[Haskell-cafe] "SameConstraints?" type constraints

[MigMit]

No, constraints aren't a part of type e. But you can specify the constraint in SC itself:

data SC c e where SC :: c d => d -> e -> S e

(or SC :: (c d, c e) => d -> e -> S e, if you like it better). Of course, you would need ConstraintKind for that.

Отправлено с iPad

> 19 июня 2015 г., в 7:16, Leza Morais Lutonda <leza.ml at fecrd.cujae.edu.cu> написал(а):
> 
> 
> Hi All,
> 
> I have the following data type:
> 
> data S e where
> 
>     SC :: d -> e -> S e
> 
> And I want to declare a Show instance for it in the following way:
> 
> instance Show e => Show (S e) where
> 
>    show (SC x y) = show x ++ show y
> 
> But, of course it don't typechecks because: could not deduce `Show d` arising from a use of `show`.
> Is there a way to constraint the `d` type in the `SC` constructor definition to have the same constraints of the `e` type? Something like:
> 
>  SC :: SameConstraints d e => d -> e -> S e ???
> 
> 
> Thanks!
> 
> 
> -- 
> Leza Morais Lutonda, Lemol-C
> http://lemol.github.io
> 
> 
> 
> 50 Aniversario de la Cujae. Inaugurada por Fidel el 2 de diciembre de 1964  http://cujae.edu.cu
> 
> 
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