Re: [ceylon-users] Enumerated objects with type parameters

[Paco Soberón]

Well, the problematic part is "of genEnum" in the definition of Enum<Gen>. What the error is saying is that genEnum satisfies Enum<String>, but not Enum<Gen> for every Gen.

El 27 sept. 2017 12:13 p. m., "Voitech" <wojciech...@gmail.com> escribió:

Hello I'm in need to have my enumerated objects to have type parameters. It should expose this parameters to satisfied interface. Example:

shared interface IFace<Gen> {}
shared abstract class Enum<Gen>() of genEnum satisfies IFace<Gen> {}
shared object genEnum extends Enum<String>() {}

This dosn't compile with error:

Case type must be a subtype of enumerated type for every type argument of the generic enumerated type: genEnum is not assignable to Enum<Gen>

Err why ? I don't understand.

Thank You

Voitech

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[Gavin King]

So the restrictions applying here are described in §3.4.3 of the
language specification:

https://ceylon-lang.org/documentation/1.3/spec/html/typesystem.html#genericenumeratedtypes

The feature you're looking for here is called "GADT" (Generalized
Algebraic Data Types) but Ceylon doesn't support them today.

On Wed, Sep 27, 2017 at 6:13 AM, Voitech <wojciech...@gmail.com> wrote:
> Hello I'm in need to have my enumerated objects to have type parameters. It
> should expose this parameters to satisfied interface. Example:
>
> shared interface IFace<Gen> {}
> shared abstract class Enum<Gen>() of genEnum satisfies IFace<Gen> {}
> shared object genEnum extends Enum<String>() {}
>
> This dosn't compile with error:
>>
>> Case type must be a subtype of enumerated type for every type argument of
>> the generic enumerated type: genEnum is not assignable to Enum<Gen>
>
>
> Err why ? I don't understand.
>
> Thank You
> Voitech
>
> --
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> "ceylon-users" group.
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Gavin King
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[Gavin King]

Of course, you can get your code to compile by removing "of genEnum"
but of course that's still not going to give you a GADT.

[Lucas Werkmeister]

You can also get it to compile by moving the type argument up by one level:

shared interface IFace<Gen> {}
shared abstract class Enum() of genEnum satisfies IFace<String> {}
shared object genEnum extends Enum() {}

But that only works as long as you only have one case type (or several, but all with the same Gen type argument).