Re^2: Hope a subroutine will return undef by default

From above cert.org link ...

In list context, things are slightly more complicated. An empty list, when evaluated in a boolean condition, evaluates to false. But the value undef, when evaluated in list context, evaluates to true because it is converted to a list with the singleton value undef. Therefore, a function should not return undef if it might ever be invoked in list context.

... that very thing -- an empty list collapsing to nothing -- has caused me irritation to work around it when something is expected, even undef: by calling the offending code in scalar context. And that was/is the *only* purpose of that damn separate call. I absolutely hate context based returned values in such cases.

Comment on Re^2: Hope a subroutine will return undef by default

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Re^3: Hope a subroutine will return undef by default ("boolean list context") by tye (Sage) on Feb 12, 2014 at 03:42 UTC
But the value undef, when evaluated in list context, evaluates to true because it is converted to a list with the singleton value undef. Therefore, a function should not return undef if it might ever be invoked in list context. Utter nonsense! "In list context" is almost the opposite of "evaluate to a boolean value". Probably the only tiny grain of truth to that is how "list assignment in scalar context" evaluates to the size of the right-hand list so "return;" is required to exit a loop written like: `while( my( $garthok ) = narfle() ) {` [download] But, of course, you can write (and are more likely to write) that loop like: `while( my $garthok = narfle() ) {` [download] Functions that return more than one value or already return a variable number of values should indeed return an empty list when wanting to convey "false". Functions that only ever return a single value, should usually "return undef;" or "return 0;" rather than be transformed into functions that return a variable number of values. IME, you are much more likely to get bitten by a "get a scalar" function returning an empty list in lots of different types of code like: `my %hash = ( gnarfle => get1garthok(), tool => a_herring() ); feed( get1garthok(), a_herring(), $spoon );` [download] than to get bitten by deciding to uselessly use a list assignment directly in a conditional expression when dealing with a "get a scalar" function. Note how the exact problem with doing what the quoted text suggests is that it can easily screw you up when you call it in list context (the opposite of what they claim). Now, if you want to be able to distinguish "successfully return 'undef'" from "failure which returns nothing", then you might opt for "return;" for the latter case (and you'll have to be extra careful when using such a function). - tye	[reply] [d/l] [select]

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Re^3: Hope a subroutine will return undef by default ("boolean list context")
by tye (Sage) on Feb 12, 2014 at 03:42 UTC

But the value undef, when evaluated in list context, evaluates to true because it is converted to a list with the singleton value undef. Therefore, a function should not return undef if it might ever be invoked in list context.

Utter nonsense!

"In list context" is almost the opposite of "evaluate to a boolean value". Probably the only tiny grain of truth to that is how "list assignment in scalar context" evaluates to the size of the right-hand list so "return;" is required to exit a loop written like:

    while( my( $garthok ) = narfle() ) {
[download]

But, of course, you can write (and are more likely to write) that loop like:

    while( my $garthok = narfle() ) {
[download]

Functions that return more than one value or already return a variable number of values should indeed return an empty list when wanting to convey "false". Functions that only ever return a single value, should usually "return undef;" or "return 0;" rather than be transformed into functions that return a variable number of values.

IME, you are much more likely to get bitten by a "get a scalar" function returning an empty list in lots of different types of code like:

    my %hash = ( gnarfle => get1garthok(), tool => a_herring() );
    feed( get1garthok(), a_herring(), $spoon );
[download]

than to get bitten by deciding to uselessly use a list assignment directly in a conditional expression when dealing with a "get a scalar" function.

Note how the exact problem with doing what the quoted text suggests is that it can easily screw you up when you call it in list context (the opposite of what they claim).

Now, if you want to be able to distinguish "successfully return 'undef'" from "failure which returns nothing", then you might opt for "return;" for the latter case (and you'll have to be extra careful when using such a function).

- tye

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