Re: perldelta unclear on 'given's fate

I imagine that given/when will be kept. However, the perception is that their current implementation is highly flawed. A new implementation will probably break some existing uses of given/when, so it makes sense to start warning people about this.

given originally used the lexical $_ rather than the global $_ (but this has changed in 5.18). This was confusing for many people.

when adds a whole extra layer of weird.

Quick quiz! What does this output?

use v5.10;
use strict;
no warnings;

sub two () { 2 }
sub three  { 3 }

sub english_number {
    my $number = shift;
    given ($number) {
        when ( 1 )      { return 'one' }
        when ( two )    { return 'two' }
        when ( three )  { return 'three' }
        when ( 4 )      { return 'four' }
        default         { return 'more' }
    }
}

say english_number($_) for 1..5;
[download]

use Moops; class Cow :rw { has name => (default => 'Ermintrude') }; say Cow->new->name

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Re^2: perldelta unclear on 'given's fate by morelenmir (Beadle) on Jan 04, 2014 at 18:45 UTC
Hmmm... I actually had to run that to work it out! It prints "one, two, three, three, three" because the 'three' routine resets the global $_ every time the flow of execution reaches it I think? But... why doesn't the same happen with the 'two ()' routine... Something to do with the parentheses - does that sub not effect the global $_? That's quite a mind-twister!!!	[reply]
Re^2: perldelta unclear on 'given's fate by iguanodon (Priest) on Jan 04, 2014 at 18:37 UTC
A whole extra layer of weird indeed. I couldn't guess so I ran it... weirder than I expected. Can you please explain what's going on here?	[reply]
Re^3: perldelta unclear on 'given's fate by tobyink (Canon) on Jan 04, 2014 at 20:38 UTC
It basically comes down to a question of why does `when(two)` seem to work as expected, but `when(three)` screw up everything below it? `when(EXPR)` can mean two different things. It sometimes means `if($_ ~~ EXPR)`, and it sometimes means `if(EXPR)`. The rules by which the interpretation of `when` is decided are fairly convoluted, and have changed from Perl version to Perl version. In this case they're fairly easily explained. `three` is a sub call, so `when(three)` is interpreted as `if(three)`. The `three` sub returns 3, so this condition always evaluates to true. Thus the subsequent condition `when(4)` and the `default` clause will never get triggered. `two` is a simple sub with a prototype of `()` though. Thus Perl treats it as a constant. It inlines the return value `2` when the `when(two)` is compiled. So Perl sees the condition as `when(2)` - that is, not a sub call at all - and interprets it as meaning `when($_ ~~ 2)`. `use Moops; class Cow :rw { has name => (default => 'Ermintrude') }; say Cow->new->name`	[reply] [d/l] [select]


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