Dear Haskell experts,

 

I working  on a large Haskell project on Model Based Testing (see https://github.com/TorXakis/TorXakis)

and I would like to get feedback on a design issue I have related to extension and parameterized classes.

I will describe a small simplified example to illustrate the issue

and I appreciate any feedback.

 

I have a class that extends another class as follows:

 

class Base a => Extend a where

    -- | Constructor from Base

    fromBase :: Base b => b -> a

 

I can define an instance of Extend that hides the Base implementation as follows:

 

{-# LANGUAGE ExistentialQuantification #-}

data BaseHidden = forall a . Base a =>

                            BaseHidden { _base :: a

                                       , ... -- additional extension info

                                       }

 

instance Extend BaseHidden where

    fromBase b = BaseHidden b ... -- additional extension initialization

 

I can define a parameterized data class of which the parameter is the type of the Base class as follows:

data BaseExpose a = BaseExpose { _base :: a

                               , ... -- additional extension info

                               }

 

With this parameterized data class, I can make the following fromBase' function

fromBase' :: Base a => a -> BaseExpose a

fromBase' a = BaseExpose a ... -- additional extension initialization

 

My question is can I make BaseExpose an instance of Extend?

 

I am aware that the signature of fromBase states that for all 'b' of Base

an Extend of type 'a' must be able to be made.

Yet, in my case 'a' is a parameterized class 'BaseExpose x' and

since the type of Extend is provided (it is a constructor)

I think it is acceptable that 'BaseExpose b' is returned as type.

I have looked at some extensions, such as InstanceSigs, yet I found no solution.

 

Does anybody on this mailing list have an answer or solution for me?

Also experience in a large project related to maintenance for the two alternatives (hiding or exposing types) is appreciated!

 

Thanks in advance,

    Pierre van de Laar