Hello Monks,

I'm reading Higher Order Perl and one of the examples is an "imap" function which acts like map only it takes a block and an iterator.

sub imap {
    my ($transform, $it) = @_;
    return Iterator {
        my $next = NEXTVAL($it);
        return unless defined $next;
        return $transform->($next);
    }
}
[download]

That code may as well be in greek because it makes no sense to me. So I figured I'd try to write my own map function to see how it works.

#!/usr/bin/env perl
use 5.010;
use strict;
use warnings;

sub foo(&@) {
    my $coderef = shift;
    my @params = @_;
    my @ret;

    while(@params) {
        push @ret, $coderef->($_);
    }
    return @ret;
}

my @a = ( "1", "2", "3", "4", );
my @b = foo { $_ ** 2 } @a;
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But that produces the error "Use of uninitialized value $_ in exponentiation (**) at coderef.pl line 19."

I know when I pass parameters to a subroutine or closure they are passed in in @_, so I tried to shift the parameter off @_, but that produces the same error (based on my understanding of perl, this second attempt should work... "Should" is funny word).

my @b = foo { $_ = shift; $_ ** 2 } @a;
my @b = foo { my $var = shift; $var ** 2 } @a;
[download]

What gives? How does map/grep work it's magic? And why doesn't my attempt work? I realize that all of the useful map functions have probably already been written, so this is more of an exercise in understanding, how could I write my own map function?

Thanks Monks!

Edit: Seems my problem was with my use of while instead of for, I thought while set $_, but I guess not.

Here's the version that works:

#!/usr/bin/env perl
use 5.010;
use strict;
use warnings;

sub foo(&@) {
    my $coderef = shift;
    my @params = @_;
    my @ret;

    for(@params) {
        push @ret, $coderef->();
    }
    return @ret;
}

my @a = ( "1", "2", "3", "4", );
my @b = foo { $_ ** 2 } @a;
[download]