Can anyone explain why the line

map2{ print "$a $b"; } qw[A B C D];

would produce the error 

Type of arg 1 to main::map2 must be block or sub {} (not list) at C:\t
+est\test.pl line 17, near "qw[A B C D];"

[download]

The error implies that it has a problem with argument 1 to the sub, but this is manifestly wrong as swaping the qw[..] for an array with the same contents and it works fine.

I'm guessing that this is something to do with the prototype, which I know can be considered evil, but there is no other way to get the map style syntax which we all love. 

#! perl -slw
use strict;

sub map2 (&@) {
    use Carp;
    my $code = shift;
    croak 'Odd number of values in list' if @_ & 1;
    map { local ($a, $b) = (shift,shift); $code->() } 1 .. (@_>>1);
}

#! This works.
my @array = ('A'..'J');
map2{ print "$a $b"; } @array;

=pod This code gives compile time error.

map2{ print "$a $b"; } qw[A B C D];

C:\test>test
Type of arg 1 to main::map2 must be block or sub {} (not list) at C:\t
+est\test.pl line 17, near "qw[A B C D];"
Execution of C:\test\test.pl aborted due to compilation errors.

=cut

#! But this works fine??
map{ print $_; } qw[A B C D];

[download]

Examine what is said, not who speaks.

The 7th Rule of perl club is -- pearl clubs are easily damaged. Use a diamond club instead.

In reply to Problem emulating built-ins like map and grep. by BrowserUk

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