[Haskell-cafe] "SameConstraints?" type constraints

Leza Morais Lutonda leza.ml at fecrd.cujae.edu.cu
Fri Jun 19 20:13:09 UTC 2015

On 06/19/2015 08:44 AM, Sean Leather wrote:
> Note this won't work for a Num instance mentioned earlier because the 
> existentially quantified d types in two SC values are not provably the 
> same type. In other words, you can't write
> instance Num e => Show (S Num e) where
>     SC x1 y1 + SC x2 y2 = SC (x1 + x2) (y1 + y2)
> because x1 and x2 can have different types.
Another issue is: what if I want to constraint the type `e` to more 
classes and make `d` to have the same constrains? I have to re-declare 
the `S` data type like?:

data S c1 c2 ... cN e where

     SC :: (c1 d, c2 d, ..., cN d) -> d -> e -> S c1 c2 ... cN e

Does anyone ever needed such a feature?

Leza Morais Lutonda, Lemol-C

50 Aniversario de la Cujae. Inaugurada por Fidel el 2 de diciembre de 1964  http://cujae.edu.cu

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