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Perl6-Currying-0.05
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Author image Damian Conway

NAME

Perl6::Currying - Perl 6 subroutine currying for Perl 5

VERSION

This document describes version 0.05 of Perl6::Currying, released May 29, 2002.

SYNOPSIS

        use Perl6::Currying;

        sub add ($a,$b) { $a + $b }     # Define a sub with named params

        print add(1,2);                 # Call it

        my $incr = &add.prebind(a=>1);  # Bind the $a argument to 1
                                        # to create an increment subroutine

        print $incr->(3), "\n";         # Increment a number

DESCRIPTION

The Perl6::Currying module lets you try out the new Perl 6 explicit higher-order function syntax in Perl 5.

In Perl 6 any subroutine can be "partially bound". That is, you can supply some of its arguments and thereby create another subroutine that calls the original with those arguments automatically supplied.

Subroutine parameters are partially bound by calling the prebind method on the subroutine. This method call returns a reference to a new subroutine that calls the original subroutine, inserting into its argument list the prebound arguments. For example:

        # Perl 6 code
        sub divide ($numerator, $denominator) {
                return $numerator / $denominator;
        }

        my $halve = &divide.prebind(denominator=>2);

Note that it's necessary to use the & sigil to indicate that the method CODE::prebind is to be called on a CODE object &divide, not the Whatever::prebind of the Whatever object returned by calling divide. To get the latter, we would write:

        divide().prebind(...)

or:

        divide.prebind(...)

Having prebound the denominator, if we now call the subroutine referred to by $halve the effect is to call divide with an automagically supplied denominator of 2. That is:

        # Perl 6 code
        print divide(42,2);     # calls &divide...prints 21
        print $halve(42);       # calls &divide...prints 21

It's also possible to prebind all the arguments of a subroutine, either all at once:

        # Perl 6 code
        my $pi_approx = &divide.prebind(numerator=>22,denominator=>7);

        print $pi_approx();     # prints 3.14285714285714

or in stages:

        # Perl 6 code
        my $pi_legislated = $halve.prebind(numerator=>6);

        print $pi_legislated(); # prints 3

Note that we didn't need the & sigil before $halve since this syntax is unambiguously a call (through a reference to a CODE object) to CODE::prebind.

You can also use the Perl 6 aliasing operator (:=) to create new named subroutines by partially binding existing ones. For example:

        # Perl 6 code
        
        &reciprocal := &divide.prebind(numerator=>1);

        print reciprocal(10)    # prints 0.1

Parameter binding in Perl 5

The Perl6::Currying module allows you to use the same syntax in Perl 5.

That is, you can supply some of the arguments to a (specially prototyped) Perl 5 subroutine and thereby create another subroutine that calls the original with those arguments automatically supplied.

The new subroutine is created by calling the prebind method on the original subroutine. For example:

        # Perl 5 code
        use Perl6::Currying;

        sub divide ($numerator, $denominator) {
                return $numerator / $denominator;
        }

        my $halve = &divide.prebind(denominator=>2);

Notes:

  1. As the above example implies, Perl6::Currying gives you the (limited) ability to declare Perl 5 subroutines with named parameters. Currently those parameters must be a list of comma-separated scalars, as shown above. Each parameter becomes a lexical scalar variable within the body of the subroutine.

  2. For forward compatibility, to prebind parameters in Perl 5, the Perl 6 method call syntax ($objref.methodname(...)) is used, rather than the Perl 5 syntax ($objref->methodname(...)).

  3. To be consistent with Perl 6, it's still necessary to use the & sigil to indicate that the method to be called is CODE::prebind, not the prebind of the object returned by calling divide.

Having prebound the denominator, if we now call the subroutine referred to by $halve the effect is to call divide with an automagically supplied denominator of 2. That is:

        # Perl 5 code
        print divide(42,2);     # calls &divide...prints 21
        print $halve->(42);     # calls &divide...prints 21

Note that since these are just normal Perl 5 subroutine calls, the Perl 5 call-through-reference syntax ($subref->(...)) is used, rather than the Perl 6 syntax ($subref.(...)).

It's also possible to prebind all the arguments of a subroutine, either all at once:

        # Perl 5 code
        use Perl6::Currying;

        my $pi_approx = &divide.prebind(numerator=>22,denominator=>7);

        print $pi_approx->();   # prints 3.14285714285714

or in stages:

        # Perl 5 code
        use Perl6::Currying;

        my $pi_legislated = $halve.prebind(numerator=>6);

        print $pi_legislated(); # prints 3

You can also use Perl 5 typeglobs to create new named subroutines by partially binding existing ones. For example:

        # Perl 5 code
        
        *reciprocal = &divide.prebind(numerator=>1);

        print reciprocal(10)    # prints 0.1

REFERENCES

A quick introduction: http://www.tunes.org/~iepos/introduction-to-logic/chap00/sect00.html

Definition of currying: http://www.cs.nott.ac.uk/~gmh//faq.html#currying

Implementation in Haskell: http://www.haskell.org/tutorial/functions.html

DEPENDENCIES AND INTERACTION

The module is implemented using Filter::Simple and requires that module to be installed.

This module can be used in conjunction with the Perl6::Placeholders module. For example:

        use Perl6::Currying;
        use Perl6::Placeholders;

        $add = { $^a + $^b };

        my $incr = $add.prebind(b=>1);

        print $incr->(7), "\n";

        my $div =  { $^x / $^y };

        print $div->(22,7), "\n";

        my $half_of = &$div.prebind(y=>2);
        my $reciprocal = $div.prebind(x=>1);

        print $half_of->(7), "\n";
        print $reciprocal->(7), "\n";

When using both modules, this module must be loaded first.

DIAGNOSTICS

Odd list of bindings for prebind

prebind expects a list of parameter_name => value pairs as its arguments. Instead it detected a non-even number of arguments.

Can't prebind sub with prototype (%s)

Currently the module only supports scalar named parameters. It has detected an attempt to bind a subroutine that has some other type of parameter specified.

Can't prebind nonexistent parameter %s of sub(%s)

You can only bind parameters that were actually declared in the subroutine's prototype.

Perl6::Placeholders should not be loaded before Perl6::Currying

When using both modules, Perl6::Placeholders should be loaded after Perl6::Currying.

AUTHOR

Damian Conway (damian@conway.org)

BUGS

This module is not designed for serious implementation work.

It uses some relatively sophisticated heuristics to translate Perl 6 syntax back to Perl 5. It will make mistakes if your code gets even moderately tricky.

Nevertheless, bug reports are most welcome.

COPYRIGHT

Copyright (c) 2002, Damian Conway. All Rights Reserved. This module is free software. It may be used, redistributed and/or modified under the terms of the Perl Artistic License (see http://www.perl.com/perl/misc/Artistic.html)