NAME
Dispatch::Fu - Converts any complicated conditional dispatch situation into familiar static hash-key based dispatch
SYNOPSIS
use strict;
use warnings;
use Dispatch::Fu; # 'dispatch', 'cases', 'xdefault', and 'on' are exported by default, just for show here
my $INPUT = [qw/1 2 3 4 5/];
my $results = dispatch { # <~ start of 'dispatch' construct
my $input_ref = shift; # <~ input reference
return ( scalar @$input_ref > 5 ) # <~ return a string that must be
? q{case5} # <~ defined below using the 'on'
: sprintf qq{case%d}, scalar @$input_ref; # <~ dynamic dispatch here
} $INPUT, # <~ input reference, SCALAR passed to dispatch BLOCK
on case0 => sub { my $INPUT = shift; return qq{case 0}}, # <~ if dispatch returns 'case0', run this CODE
on case1 => sub { my $INPUT = shift; return qq{case 1}}, # <~ if dispatch returns 'case1', run this CODE
on case2 => sub { my $INPUT = shift; return qq{case 2}}, # ... ... ... ... ... ... ...
on case3 => sub { my $INPUT = shift; return qq{case 3}}, # ... ... ... ... ... ... ... ...
on case4 => sub { my $INPUT = shift; return qq{case 4}}, # ... ... ... ... ... ... ...
on case5 => sub { my $INPUT = shift; return qq{case 5}}; # <~ if dispatch returns 'case5', run this CODEDESCRIPTION
Dispatch::Fu provides an idiomatic and succinct way to organize a HASH-based dispatch table by first computing a static key using a developer defined process. This static key is then used immediate to execute the subroutine reference registered to the key.
This module presents a generic structure that can be used to implement all of the past attemts to bring things to Perl like, switch or case statements, given/when, smartmatch, etc.
The Problem
HASH based dispatching in Perl is a very fast and well established way to organize your code. A dispatch table can be fashioned easily when the dispatch may occur on a single variable that may be one or more static strings suitable to serve also as HASH a key.
For example, the following is more or less a classical example of this approach that is fundamentally based on a 1:1 mapping of a value of $action to a HASH key defined in $dispatch:
my $CASE = get_case(); # presumed to return one of the hash keys used below
my $dispatch = {
do_dis => sub { ... },
do_dat => sub { ... },
do_deez => sub { ... },
do_doze => sub { ... },
};
if (not $CASE or not exists $dispatch->{$CASE}) {
die qq{case not supported\n};
}
my $results = $dispatch->{$CASE}->();But this nice situation breaks down if $CASE is a value that is not suitable for us as a HASH key, is a range of values, or a single variable (e.g., $CASE) is not sufficient to determine what case to dispatch. Dispatch::Fu solves this problem by providing a stage where a static key might be computed or classified.
The Solution
Dispatch::Fu solves the problem by providing a Perlish and idiomatic hook for computing a static key from an arbitrarily defined algorithm written by the developer using this module.
The dispatch keyword and associated lexical block (that should be treated as the body of a subroutine that receives exactly one parameter), determines what case defined by the on keyword is immediately executed.
The simple case above can be trivially replicated below using Dispatch::Fu, as follows:
my $results = dispatch {
my $case = shift;
return $case;
},
$CASE,
on do_dis => sub { ... },
on do_dat => sub { ... },
on do_deez => sub { ... },
on do_doze => sub { ... };The one difference here is, if $case is defined but not accounted for using the on keyword, then dispatch will throw an exception via die. Certainly any logic meant to deal with the value (or lack thereof) of $CASE should be handled in the dispatch BLOCK.
An example of a more complicated scenario for generating the static key might be defined, follows:
my $results = dispatch {
my $input_ref = shift;
my $rand = $input_ref->[0];
if ( $rand < 2.5 ) {
return q{do_dis};
}
elsif ( $rand >= 2.5 and $rand < 5.0 ) {
return q{do_dat};
}
elsif ( $rand >= 5.0 and $rand < 7.5 ) {
return q{do_deez};
}
elsif ( $rand >= 7.5 ) {
return q{do_doze};
}
},
[ rand 10 ],
on do_dis => sub { ... },
on do_dat => sub { ... },
on do_deez => sub { ... },
on do_doze => sub { ... };The approach facilited by Dispatch::Fu is one that requires the programmer to define each case by a static key via on, and define a custom algorithm for picking which case (by way of return'ing the correct static key as a string) to execute using the dispatch BLOCK.
USAGE
For more working examples, look at the tests in the ./t directory. It should quickly become apparent how to use this method and what it's for by trying it out. But if in doubt, please inquire here, there, everywhere.
dispatchBLOCK-
BLOCKis required, and is coerced to be an anonymous subroutine that is passed a single scalar reference; this reference can be a single value or point to anything a Perl scalar reference can point to. It's the single point of entry for input.my $results = dispatch { my $input_ref = shift; # <~ there is only one parameter, but can a reference to anything my $key = q{default}; # <~ initiate the default key to use, 'default' by convention not required ... # <~ compute $key (yada yada) return $key; # <~ key must be limited to the set of keys added with C<on> } ...The
dispatchimplementation must return a static string, and that string should be one of the keys added using theonkeyword. Otherwise, an exception will be thrown viadie.Note:
dispatchfaithfully returns whatever the dispatchedsubroutineis written to return; including a single valueSCALAR,SCALARrefernce,LIST, etc.my @results = dispatch { my $input_ref = shift; # <~ there is only one parameter, but can a reference to anything my $key = q{default}; # <~ initiate the default key to use, 'default' by convention not required ... # <~ compute $key (yada yada) return $key; # <~ key must be limited to the set of keys added with C<on> } ... on default => sub { return qw/1 2 3 4 5 6 7 8 9 10/ }, ...If the
defaultcase is the one dispatched, then@resultswill contain the digits1 .. 10returned as a LIST viaqw//, above. cases-
This routine is for introspection inside of the
dispatchBLOCK. It returns the list of all cases added by theonroutine. Outside of thedispatchBLOCK, it returns an emptyHASHreference. It is available within theCODEbody of each case handler registered via `on`.Note: do not rely on the ordering of these cases to be consistent; it relies on the
keyskeyword, which operates onHASHes and key order is therefore not deterministic.Given the full example above,
my $results = dispatch { my $input_ref = shift; ... my @cases = cases; # (qw/do_dis do_dat do_deez do_doze/) ... }, [ rand 10 ], on do_dis => sub { ... }, on do_dat => sub { ... }, on do_deez => sub { ... }, on do_doze => sub { ... }; xdefaultSCALAR, [DEFAULT_STRING]-
Note: SCALAR must be an actual value (string, e.g.) or
undef.Provides a shortcut for the common situation where one static value really define the case key. Used idiomatically without the explicit return provided it is as the very last line of the
dispatchBLOCK.my $results = dispatch { my $input_str = shift; xdefault $input_str, q{do_default}; # if $input_str is not in supported cases, return the string 'default' }, $somestring, on do_default => sub { ... }, #<~ default case on do_dis => sub { ... }, on do_dat => sub { ... }, on do_deez => sub { ... }, on do_doze => sub { ... };xdefaultcan be passed just the string that is checked for membership incases, if just provided the default case key, the stringdefaultwill be used if the string being tested is not in the set of cases defined usingon.my $results = dispatch { my $input_str = shift; xdefault $input_str; # if $input_str is not in the set of supported cases, it will return the string 'default' }, $somestring, on default => sub { ... }, #<~ default case on do_dis => sub { ... }, on do_dat => sub { ... }, on do_deez => sub { ... }, on do_doze => sub { ... };And just for the sake of minimization, we can get rid of one more line here:
my $results = dispatch { xdefault shift; # if $input_str is not in supported cases, return the string 'default' }, $somestring, on default => sub { ... }, #<~ default case on do_dis => sub { ... }, on do_dat => sub { ... }, on do_deez => sub { ... }, on do_doze => sub { ... }; REF-
This is the singular scalar reference that contains all the stuff to be used in the
dispatchBLOCK. In the example above it is,[rand 10]. It is the way to pass arbitrary data intodispatch. E.g.,my $INPUT = [qw/foo bar baz 1 3 4 5/]; my $result = dispatch { my $input_ref = shift; # <~ there is only one parameter, but can a reference to anything my $key = q{default}; # <~ initiate the default key to use, 'default' by convention not required ... # <~ compute $key (yada yada) return $key; # <~ key must be limited to the set of keys added with C<on> } $INPUT, ### <><~ the single scalar reference to be passed to the C<dispatch> BLOCK ... on-
This keyword builds up the dispatch table. It consists of a static string and a subroutine reference. In order for this to work for you, the
dispatchBLOCK must return strictly only the keys that are defined viaon.my $INPUT = [qw/foo bar baz 1 3 4 5/]; my $results = dispatch { my $input_ref = shift; # <~ there is only one parameter, but can a reference to anything my $key = q{default}; # <~ initiate the default key to use, 'default' by convention not required ... # <~ compute $key (yada yada) return $key; # <~ key must be limited to the set of keys added with C<on> } $INPUT, ### <><~ the single scalar reference to be passed to the C<dispatch> BLOCK on case1 => sub { my $INPUT = shift; ... }, on case2 => sub { my $INPUT = shift; ... }, on case3 => sub { my $INPUT = shift; ... }, on case4 => sub { my $INPUT = shift; ... }, on case5 => sub { my $INPUT = shift; ... };Note: when the subroutine associated with each case is dispatched, the
$INPUTscalar is provide as input.my $INPUT = [qw/foo bar baz 1 3 4 5/]; my $results = dispatch { my $input_ref = shift; # there is only one parameter, but can a reference to anything my $key = q{default}; # initiate the default key to use, 'default' by convention not required ... # compute $key return $key; # key must be limited to the set of keys added with C<on> } $INPUT, # <~ the single scalar reference to be passed to the C<dispatch> BLOCK on default => sub { my $INPUT = shift; do_default($INPUT); }, on key1 => sub { my $INPUT = shift; do_key1(cases => $INPUT); }, on key2 => sub { my $INPUT = shift; do_key2(qw/some other inputs entirely/); };
Diagnostics and Warnings
The on method must always follow a comma! Commas and semicolons look a lot alike. This is why a wantarray check inside is able to warn when it's being used in a useless void or scalar contexts. Experience has show that it's easy for a semicolon to sneak into a series of on statements as they are added or reorganized. For example, how quickly can you spot a the misplaced semicolon below:
my $results = dispatch {
my $input_ref = shift;
...
return $key;
} $INPUT,
on case01 => sub { my $INPUT = shift; ... },
on case02 => sub { my $INPUT = shift; ... },
on case03 => sub { my $INPUT = shift; ... },
on case04 => sub { my $INPUT = shift; ... },
on case05 => sub { my $INPUT = shift; ... },
on case06 => sub { my $INPUT = shift; ... };
on case07 => sub { my $INPUT = shift; ... },
on case08 => sub { my $INPUT = shift; ... },
on case09 => sub { my $INPUT = shift; ... },
on case10 => sub { my $INPUT = shift; ... },
on case11 => sub { my $INPUT = shift; ... },
on case12 => sub { my $INPUT = shift; ... },
on case13 => sub { my $INPUT = shift; ... },
on case14 => sub { my $INPUT = shift; ... },
on case15 => sub { my $INPUT = shift; ... },
on case16 => sub { my $INPUT = shift; ... },
on case17 => sub { my $INPUT = shift; ... },
on case18 => sub { my $INPUT = shift; ... },
on case19 => sub { my $INPUT = shift; ... },
on case20 => sub { my $INPUT = shift; ... };BUGS
Please report any bugs or ideas about making this module an even better basis for doing dynamic dispatching.
AUTHOR
O. ODLER 558 <oodler@cpan.org>.
LICENSE AND COPYRIGHT
Same as Perl.