Combine two functions to create a function with a union type input

[manuel rascioni]

Hi,

I've wrote this code, in which I combine a function from String to String and another one from Integer to String to create a new function that has input String|Integer and returns a String:

String(Integer|String) mix(String(Integer) f, String(String) s) {

    return ((Integer|String arg) => 

        switch(arg)

        case (is Integer)

            f(arg)

        case (is String)

            s(arg)

    );

}

value fn4 = mix(Integer.string, String.uppercased);

print(fn4("ciao"));

print(fn4(160));

Then I tried to generalize it to have generics inputs of the functions:

String(F|S) mix<F, S>(String(F) f, String(S) s) {

    return ((F|S arg)

        => switch (arg)

        case (is F)

            f(arg)

        case (is S)

            s(arg)

    );

}

But (sadly) this piece of code doesn't compiles, saying that cases are not disjoint.

I was wondering why?

Is it because I can't guarantee that F doesn't extends S?

Is there a way to do it?

[Gavin King]

Right, the issue is that if we have two type parameters, we just can't
be sure that they don't refer to overlapping types. The branches of a
switch are required to be disjoint.

OTOH, the branches of an if statement are *not* required to be
disjoint, so you can use "if (is)" instead of switch.

> --
> You received this message because you are subscribed to the Google Groups
> "ceylon-users" group.
> To unsubscribe from this group and stop receiving emails from it, send an
> email to ceylon-users...@googlegroups.com.
> To post to this group, send email to ceylon...@googlegroups.com.
> Visit this group at https://groups.google.com/group/ceylon-users.
> To view this discussion on the web visit
> https://groups.google.com/d/msgid/ceylon-users/b471d7fa-550a-45aa-97b4-12b2b0991d24%40googlegroups.com.
> For more options, visit https://groups.google.com/d/optout.



--
Gavin King
ga...@ceylon-lang.org
http://profiles.google.com/gavin.king
http://ceylon-lang.org
http://hibernate.org
http://seamframework.org

[manuel rascioni]

I've tried, but with the if statement it seems that the union type can't be "cast" (don't know if is the right term) to F.

<String|Exception>(F|S) mix<F, S>(String(F) f, String(S) s) {

    return ((F|S arg) {

        if (arg is F) {

            return f(arg);

        }

        else if (arg is S) {

            return s(arg);

        }

        else {

            return Exception("");

        }

    });

}

Saying:

argument must be assignable to parameter type: 'F|S' is not assignable to 'F'

I'm pretty new to Ceylon, the type system seems very interesting, I'm exploring it :)

[John Vasileff]

There are actually two ways `is` can be used. The `arg is F` form is just a Boolean expression. The other, `is F arg`, is a condition that will narrow `arg` to the type `F`.

More on narrowing can be found here https://ceylon-lang.org/documentation/1.3/tour/types/#narrowing_the_type_of_an_object_reference

HTH,

John

To view this discussion on the web visit https://groups.google.com/d/msgid/ceylon-users/c5513aeb-fbf7-48c4-9240-9fb3e2e38638%40googlegroups.com.

[manuel rascioni]

Thank you!

Now I made it works!