Re: Preventing unintended list expansion inside hash literals.

G'day gregory-nisbet,

"What's the idiomatic way to guard against unintentionally expanding a non-singleton list into the bodies of your hashes?"

I don't know if there's an idiom for that specific case; however, as a general case, you can use scalar to force scalar context. So, if you're happy to have undef values in your hash:

key1 => scalar(some_key($bar))
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If you don't want undef values and are using perl v5.10, or higher, you can do something like these:

key1 => (some_key($bar) // 0)
key1 => (some_key($bar) // [])
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If none of those suit, then what you're currently doing ("use an intermediate scalar variable") seems like a perfectly fine solution.

— Ken

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Re^2: Preventing unintended list expansion inside hash literals.
by Athanasius (Archbishop) on Jan 05, 2017 at 09:55 UTC

Hello kcott,

I considered using scalar too, but then the comma operator returns the right-most value in the list, which doesn’t seem to be what the OP is looking for:

19:53 >perl -MData::Dump -wE "sub f { return (17, 42); } my %h = ( key
+1 => scalar f() ); dd \%h;"
{ key1 => 42 }

19:54 >
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Cheers,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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Re^3: Preventing unintended list expansion inside hash literals.

by kcott (Archbishop) on Jan 05, 2017 at 10:38 UTC

I was working on this from the OP:

"However, when the hash table is empty, we just call return."

That appears to be confirmed in his code example:

...
%$hash_ref or return; # just return
...
$key;                 # or last line in sub returns a scalar
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So, %some_hash is ending up as one of:

( key1 => ($val1), key2 => ($val2) )
( key1 => (),      key2 => ()      )
( key1 => ($val1), key2 => ()      )
( key1 => (),      key2 => ($val2) )
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These will be flattened to:

( 'key1', $val1, 'key2', $val2 )
( 'key1', 'key2'               )
( 'key1', $val1, 'key2'        )
( 'key1', 'key2', $val2        )
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The first is fine; the second is what the OP had a problem with; the last two are problematic but, assuming he's using warnings, should generate an "Odd number of elements in hash assignment" diagnostic (from perldiag).

I don't think the "comma operator returns the right-most value in the list" comes into play in any of these scenarios: all lists are either empty or only have one value.

Am I missing something with your "return (17, 42);" example?

— Ken

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Re^4: Preventing unintended list expansion inside hash literals.

by Athanasius (Archbishop) on Jan 05, 2017 at 13:00 UTC

Hello Ken,

No, you’re right: in the context of the OP’s example code, scalar works fine. I was looking to the more general question:

What's the idiomatic way to guard against unintentionally expanding a non-singleton list into the bodies of your hashes?

A “non-singleton list” can be an empty list (as in the example code), OR a list with more than one element. Use of scalar for the latter case returns the last element of the list, not the first. From the OP’s use of $array_ref->[0] as his example of “a single-element list,” I assumed that in this case he would want the first element of the list. But admittedly this is little more than a guess. :-( Sorry for the confusion.

Cheers,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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