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;
[download]

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]

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.