General conversion type class

I'd like to see if it is feasible to have a type class for converting one thing into another and back again from a mapping of [(a,b)].

This example should illustrate what I'd like to do:

data XX = One | Two | Three deriving (Show, Eq)
data YY = Eno | Owt | Eerht deriving (Show, Eq)

instance Convert XX YY where
   mapping = [(One, Eno), (Two, Owt), (Three, Eerht)]

-- // How can I make this work?:
main = do print $ (convert One :: YY) -- Want to output: Eno
          print $ (convert Owt :: XX) -- Want to output: Two

Here's my stab at making this work:

{-# LANGUAGE MultiParamTypeClasses #-}    
import Data.Maybe(fromJust)

lk = flip lookup
flipPair = uncurry $ flip (,)

class (Eq a, Eq b) => Convert a b where

   mapping :: [(a, b)]
   mapping = error "No mapping defined"

   convert :: a -> b
   convert = fromJust . lk mapping

-- // This won't work:
instance (Convert a b) => Convert b a where
   convert = fromJust . lk (map flipPair mapping)

It is easy to do this with defining two instances for the conversion going either way but I'd like 开发者_JS百科to only have to declare one as in the first example. Any idea how I might do this?

Edit: By feasible I mean, can this be done without overlapping instances any other nasty extensions?

I, er... I almost hate to suggest this, because doing this is kinda horrible, but... doesn't your code work as is?

{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE OverlappingInstances #-}
import Data.Maybe(fromJust)

lk x = flip lookup x
flipPair = uncurry $ flip (,)

class (Eq a, Eq b) => Convert a b where
   mapping :: [(a, b)]
   mapping = error "No mapping defined"
   convert :: a -> b
   convert = fromJust . lk mapping

instance (Convert a b) => Convert b a where
   convert = fromJust . lk (map flipPair mapping)

data XX = One | Two | Three deriving (Show, Eq)
data YY = Eno | Owt | Eerht deriving (Show, Eq)

instance Convert XX YY where
   mapping = [(One, Eno), (Two, Owt), (Three, Eerht)]

main = do print $ (convert One :: YY)
          print $ (convert Owt :: XX)

And:

[1 of 1] Compiling Main             ( GeneralConversion.hs, interpreted )
Ok, modules loaded: Main.
*Main> main
Eno
Two
*Main> 

I'm not sure how useful such a type class is, and all the standard disclaimers about dubious extensions apply, but that much seems to work. Now, if you want to do anything fancier... like Convert a a or (Convert a b, Convert b c) => Convert a c... things might get awkward.

I suppose I might as well leave a few thoughts about why I doubt the utility of this:

• In order to use the conversion, both types must be unambiguously known; likewise, the existence of a conversion depends on both types. This limits how useful the class can be for writing very generic code, compared to things such as fromIntegral.

• The use of error to handle missing conversions, combined with the above, means that any allegedly generic function using convert will be a seething pit of runtime errors just waiting to happen.

• To top it all off, the generic instance being used for the reversed mapping is in fact a universal instance, only being hidden by overlapped, more specific instances. That (Convert a b) in the context? That lets the reversed mapping work, but doesn't restrict it to only reversing instances that are specifically defined.