Question

Some time ago I was asked the “strange” question how would I implement map with grep.
Today I tried to do it, and here is what came out. Did I squeeze everything from Perl, or there are other more clever hacks?

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

sub my_map(&@) {
    grep { $_= $_[0]->($_) } @_[1..$#_];
}

my @arr = (1,2,3,4);

#list context
say (my_map sub {$_+1}, @arr);
#scalar context
say "".my_map {$_+1} @arr;
say "the array from outside: @arr";
say "builtin map:", (map {$_+1} @arr);

Answer 1

Are you sure they didn’t ask how to implement grep with map? That’s actually useful sometimes.

grep { STMTs; EXPR } LIST

can be written as

map { STMTs; EXPR ? $_ : () } LIST

(With one difference: grep returns lvalues, and map doesn’t.)

Knowing this, one can compact

map { $_ => 1 } grep { defined } @list

to

map { defined ? $_ => 1 : () } @list

(I prefer the “uncompressed” version, but the “compressed” version is probably a little faster.)

---

As for implementing map using grep, well, you can take advantage of grep‘s looping and aliasing properties.

map { STMTs; EXPR } LIST

can be written as

my @rv;
grep { STMTs; push @rv, EXPR } LIST;
@rv