Apply function "keys" directly to function returning hash

wirrwarr has asked for the wisdom of the Perl Monks concerning the following question:

Dear monks,
I have a function returning a hash, like

sub func { return ('a' => 'ah', 'b' => 'be') };
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now I'd like to apply the internal perl function keys to this function. I tried

keys func();
keys ( func() );
keys ( &func() );
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all of which give the following error:

Type of arg 1 to keys must be hash (not subroutine entry)
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It works if I use a temporary variable though:

my %hash = func();
keys %hash;
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Now is there a way I can use the function directly? Or doesn't Perl's syntax allow this? perldoc perlfunc gives following description:

Returns a list consisting of all the keys of the named hash. ...

which seems to imply I have to use a variable.

Thanks for any insights on this!

Comment on Apply function "keys" directly to function returning hash Select or Download Code

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Re: Apply function "keys" directly to function returning hash by benn (Vicar) on Aug 25, 2003 at 16:48 UTC
The problem is that you're not returning a hash - you're returning a list, which `my %hash = func()` turns into a hash for you. One way round this would be to return a hashref instead - something like this... `sub func { return {'a' => 'ah', 'b' => 'be'} }; print keys %{func()}; # a b` [download] Hope this helps, Ben.	[reply] [d/l] [select]
Re: Apply function "keys" directly to function returning hash by herveus (Prior) on Aug 25, 2003 at 16:50 UTC
Howdy! I got somewhere with `keys %{{func()}}` func() returns a list\ {func()} constructs an anonymous hash from that list %{} around that tries to dereference what is inside it as a hash reference (ending up with the hash keys wants) `keys # take keys of following HASH %{ # dereference HASHREF { # construct HASHREF from list func() # returns list } }` [download] yours, Michael	[reply] [d/l] [select]
Re: Re: Apply function "keys" directly to function returning hash by wirrwarr (Monk) on Aug 25, 2003 at 16:56 UTC
Oh yes, that's the solution. I already tried `%{\(func())}`, but I always forget that \() is not the same as {}. :( Thanks a lot!	[reply]
Re: Apply function "keys" directly to function returning hash by sgifford (Prior) on Aug 25, 2003 at 19:19 UTC
Essentially you want to keep all of the even-numbered array elements, and discard all of the odd ones. Something like this will do that: `my $i = 1; print join(" ",grep { $i++ % 2 } l()),"\n";` [download]	[reply] [d/l]
Re: Re: Apply function "keys" directly to function returning hash by wirrwarr (Monk) on Aug 26, 2003 at 10:35 UTC
Hmm. Well, yes, this does get me the keys of the hash, but my intent was rather more general: I wanted to be able to call a perl internal function which takes a hash directly with a function call as a parameter, i.e. "somefunc func()". But thanks anyways!. daniel.	[reply]
Re: Re: Re: Apply function "keys" directly to function returning hash by sgifford (Prior) on Aug 26, 2003 at 18:07 UTC
It's not an internal function, but it does work exactly as you describe: `#!/usr/bin/perl -sw use strict; sub somefunc(@) { my $i = 1; grep { $i++ % 2 } @_; } sub func { return ( 1 => "one", 2 => "two", 3 => "three", ); } print join(" ",somefunc func());` [download]	[reply] [d/l]