NAME

Scope::Upper - Act on upper scopes.

VERSION

Version 0.16

SYNOPSIS

"reap", "localize", "localize_elem", "localize_delete" and "WORDS" :

package Scope;

use Scope::Upper qw<reap localize localize_elem localize_delete :words>;

sub new {
 my ($class, $name) = @_;

 localize '$tag' => bless({ name => $name }, $class) => UP;

 reap { print Scope->tag->name, ": end\n" } UP;
}

# Get the tag stored in the caller namespace
sub tag {
 my $l   = 0;
 my $pkg = __PACKAGE__;
 $pkg    = caller $l++ while $pkg eq __PACKAGE__;

 no strict 'refs';
 ${$pkg . '::tag'};
}

sub name { shift->{name} }

# Locally capture warnings and reprint them with the name prefixed
sub catch {
 localize_elem '%SIG', '__WARN__' => sub {
  print Scope->tag->name, ': ', @_;
 } => UP;
}

# Locally clear @INC
sub private {
 for (reverse 0 .. $#INC) {
  # First UP is the for loop, second is the sub boundary
  localize_delete '@INC', $_ => UP UP;
 }
}

...

package UserLand;

{
 Scope->new("top");      # initializes $UserLand::tag

 {
  Scope->catch;
  my $one = 1 + undef;   # prints "top: Use of uninitialized value..."

  {
   Scope->private;
   eval { require Cwd };
   print $@;             # prints "Can't locate Cwd.pm in @INC (@INC contains:) at..."
  }

  require Cwd;           # loads Cwd.pm
 }

}                        # prints "top: done"

"unwind" and "want_at" :

package Try;

use Scope::Upper qw<unwind want_at :words>;

sub try (&) {
 my @result = shift->();
 my $cx = SUB UP; # Point to the sub above this one
 unwind +(want_at($cx) ? @result : scalar @result) => $cx;
}

...

sub zap {
 try {
  my @things = qw<a b c>;
  return @things; # returns to try() and then outside zap()
  # not reached
 };
 # not reached
}

my @stuff = zap(); # @stuff contains qw<a b c>
my $stuff = zap(); # $stuff contains 3

"uplevel" :

package Uplevel;

use Scope::Upper qw<uplevel CALLER>;

sub target {
 faker(@_);
}

sub faker {
 uplevel {
  my $sub = (caller 0)[3];
  print "$_[0] from $sub()";
 } @_ => CALLER(1);
}

target('hello'); # "hello from Uplevel::target()"

DESCRIPTION

This module lets you defer actions at run-time that will take place when the control flow returns into an upper scope. Currently, you can:

hook an upper scope end with "reap" ;
localize variables, array/hash values or deletions of elements in higher contexts with respectively "localize", "localize_elem" and "localize_delete" ;
return values immediately to an upper level with "unwind", and know which context was in use then with "want_at" ;
execute a subroutine in the context of an upper subroutine stack frame with "uplevel".

FUNCTIONS

In all those functions, $context refers to the target scope.

You have to use one or a combination of "WORDS" to build the $context passed to these functions. This is needed in order to ensure that the module still works when your program is ran in the debugger. The only thing you can assume is that it is an absolute indicator of the frame, which means that you can safely store it at some point and use it when needed, and it will still denote the original scope.

`reap $callback, $context`

Adds a destructor that calls $callback (in void context) when the upper scope represented by $context ends.

`localize $what, $value, $context`

Introduces a local delayed to the time of first return into the upper scope denoted by $context. $what can be :

A glob, in which case $value can either be a glob or a reference. "localize" follows then the same syntax as local *x = $value. For example, if $value is a scalar reference, then the SCALAR slot of the glob will be set to $$value - just like local *x = \1 sets $x to 1.
A string beginning with a sigil, representing the symbol to localize and to assign to. If the sigil is '$', "localize" follows the same syntax as local $x = $value, i.e. $value isn't dereferenced. For example,
```
localize '$x', \'foo' => HERE;
```
will set $x to a reference to the string 'foo'. Other sigils ('@', '%', '&' and '*') require $value to be a reference of the corresponding type.

When the symbol is given by a string, it is resolved when the actual localization takes place and not when "localize" is called. Thus, if the symbol name is not qualified, it will refer to the variable in the package where the localization actually takes place and not in the one where the "localize" call was compiled. For example,
```
{
 package Scope;
 sub new { localize '$tag', $_[0] => UP }
}

{
 package Tool;
 {
  Scope->new;
  ...
 }
}
```
will localize $Tool::tag and not $Scope::tag. If you want the other behaviour, you just have to specify $what as a glob or a qualified name.

Note that if $what is a string denoting a variable that wasn't declared beforehand, the relevant slot will be vivified as needed and won't be deleted from the glob when the localization ends. This situation never arises with local because it only compiles when the localized variable is already declared. Although I believe it shouldn't be a problem as glob slots definedness is pretty much an implementation detail, this behaviour may change in the future if proved harmful.

`localize_elem $what, $key, $value, $context`

Introduces a local $what[$key] = $value or local $what{$key} = $value delayed to the time of first return into the upper scope denoted by $context. Unlike "localize", $what must be a string and the type of localization is inferred from its sigil. The two only valid types are array and hash ; for anything besides those, "localize_elem" will throw an exception. $key is either an array index or a hash key, depending of which kind of variable you localize.

If $what is a string pointing to an undeclared variable, the variable will be vivified as soon as the localization occurs and emptied when it ends, although it will still exist in its glob.

`localize_delete $what, $key, $context`

Introduces the deletion of a variable or an array/hash element delayed to the time of first return into the upper scope denoted by $context. $what can be:

A glob, in which case $key is ignored and the call is equivalent to local *x.
A string beginning with '@' or '%', for which the call is equivalent to respectiveley local $a[$key]; delete $a[$key] and local $h{$key}; delete $h{$key}.
A string beginning with '&', which more or less does undef &func in the upper scope. It's actually more powerful, as &func won't even exists anymore. $key is ignored.

`unwind @values, $context`

Returns @values from the context pointed by $context, i.e. from the subroutine, eval or format at or just above $context, and immediately restart the program flow at this point - thus effectively returning to an upper scope.

The upper context isn't coerced onto @values, which is hence always evaluated in list context. This means that

my $num = sub {
 my @a = ('a' .. 'z');
 unwind @a => HERE;
 # not reached
}->();

will set $num to 'z'. You can use "want_at" to handle these cases.

`want_at $context`

Like wantarray, but for the subroutine/eval/format at or just above $context.

The previous example can then be "corrected" :

my $num = sub {
 my @a = ('a' .. 'z');
 unwind +(want_at(HERE) ? @a : scalar @a) => HERE;
 # not reached
}->();

will rightfully set $num to 26.

`uplevel $code, @args, $context`

Executes the code reference $code with arguments @args as if it were located at the subroutine stack frame pointed by $context, effectively fooling caller and die into believing that the call actually happened higher in the stack. The code is executed in the context of the uplevel call, and what it returns is returned as-is by uplevel.

sub target {
 faker(@_);
}

sub faker {
 uplevel {
  map { 1 / $_ } @_;
 } @_ => CALLER(1);
}

my @inverses = target(1, 2, 4); # @inverses contains (0, 0.5, 0.25)
my $count    = target(1, 2, 4); # $target is 3

Sub::Uplevel also implements a pure-Perl version of uplevel. Both are identical, with the following caveats :

The Sub::Uplevel implementation of uplevel may execute a code reference in the context of any upper stack frame. The Scope::Upper version only allows to uplevel to a subroutine stack frame, and will croak if you try to target an eval or a format.
Exceptions thrown from the code called by this version of uplevel will not be caught by eval blocks between the target frame and the uplevel call, while they will for Sub::Uplevel's version. This means that :
```
eval {
 sub {
  local $@;
  eval {
   sub {
    uplevel { die 'wut' } CALLER(2); # for Scope::Upper
    # uplevel(3, sub { die 'wut' })  # for Sub::Uplevel
   }->();
  };
  print "inner block: $@";
  $@ and exit;
 }->();
};
print "outer block: $@";
```
will print "inner block: wut..." with Sub::Uplevel and "outer block: wut..." with Scope::Upper.
Sub::Uplevel globally overrides CORE::GLOBAL::caller, while Scope::Upper does not.

A simple wrapper lets you mimic the interface of "uplevel" in Sub::Uplevel :

use Scope::Upper;

sub uplevel {
 my $frame = shift;
 my $code  = shift;
 my $cxt   = Scope::Upper::CALLER($frame);
 &Scope::Upper::uplevel($code => @_ => $cxt);
}

Albeit the three exceptions listed above, it passes all the tests of Sub::Uplevel.

CONSTANTS

`SU_THREADSAFE`

True iff the module could have been built when thread-safety features.

WORDS

Constants

`TOP`

Returns the context that currently represents the highest scope.

`HERE`

The context of the current scope.

Getting a context from a context

For any of those functions, $from is expected to be a context. When omitted, it defaults to the the current context.

`UP $from`

The context of the scope just above $from.

`SUB $from`

The context of the closest subroutine above $from. Note that $from is returned if it is already a subroutine context ; hence SUB SUB == SUB.

`EVAL $from`

The context of the closest eval above $from. Note that $from is returned if it is already an eval context ; hence EVAL EVAL == EVAL.

Getting a context from a level

Here, $level should denote a number of scopes above the current one. When omitted, it defaults to 0 and those functions return the same context as "HERE".

`SCOPE $level`

The $level-th upper context, regardless of its type.

`CALLER $level`

The context of the $level-th upper subroutine/eval/format. It kind of corresponds to the context represented by caller $level, but while e.g. caller 0 refers to the caller context, CALLER 0 will refer to the top scope in the current context.

Examples

Where "reap" fires depending on the $cxt :

sub {
 eval {
  sub {
   {
    reap \&cleanup => $cxt;
    ...
   }     # $cxt = SCOPE(0), or HERE
   ...
  }->(); # $cxt = SCOPE(1), or UP, or SUB, or CALLER, or CALLER(0)
  ...
 };      # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1)
 ...
}->();   # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2)
...

Where "localize", "localize_elem" and "localize_delete" act depending on the $cxt :

sub {
 eval {
  sub {
   {
    localize '$x' => 1 => $cxt;
    # $cxt = SCOPE(0), or HERE
    ...
   }
   # $cxt = SCOPE(1), or UP, or SUB, or CALLER, or CALLER(0)
   ...
  }->();
  # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1)
  ...
 };
 # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2)
 ...
}->();
# $cxt = SCOPE(4), UP SUB UP SUB, or UP SUB EVAL, or UP CALLER(2), or TOP
...

Where "unwind", "want_at" and "uplevel" point to depending on the $cxt:

sub {
 eval {
  sub {
   {
    unwind @things => $cxt;     # or uplevel { ... } $cxt;
    ...
   }
   ...
  }->(); # $cxt = SCOPE(0 .. 1), or HERE, or UP, or SUB, or CALLER(0)
  ...
 };      # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1) (*)
 ...
}->();   # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2)
...

# (*) Note that uplevel() will croak if you pass that scope frame,
#     because it can't target eval scopes.

EXPORT

The functions "reap", "localize", "localize_elem", "localize_delete", "unwind", "want_at" and "uplevel" are only exported on request, either individually or by the tags ':funcs' and ':all'.

The constant "SU_THREADSAFE" is also only exported on request, individually or by the tags ':consts' and ':all'.

Same goes for the words "TOP", "HERE", "UP", "SUB", "EVAL", "SCOPE" and "CALLER" that are only exported on request, individually or by the tags ':words' and ':all'.

CAVEATS

Be careful that local variables are restored in the reverse order in which they were localized. Consider those examples:

local $x = 0;
{
 reap sub { print $x } => HERE;
 local $x = 1;
 ...
}
# prints '0'
...
{
 local $x = 1;
 reap sub { $x = 2 } => HERE;
 ...
}
# $x is 0

The first case is "solved" by moving the local before the reap, and the second by using "localize" instead of "reap".

The effects of "reap", "localize" and "localize_elem" can't cross BEGIN blocks, hence calling those functions in import is deemed to be useless. This is an hopeless case because BEGIN blocks are executed once while localizing constructs should do their job at each run. However, it's possible to hook the end of the current scope compilation with B::Hooks::EndOfScope.

Some rare oddities may still happen when running inside the debugger. It may help to use a perl higher than 5.8.9 or 5.10.0, as they contain some context-related fixes.

DEPENDENCIES

XSLoader (standard since perl 5.006).

AUTHOR

Vincent Pit, <perl at profvince.com>, http://www.profvince.com.

You can contact me by mail or on irc.perl.org (vincent).

BUGS

Please report any bugs or feature requests to bug-scope-upper at rt.cpan.org, or through the web interface at http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Scope-Upper. I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.

SUPPORT

You can find documentation for this module with the perldoc command.

perldoc Scope::Upper

Tests code coverage report is available at http://www.profvince.com/perl/cover/Scope-Upper.

ACKNOWLEDGEMENTS

Inspired by Ricardo Signes.

Thanks to Shawn M. Moore for motivation.

COPYRIGHT & LICENSE

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

To install Scope::Upper, copy and paste the appropriate command in to your terminal.

cpanm

cpanm Scope::Upper

CPAN shell

perl -MCPAN -e shell
install Scope::Upper

For more information on module installation, please visit the detailed CPAN module installation guide.

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NAME

VERSION

SYNOPSIS

DESCRIPTION

FUNCTIONS

reap $callback, $context

localize $what, $value, $context

localize_elem $what, $key, $value, $context

localize_delete $what, $key, $context

unwind @values, $context

want_at $context

uplevel $code, @args, $context