Question

#!/usr/bin/perl
use Modern::Perl;
while (<>)
{ chomp;
say reverse;  
}

The above code doesn’t work but when I change 2nd last line to say scalar reverse; then it works fine. Why do I need to force it to be a scalar explicitly? Can’t Perl DWIM?

Answer 1

If I understand the documentation right, reverse normally operates on lists. In a list context used without arguments, it returns an empty list and by default doesn’t assign it anywhere. In your example, say outputs the unchanged $_;

Forcing reverse into scalar context changes its behaviour and makes it reverse character strings, and use $_ by default. Because say can be used to print lists as well as scalars, it doesn’t force its arguments into scalar context.

Perl probably does DWIM, just for given values of “I”.

A breakdown of what reverse does when:

#!/usr/bin/env perl
use strict;
use v5.12;

my $inscalar = "foo bar";

# `reverse` is in list context, $in is coerced to a single-element list
my @outlist = reverse $inscalar;

# `reverse` is in scalar context, so it reverses strings
my $outscalar = reverse $inscalar;

say join(" | ", @outlist); # prints "foo bar"
say $outscalar; # prints "rab oof"

# the "normal" behaviour of `reverse`
my @inlist = qw(foo bar);
@outlist = reverse @inlist;
say join(" | ", @outlist); # prints "bar | foo"