How to get the `< ... >` syntax on an object method?

Question

In Raku: when I create an object with a CALL-ME method, I would like to use the < ... > syntax for the signature instead of ( '...' ) when ... is a Str.
In the documentation, there is an example of creating an operator, which is an ordinary Raku sub with a special syntax. This sort of sub automatically converts < ... > into a Str.
For example, if I have

use v6.d;
class A {
   has %.x;
   method CALL-ME( Str:D $key ) { say %!x{ $key } }
}

my A $q .= new( :x( <one two three> Z=> 1..* ) );
$q('two');
$q<two>;

But this produces

2
Type A does not support associative indexing.
  in block <unit> at test.raku line 10

Changing has %.x; to has %.x handles <AT-KEY>; means your example works. This follows logically from Liz's comment in the discussion I linked. But, based on what AJS said in that thread, I imply no warranties on the general fitness of this solution. — 15 hours ago
@raiph, the r/rakulang thread is closed, but I agree with what you said there about class Foo is Hash. The new issue can be solved with method new(|c) { self.Any::new(|c) } — 14 hours ago

codesectionscodesections 9,34016 silver badges50 bronze badges · Accepted Answer · 2023-11-12 20:46:09Z

5

An answer based on @raiph’s comment:

You need to implement the AT-KEY method on your type.

You can do so directly like this:

class A {
   has %.x;
   method CALL-ME( Str:D $key ) { say %!x{ $key } }
   method AT-KEY($key) { self.($key) }
}

Or you can do so by delegating AT-KEY to %!x using the handles trait:

class A {
   has %.x handles <AT-KEY>;
   method CALL-ME( Str:D $key ) { say %!x{ $key } }
}

answered 14 hours ago

codesections

9,34016 silver badges50 bronze badges

4

@raiph but if I'm reading the reddit thread correctly, neither your AJS's suggested change nor yours would change the semantics of handles <AT-KEY>, would they? (It's just about handles * or handles '*', right?)

– codesections

13 hours ago
In the question, I used has %.x as the minimal structure. In the code I'm working on, the hash does exist a couple of layers down, so a handles is not really an option. The method AT-KEY is the more generic route.

– Richard Hainsworth

12 hours ago
@codesections Correct.

– raiph

11 hours ago
@codesections My current understanding is that an explicit method AT-KEY (as Richard has chosen to go with) is problematic, adding handles <AT-KEY> (explicitly delegating) is problematic, and handles * (which is trumped not only by methods in the enclosing class but also classes it inherits from) is problematic. That's why I was hesitant to write an answer. But having figured out enough to feel my comment link plus an answer about handles <AT-KEY> would be enough to help while I worked to nail down the problems and write a nuanced answer, I posted my answer — and then saw yours. 👍 🙃

– raiph

11 hours ago

How to get the `< ... >` syntax on an object method?