in reply to Raku: * v. $_ (asterisk v. the topic variable)

Hello 7stud,

Consider the following:

16:46 >raku -e "for 1 .. * { qq[$_ is { $_.^name }].put; last if $_ >=
+ 3; }"
1 is Int
2 is Int
3 is Int

16:46 >

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$_ is the topic variable which, as in Perl, acts like a pronoun: we can read the code as “for each value from 1 to infinity, print it and its type, ...” The topic variable $_ is documented in https://docs.raku.org/language/variables#The_$__variable.

The star, on the other hand, is not a variable, but rather an object — specifically, an instance of Raku’s built-in Whatever class. Its meaning depends on the context in which it is used. In this case it is being used as the end-point of a range, so its meaning is defined by the Range class, which interprets it as Inf (infinity). This use of the star in Raku is documented in https://docs.raku.org/type/Whatever.

So although $_ and * can sometimes be used interchangeably, they are in fact quite different things.

Hope that helps,

Athanasius <°(((>< contra mundum סתם עוד האקר של פרל,

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Re^2: Raku: * v. $_ (asterisk v. the topic variable)
by 7stud (Deacon) on Feb 19, 2024 at 14:34 UTC

    Thanks! I do think the * is prettier than $_. :)

    According to the Whatever docs, if you call a method on * then that creates a lambda (anonymous function): 

    <a b c>.map: *.uc;     # same as <a b c>.map: -> $char { $char.uc }

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    Let's test that out on one of the examples in my question. I'll test whether:

     Str $file where *.IO.f = 'file.dat',

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    ...is the same as:

    Str $file where -> $x {$x.IO.f} = 'file.dat',

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    Here's the full code with a file name, a.txt, that exists in the current directory:

    #b.raku

sub MAIN(
  Str $file where -> $x {$x.IO.f} = 'a.txt',
  Int  :$length = 24,
  Bool :$verbose
) {
    say $length if $length.defined;
    say $file   if $file.defined;
    say 'Verbosity ', ($verbose ?? 'on' !! 'off');
}

--output:--
$ raku b.raku
24
a.txt
Verbosity off


$ raku b.raku --verbose --length=7 a.raku
7
a.raku
Verbosity on

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    It looks to me like when you specify a code block on the right of where, the code block is called with the term to the left of where as the argument.

    I don't think the if statements on the end of $length and $file actually do anything: with those default values both variables will be defined.

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        That's some comment! Thanks!
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