Re: Re: Did I get what I expected (an array)?

Ok, I'll try a better translation from my thoughts in italian to a better english :-)

Maybe I should restate the question this way:

is it possible , and how, to distinguish if a sub/method is returning a single scalar (

return
$this
[download]

, which my test script should consider wrong) or a list (return ($this) or return qw(a bunch of values)) or an array (return @this) which my test script should both consider good?

I'm writing a test script (one of those that make test runs) that checks if a class of mine or a subclass of it behaves like it should.

My check detects the case return @this but fails on return qw(this list).

use strict ;
use warnings ;

sub test {  return ('a','test') }

my $test = test() ;
my @test = test() ;

print ">>> $test\n" ;
print map "[$_]\n",@test ;
print $test == scalar(@test)? "Yepa!\n" : "Nope!\n" ;
[download]

This one fails:

$ perl test.pl
>>> test
[a]
[test]
Argument "test" isn't numeric in numeric eq (==) at test.pl line 11.
Nope!
[download]

(even if I consider test's return value ok for my class...)

I hope I explained now. Do I?

Ciao!
--bronto

# Another Perl edition of a song:
# The End, by The Beatles
END {
  $you->take($love) eq $you->made($love) ;
}

Comment on Re: Re: Did I get what I expected (an array)? Select or Download Code

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Re: Did I get what I expected (an array)? by Abigail-II (Bishop) on Jul 16, 2002 at 17:31 UTC
Yes, you explained what you want. It's also clear you lack some fundamental knowledge about Perl. You need to make yourself more familiar with the concept context. A subroutine will return a list if and only if it was called in list context. A subroutine will return a scalar if and only if it was called in scalar context. A subroutine will return nothing if and only if it was called in void context. The crucial point is that it is the context that determines whether the subroutine returns a list or a scalar - not the subroutine. Furthermore, parenthesis do not create lists (except in some very specific syntax constructs). Parenthesis are used for grouping. `return $this; return ($this); return (($this)); return ((($this)));` [download] all mean the same. The do not differ just like there is no difference between: `3 + 4; (3 + 4); (3) + (4); ((3) + (4));` [download] As for `return @x;` [download] that will return the number of elements of `@x` in scalar context, and a list consisting of the elements of `@x` in list context. Perl will never return an array. You can check for yourself: `my @x = qw /foo bar baz/; sub my_func {return @x}; (my_func) [1] .= "hello";` [download] This gives a compile error: `Can't modify list slice in concatenation (.) or string` [download] Note that it says "list slice". A list, not an array. Abigail	[reply] [d/l] [select]
Re: Re: Did I get what I expected (an array)? by bronto (Priest) on Jul 17, 2002 at 08:21 UTC
First of all: thanks Abigail! Second: sorry for posting as Anonymous Monk, unfortunately Konqueror had me and broke my authentication. If I can recover that node by any mean I'll be thankful Now back to Perl. Actually I am guilty, yes, but not of the sin you are accusing me `:-)`. I didn't get, and probabily I still have to meditate about, the difference between lists and arrays. But I know what "context" means. Otherwise, I wouldn't write: `$test = test() ; @test = test() ; print $test == scalar(@test)? "Yepa!\n" : "Nope!\n" ;` [download] The idea under that snippet was: retrieve in scalar context and in list context; if `$test` matches `scalar @test` then surely we `return @x`. About the `return` thing, you got a point: I forgot the golden rule if it seems a function, it is a function; moreover, I remembered `return LIST` from the docs, but it's `return EXPR`. I'm a sinner `;)` Last but not least, for everybody telling me about `wantarray`: you completely missed the point, but it probabily was my english's fault. I didn't tell I was looking for a way to write a better `return` statement. Instead, I was trying to test a return value of a sub/method, whoever wrote it. I can't enforce `wantarray` usage on people that is subclassing a class of mine. Anyway, another lesson learnt. Thanks to everybody Ciao! `--bronto` # Another Perl edition of a song: # The End, by The Beatles END { $you->take($love) eq $you->made($love) ; }	[reply] [d/l] [select]