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Re: On Scalar Context ("list")

by tye (Cardinal)
on Apr 23, 2004 at 18:14 UTC ( #347699=note: print w/ replies, xml ) Need Help??


in reply to On Scalar Context

Thanks, merlyn++.

Note that in the following cases, "last element" doesn't mean the same thing each time.

(10, 20, 30)last element
@foo[3..5]last element
(10, 20, 30)[2, 1]last element

I'd actually change one entry:

(10, 20, 30)last expression

Because it isn't the "last element" of the "list of scalar values that this operation would produce if called in a list context" but is instead the "last element" in the "list of Perl expressions that are separated by commas". Likewise, I might change the other two to "last scalar value" to be clearer.

This is another perfect example of why "list" doesn't just mean one thing when talking about Perl. Each of those cases return the "last element" of some "list", it is just that "list" means two different things in those two cases.

I think this is a big part of why people get confused and people argue when talking about "lists" in Perl. Since "list context", "scalar context", and "scalar" all have precise definitions in Perl, it is natural to think that "list" has a precise definition as well.

#!/usr/bin/perl -wl use strict; my @idx= ( 2, 99, 5 ); my @a2z= ( 'a'..'z' ); my @arr= ( 10..16 ); print '(@arr,@a2z): ', scalar( (@arr,@a2z) ); print '(@arr,@a2z)[@idx]: ', scalar( (@arr,@a2z)[@idx] ); print '@arr[@idx]: ', scalar( @arr[@idx] );

produces

Useless use of private array in void context at scalar.pl line 9. (@arr,@a2z): 26 (@arr,@a2z)[@idx]: 15 @arr[@idx]: 15

So only the first item is lazy (passing the scalar context inside such that a "list of scalar values on the stack" doesn't get generated just to be mostly thrown away).

I still believe that the other two "should" have been lazy (pass their scalar context into @idx) but instead the developers of Perl were lazy and just wrote code to generate the list of scalar values and simply pick the last one at the last minute. But since slices in scalar context are more "edge" or "corner" cases, being lazy with the design was probably the right choice. (:

So $s = (10,20,30) does contain "a list in scalar context". Either one has to admit that or one has to not say that $s gets set the the "last element of the list". :)

- tye        


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Re: Re: On Scalar Context ("list")
by Errto (Vicar) on Apr 24, 2004 at 02:04 UTC

    Although it gets a bit hairy, one can explain that behavior by saying that my $a = (10,20,30); is merely an application of the comma operator in scalar context, whereas the rhs of my $a = (10,20,30)[0..2] is really a list, and so the behavior of taking the last value has to be explicitly defined. The distinction is important because I believe that  my $a = (1,2,foo) would call foo in scalar context, but my $a = (1,2,foo)[0..2] would call it in list context.

    The preceding paragraph will be moot someday because according to Apocalypse 3, there will be no scalar comma operator in Perl 6 - it will always be a list, and a list in scalar context will return an array reference

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