Hello,

I would say that type families should do the joke:

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE GADTs #-}
type family Prod a where
  Prod String    = Int
  Prod (Maybe a) = a

data Foo e a where                                                                                                                           
   Foo :: e -> Foo (Prod e) a

Regards.

2016-07-04 22:43 GMT+02:00 Corentin Dupont <corentin.dupont@gmail.com>:
Hi all,
I have a data type looking like this:

data Foo e a where                                                                                                                            
   Foo :: e →  Foo e a    

I would like to instantiate it to make it equivalent to:

data Bar a where                                                                                                                              
   A :: String    →  Bar Int
   B :: Maybe a     →  Bar a

How can I do that? With a functional dependency?
I probably need to change the definition of Foo.

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