= and use, because the macro caller's compile-time is the macro's runtime.

Splicing

Bare AST variables (such as the arguments to the macro) may not be spliced directly into a quasiquote because they would be taken as normal bindings. Likewise, program text strings to be inserted need to be specially marked or they will be bound normally. To insert a "unquoted" expression of either type within a quasiquote, use the quasiquote delimiter tripled, typically a bracketing quote of some sort:

return q:code { say $a + {{{ $ast }}} }
return q:code [ say $a + [[[ $ast ]]] ]
return q:code < say $a + <<< $ast >>> >
return q:code ( say $a + ((( $ast ))) )

The delimiters don't have to be bracketing quotes, but the following is probably to be construed as Bad Style:

return q:code / say $a + /// $ast /// /

(Note to implementors: this must not be implemented by finding the final closing delimiter and preprocessing, or we'll violate our one-pass parsing rule. Perl 6 parsing rules are parameterized to know their closing delimiter, so adding the opening delimiter should not be a hardship. Alternately the opening delimiter can be deduced from the closing delimiter. Writing a rule that looks for three opening delimiters in a row should not be a problem. It has to be a special grammar rule, though, not a fixed token, since we need to be able to nest code blocks with different delimiters. Likewise when parsing the inner expression, the inner parser subrule is parameterized to know that }}} or whatever is its closing delimiter.)

Unquoted expressions are inserted appropriately depending on the type of the variable, which may be either a syntax tree or a string. (Again, syntax tree is preferred.) The case is similar to that of a macro called from within the quasiquote, insofar as reparsing only happens with the string version of interpolation, except that such a reparse happens at macro call time rather than macro definition time, so its result cannot change the parser's expectations about what follows the interpolated variable.

Hence, while the quasiquote itself is being parsed, the syntactic interpolation of a unquoted expression into the quasiquote always results in the expectation of an operator following the variable. (You must use a call to a submacro if you want to expect something else.) Of course, the macro definition as a whole can expect whatever it likes afterwards, according to its syntactic category. (Generally, a term expects a following postfix or infix operator, and an operator expects a following term or prefix operator. This does not matter for textual macros, however, since the reparse of the text determines subsequent expectations.)

Quasiquotes default to hygienic lexical scoping, just like closures. The visibility of lexical variables is limited to the q:code expression by default. A variable declaration can be made externally visible using the COMPILING:: pseudo-package. Individual variables can be made visible, or all top-level variable declarations can be exposed using the q:code(:COMPILING) form.

Both examples below will add $new_variable to the lexical scope of the macro call:

q:code {  my $COMPILING::new_variable;   my $private_var; ... }
q:code(:COMPILING) { my $new_variable; { my $private_var; ... } }

(Note that :COMPILING has additional effects described in Macros.)

Other matters

Anonymous hashes vs blocks

{...} is always a block. However, if it is completely empty or consists of a single list, the first element of which is either a hash or a pair, it is executed immediately to compose a Hash object.

The standard pair list operator is equivalent to:

sub pair (*@LIST) {
    my @pairs;
    for @LIST -> $key, $val {
        push @pairs, $key => $val;
    }
    return @pairs;
}

or more succinctly (and lazily):

sub pair (*@LIST) {
    gather for @LIST -> $key, $val {
        take $key => $val;
    }
}

The standard hash list operator is equivalent to:

sub hash (*@LIST) {
    return { pair @LIST };
}

So you may use sub or hash or pair to disambiguate:

$obj =  sub { 1, 2, 3, 4, 5, 6 };   # Anonymous sub returning list
$obj =      { 1, 2, 3, 4, 5, 6 };   # Anonymous sub returning list
$obj =      { 1=>2, 3=>4, 5=>6 };   # Anonymous hash
$obj =      { 1=>2, 3, 4, 5, 6 };   # Anonymous hash
$obj =  hash( 1, 2, 3, 4, 5, 6 );   # Anonymous hash
$obj =  hash  1, 2, 3, 4, 5, 6  ;   # Anonymous hash
$obj = { pair 1, 2, 3, 4, 5, 6 };   # Anonymous hash

Pairs as lvalues

Since they are immutable, Pair objects may not be directly assigned:

(key => $var) = "value";    # ERROR

However, when binding pairs, names can be used to "match up" lvalues and rvalues, provided you write the left side as a signature using :(...) notation:

:(:who($name), :why($reason)) := (why => $because, who => "me");

(Otherwise the parser doesn't know it should parse the insides as a signature and not as an ordinary expression until it gets to the :=, and that would be bad. Alternately, the my declarator can also force treatment of its argument as a signature.)

Out-of-scope names

GLOBAL::<$varname> specifies the $varname declared in the * namespace. Or maybe it's the other way around...

CALLER::<$varname> specifies the $varname visible in the dynamic scope from which the current block/closure/subroutine was called, provided that variable is declared with the "is context" trait. (Implicit lexicals such as $_ are automatically assumed to be contextual.)

CONTEXT::<$varname> specifies the $varname visible in the innermost dynamic scope that declares the variable with the "is context" trait.

MY::<$varname> specifies the lexical $varname declared in the current lexical scope.

OUR::<$varname> specifies the $varname declared in the current package's namespace.

COMPILING::<$varname> specifies the $varname declared (or about to be declared) in the lexical scope currently being compiled.

OUTER::<$varname> specifies the $varname declared in the lexical scope surrounding the current lexical scope (i.e. the scope in which the current block was defined).

Declaring a MAIN subroutine

Ordinarily a top-level Perl "script" just evaluates its anonymous mainline code and exits. During the mainline code, the program's arguments are available in raw form from the @ARGS array. At the end of the mainline code, however, a MAIN subroutine will be called with whatever command-line arguments remain in @ARGS. This call is performed if and only if:

a)

the compilation unit was directly invoked rather than by being required by another compilation unit, and

b)

the compilation unit declares a Routine named "MAIN", and

c)

the mainline code is not terminated prematurely, such as with an explicit call to exit, or an uncaught exception.

The command line arguments (or what's left of them after mainline processing) is magically converted into a Capture and passed to MAIN as its arguments, so switches may be bound as named args and other arguments to the program may be bound to positional parameters or the slurpy array:

sub MAIN ($directory, :$verbose, *%other, *@filenames) {
    for @filenames { ... }
}

If MAIN is declared as a set of multi subs, MMD dispatch is performed.

As with module and class declarations, a sub declaration ending in semicolon is allowed at the outermost file scope if it is the first such declaration, in which case the rest of the file is the body:

sub MAIN ($directory, :$verbose, *%other, *@filenames);
for @filenames { ... }

This form is allowed only for simple subs named MAIN that are intended to be run from the command line. Proto or multi definitions may not be written in semicolon form, nor may MAIN subs within a module or class. (A MAIN routine is allowed in a module or class, but is not usually invoked unless the file is run directly (see a above). This corresponds to the "unless caller" idiom of Perl 5.) In general, you may have only one semicolon-style declaration that controls the whole file.

If an attempted dispatch to MAIN fails, the USAGE routine is called. If there is no USAGE routine, a default message is printed. This usage message is automatically generated from the signature (or signatures) of MAIN. This message is generated at compile time, and hence is available at any later time as $?USAGE.

Common Unix command-line conventions are mapped onto the capture as follows:

 On command line...         $ARGS capture gets...

-name                      :name
-name=value                :name<value>
-name="spacy value"        :name«'spacy value'»
-name='spacy value'        :name«'spacy value'»
-name=val1,'val 2', etc    :name«val1 'val 2' etc»

--name                     :name            # only if declared Bool
--name=value               :name<value>     # don't care
--name value               :name<value>     # only if not declared Bool

--name="spacy value"       :name«'spacy value'»
--name "spacy value"       :name«'spacy value'»
--name='spacy value'       :name«'spacy value'»
--name 'spacy value'       :name«'spacy value'»
--name=val1,'val 2', etc   :name«val1 'val 2' etc»
--name val1 'val 2' etc    :name«val1 'val 2' etc» # only if declared @
--                                 # end named argument processing

+name                      :!name
+name=value                :name<value> but False
+name="spacy value"        :name«'spacy value'» but False
+name='spacy value'        :name«'spacy value'» but False
+name=val1,'val 2', etc    :name«val1 'val 2' etc» but False

:name                      :name
:!name                     :!name   # potential conflict with ! histchar
:/name                     :!name   # potential workaround?
:name=value                :name<value>
:name="spacy value"        :name«'spacy value'»
:name='spacy value'        :name«'spacy value'»
:name=val1,'val 2', etc    :name«val1 'val 2' etc»

Exact Perl 6 forms are okay if quoted from shell processing:

':name<value>'             :name<value>
':name(42)'                :name(42)

For security reasons, only constants are allowed as arguments, however.

The default Capture mapper pays attention to declaration of MAIN's parameters to resolve certain ambiguities. A --foo switch needs to know whether to treat the next word from the command line as an argument. (Allowing the spacey form gives the shell room to do various things to the argument.) The short -foo form never assumes a separate argument, and you must use =. For the --foo form, if there is a named parameter corresponding to the switch name, and it is of type Bool, then no argument is expected. Otherwise an argument is expected. If the parameter is of a non-slurpy array type, all subsequent words up to the next command-line switch (or the end of the list) are bound to that parameter.

As usual, switches are assumed to be first, and everything after the first non-switch, or any switches after a --, are treated as positionals or go into the slurpy array (even if they look like switches). Other policies may easily be introduced by calling MAIN explicitly. For instance, you can parse your arguments with a grammar and pass the resulting Match object as a Capture to MAIN:

@*ARGS ~~ /<MyGrammar::top>/;
MAIN(|$/);
exit;

sub MAIN ($frompart, $topart, *@rest) {
    if $frompart<foo> { ... }
    if $topart<bar><baz> { ... }
}

This will conveniently bind top-level named matches to named parameters, but still give you access to nested matches through those parameters, just as any Match object would. Of course, in this example, there's no particular reason the sub has to be named MAIN.

To give both a long and a short switch name, you may use the pair notation. The key will be considered the short switch name, while the variable name will be considered the long switch name. So if the previous declaration had been:

sub MAIN (:f($frompart), :t($topart), *@rest)

then you could invoke the program with either -f or --frompart to specify the first parameter. Likewise you could use either -t or --topart for the second parameter.

If a switch of the form -abc cannot be matched against any particular parameter, an attempt will be made to match it as if it had been written -a -b -c.

cpanm Perl6::Doc

perl -MCPAN -e shell
install Perl6::Doc