Re: Near-free function currying in Perl

[Jenda]

I don't like the syntax. When looking at the code I would definitely not expect foo_c(1,2) to be "equivalent" to sub {foo(1,2,@_}. I think the currying would have to have a very very nice syntax to be used in place of anonymous subs. A syntax that would look readable enough to me is

my $fun = foo( 1, 2, ...);
# or using your example
$app_server = AppServer->new(
        logger => log_to_handle( *STDERR, "app-server", ...),
        other_option => 5,
    );

(Except that it would colide with the common usage of three dots in pseudo code.)

It's actually doable, but AFAIK only via source filters. And it is not that long actually:

package Curry::Dots;
use Filter::Simple;

my $braces = qr{
           \(
           (?:
              (?> [^\(\)]+ )    # Non-parens without backtracking
            |
              (??{ $braces })     # Group with matching parens
           )*
           \)
        }x;

sub curry {
    my $f = shift;
    my $args = \@_;
    sub { $f->(@$args, @_) };
}

FILTER_ONLY
    code => sub {
        return unless /,\s*\.\.\.\s*\)/;
        s/\&\$(\b\w+)\s*\(([^\(\)]*(?:$braces[^\(\)]*)*)\s*,\s*\.\.\.\
+s*\)/Curry::Dots::curry (\$$1, $2)/g;
        s/(\$\b\w+)\s*->(\w+)\s*\(([^\(\)]*(?:$braces[^\(\)]*)*)\s*,\s
+*\.\.\.\s*\)/Curry::Dots::curry ($1->can('$2'), $1, $3)/g;
        s/(\b\w+)\s*->(\w+)\s*\(([^\(\)]*(?:$braces[^\(\)]*)*)\s*,\s*\
+.\.\.\s*\)/Curry::Dots::curry ($1->can('$2'), '$1', $3)/g;
        s/\$(\b\w+)\s*->\s*\(([^\(\)]*(?:$braces[^\(\)]*)*)\s*,\s*\.\.
+\.\s*\)/Curry::Dots::curry (\$$1, $2)/g;
        s/(\b\w+)\s*\(([^\(\)]*(?:$braces[^\(\)]*)*)\s*,\s*\.\.\.\s*\)
+/Curry::Dots::curry (\\\&$1, $2)/g;
    };

1;

and the examples:

use strict;
use Curry::Dots;

sub foo {
    print "@_\n";
}

sub Obj::method {print "Obj::method( @_)\n"}
sub Obj::new {bless {}, 'Obj'}

my $f1 = foo(1, 2, ...);

$f1->(3);

my $f2 = &$f1( 99, ...);

$f2->(0);

my $f3 = $f1->(7,... );

$f3->(123);

my $obj = new Obj;
my $f4 = $obj->method(987, 654, ...);

$f4->(321);

my $f5 = Obj->method(55,...);
$f5->(22);

Tested with Perl v5.8.0 (ActivePerl build 805).

It only supports the simpler types of calls like foo(params, ...), &$foo(params, ...), $foo->(params, ...), $obj->Method(params, ...) and Class->Method(params, ...) and it uses just \w+ for variable/function/method/class names which definitely is not correct regexp for identifiers in Perl.

[diotalevi]

No one should ever use source filters. Its a flakey practice.

[Jenda]

That's why I said it's AFAIK doable ONLY using source filters. It was a nice exercise writing the module, I do like the syntax that it allows, but I don't think I will ever use it in production code.

The code could be improved to match the Perl identifiers and variables better, but I'm not sure it'd be worth it. If anyone wants to extend the code, use it and even release it to CPAN it's fine with me. I doubt I ever will. Though ... you never know ;-)

Update: BTW, I searched all my Perl sources including the perl instalation and only found /,\s*\.\.\.\s*\)/ in PODs and comments and in one string printed by DBI.pm, one in Tk\X11Font.pm, one in Benchmark.pm and one in DB_File.pm.