Dear Monks,

Here's an example that uses the topic variable $_

use v6.d;

sub go(Int $x where 1 < $_ < 9) {    # $_ refers to $x
    say $x;
}

go(3);
go(0);

--output:--
3
Constraint type check failed in binding to parameter '$x'; expected an
+onymous constraint to be met but got Int (0)
  in sub go at b.raku line 4
  in block <unit> at b.raku line 9
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I've also seen examples that use a * instead of $_

sub go(Int $x where 1 < * < 9) {
    say $x;
}

go(3);
go(0);

--output:--
3
Constraint type check failed in binding to parameter '$x'; expected an
+onymous constraint to be met but got Int (0)
  in sub go at b.raku line 4
  in block <unit> at b.raku line 9
[download]

But * and $_ do not appear to be synonyms because * doesn't work all the time:

for <a b c> {say $_};  # shorter version: for <a b c> {.say}
for <a b c> {say *};

--output:--
a
b
c
*
*
*
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Here is another example in the docs that uses *:

sub MAIN(
  Str   $file where *.IO.f = 'file.dat',
  Int  :$length = 24,
  Bool :$verbose
) {
    say $length if $length.defined;
    say $file   if $file.defined;
    say 'Verbosity ', ($verbose ?? 'on' !! 'off');
}
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In that example, I can replace * with $_ or $file, and there is no error, and I get the same output. (I don't understand the where clause because $file.IO.f tests whether $file is a file and returns True or False, so the where clause evaluates to something like True = 'file.dat'. What does that do? Ahh, nevermind: 'file.dat' is the default value, and the parameter variable is $file where $file.IO.f. My tests show that the default value gets assigned to $file before the file test. It would be clearer if you could write  Str $file = 'file.dat' where *.IO.f but that results in an error. )

Here's an example that calls first() on a list/array, which allows you to look for the first() thing that matches some condition:

class Foo {
    method bar( $self: ){ "baz" }
};
say Foo.^methods.first(*.name eq 'bar').signature ~~ :($: *%) ;
# OUTPUT: «True␤»
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If I replace the * with $_, I get an error.

I can't find anything in the docs about the use of * in the examples I posted.