Re: [Haskell-cafe] "SameConstraints?" type constraints

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 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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