CODEWITHSUNDEEP
perlsmartmatch
d.k
d.k

Reputation: 4456

Why is @array ~~ $number different from @array == $number?

According to Programming Perl, using smartmatch with "any" on the left and a number on the right checks numeric equality:

------------------------------------------------------------------------------
| Left | Right |    Description    | Like (But Evaluated in Boolean Context) |
------------------------------------------------------------------------------
|  Any |   Num |  Numeric equality |               Any == Num                |

Therefore, I expect the following to output 1:

my @arr = ('aaa');
my $num = 1;
say @arr ~~ $num;

but it actually outputs the empty string.

I thought @arr would be converted to scalar 1 because it has 1 element, so say @arr ~~ $num would be equivalent to say @arr == $num.

Why is @arr ~~ $num different from @arr == $num?

Upvotes: 3

Views: 257

Answers (2)

ikegami
ikegami

Reputation: 385897

The smartmatch operator obviously doesn't take lists as operands. As such, it evaluates its operands in scalar context. If that was the end of the story, the following wouldn't work

my @a = ('aaa', 'bbb');
my $s = 'aaa';
say $s ~~ @a;

because it would be the same as

say 'aaa' ~~ 2;

But it's clear that it does work. That's because smartmatch automatically creates a reference to its operands that are arrays or hashes, just like push does to its first operand. That means

say $s ~~ @a;

is really

say $s ~~ \@a;

and (your code)

say @a ~~ $n;

is the same as

say \@a == $n;

You need to get the length explicitly. The following will do what you want:

say 0+@a ~~ $n;

Of course, so would

say 0+@a == $n;

or even

say @a == $n;

Upvotes: 3

Leon Timmermans
Leon Timmermans

Reputation: 30225

That's because smartmatch takes a reference first. @array ~~ $num would be equivalent to \@array == $num, which obviously is very very unlikely to be true.

You might want to use my Smart::Match module and do @array ~~ array_length($num), @array ~~ contains($num) and other things in a non-surprising way.

Upvotes: 2

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