Nil vs Any confusion (trap)

[briandfoy]

Which should it be? Here's what it does now.

grammar Number {
	rule TOP { \d+ }
	}

my @strings = '137', '137 ', ' 137 ';

for @strings -> $string {
	my $result = Number.parse( $string );
	say "Result is { $result.^name }";
	given $result {
		when Match { put "<$string> worked!" }
		when Any   { put "<$string> failed!" }
		}
	}

And the result:

Result is Match
<137> worked!
Result is Match
<137 > worked!
Result is Any
< 137 > failed!

[ronaldxs]

I think there may be some confusion about the behavior of Nil here. From the docs https://docs.perl6.org/type/Nil

When assigned to a container, the Nil value (but not any subclass of Nil) will attempt to revert the container to its default value; if no such default is declared, Perl 6 assumes Any.

grammar G { token TOP { a+ } };
my Nil $n = G.parse("zz");
say $n.WHAT;
my $result = G.parse("zz");
say $result.WHAT;

prints

Nil
(Any)

[jonathanstowe]

It does return Nil, what you are seeing there is the effect of assigning Nil to an Any typed variable:

grammar G { token TOP { a+ } };
say G.parse("zz").WHAT; #  -> Nil

[briandfoy]

Duh. I'm an idiot. Thanks,

[AlexDaniel]

Do we have it explained somewhere in the docs? If yes, where?

For example, maybe it deserves a section in traps?

[jonathanstowe]

Though it's not specific to Grammar, it's just what happens when you assign Nil to a variable, I don't know what happens under the hood, but the nett effect is that the variable is the same type object that it would have been without the assignment, It thus allows you to unset a variable without knowing what type it is.

I'd go with making sure this is documented clearly in the variables page and make sure that Nil is searchable with a clear explanation somewhere.

[AlexDaniel]

I don't know where this could be documented, but I've seen this confusion a couple of times.
Another interesting point is that arrays can contain Nils:

my @a = 4, 8, 15, 16;
@a[2] = Nil;
say @a; # OUTPUT: «[4 8 (Any) 16]␤»
@a[3] := Nil;
say @a; # OUTPUT: «[4 8 (Any) Nil]␤»

Let's document it as a trap perhaps? (i.e. not understand what Any or Nil do)

[tisonkun]

https://irclog.perlgeek.de/perl6/2017-10-30#i_15374966

14:38 | lizmat | m: my %h is default(Nil) = a => Nil; dd %h
14:38 | camelia | rakudo-moar 497e0582e: OUTPUT: «Hash %h = {:a(Nil)}␤»
14:38 | lizmat | jdv79: ^^^
14:38 | jdv79 | oh, that makes sense
14:38 |            | thanks
14:40 | geekosaur | the core issue is that, in an assignment, Nil usually means use the default value for the type. so you need the default to also be Nil

[tisonkun]

Text about it can be found at https://docs.perl6.org/type/Nil#index-entry-Nil_assignment

[JJ]

@briandfoy , would the text linked by @W4anD0eR96 be enough?