Home > Perl6 > Nil is a pessimist

Nil is a pessimist

Guifa was unhappy with $xml.elements returning a list with one undefined element if there are no child nodes. That led me to the conclusion that Nil is only halve Empty.

Let’s consider this piece of code.

sub nilish() { Nil }; 
for nilish() { say 'oi‽' }

my $nil := nilish();
for $nil { say 'still oi‽' }

sub no-return() { }
for no-return() { say 'even more oi‽' }

sub return-a-list( --> List:D ) { Nil }
for return-a-list() { say 'Should this happen?' }

# OUTPUT:
# oi‽
# still oi‽
# even more oi‽
# Should this happen?

We are iterating over the special container-reset-value called Nil because there is no container to reset. Since for binds to its arguments it binds to Nil. A type object, even a very special one as Nil, is a single item which is treated as a list with one element by for.

We can solve this problem by a multi sub that turn unreasonable values into the empty list.

multi sub undefined-to-empty(Nil) { Empty }
multi sub undefined-to-empty(@a where all @a.map(!*.defined)) { Empty }
multi sub undefined-to-empty(\item) { item }

for undefined-to-empty(Nil) { say 'nothing happens' }
for undefined-to-empty((Any,)) { say 'nothing happens' }

By adding a candidate that checks if there are only undefined values in a list we can also solve guifa`s problem.

This is of cause just a shim. The real solution is to stop turning nullinto Nil in native bindings. If you write a sub that has to return a list but can’t either fail or return Empty if there is nothing to return. To help avoid that mistake in the future I filed #2721.

If it looks empty or sounds emtpy or tastes emtpy make it Empty!

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Categories: Perl6
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