Does Raku have Python's Union type?

Question

In Python, Python has Union type, which is convenient when a method can accept multi types:

from typing import Union

def test(x: Union[str,int,float,]):
    print(x)

if __name__ == '__main__':
    test(1)
    test('str')
    test(3.1415926)

Raku probably doesn't have Union type as Python, but a where clause can achieve a similar effect:

sub test(\x where * ~~ Int | Str | Rat) {
    say(x)
}

sub MAIN() {
    test(1);
    test('str');
    test(3.1415926);
}

I wander if Raku have a possibility to provide the Union type as Python?

#        vvvvvvvvvvvvvvvvvvvv - the Union type doesn't exist in Raku now.
sub test(Union[Int, Str, Rat] \x) {
    say(x)
}

"If instead you'd like to write a Sum Type, there is not an exact equivalent in Raku. The closest thing would be an Enum." [link](https://docs.raku.org/language/haskell-to-p6#Data_definitions) Note that types are unenforced in Python, ```test``` will work with any type that has ```___repr___``` or ```___str___``` methods — freshpasta, Mar 08 '22 at 11:39

librasteve · Accepted Answer · 2022-03-08T23:25:01.003

My answer (which is very similar to your first solution ;) would be:

subset Union where Int | Rat | Str;

sub test(Union \x) {
   say(x) 
}

sub MAIN() {
    test(1);
    test('str');
    test(pi);
}

Constraint type check failed in binding to parameter 'x'; 
expected Union but got Num (3.141592653589793e0)

(or you can put a where clause in the call signature, as you have it)

In contrast to Python:

this is native in raku and does not rely on a package like "typing" to be imported
Python Union / SumTypes are used for static hinting, which is good for eg. IDEs
but these types are unenforced in Python (per @freshpaste comment and this SO), in raku they are checked and will fail at runtime

So - the raku syntax is there to do what you ask ... sure, it's a different language so it does it in a different way.

Personally I think that a typed language should fail if type checks are breached. It seems to me that type hinting that is not always enforced is a false comfort blanket.

On a wider point, raku also offers built in Allomorph types for IntStr, RatStr, NumStr and ComplexStr - so you can work in a mixed mode using both string and math functions

Does Raku have Python's Union type?

1 Answers1