[Haskell-cafe] Data.Data and OverlappingInstances

Timo von Holtz timo.v.holtz at gmail.com
Thu Aug 9 12:00:18 CEST 2012

Hi cafe,

in my code I use Data.Data and OverlappingInstances. My problem now is,
that I can't use functions using these instances as arguments for

This is what my problem looks like:

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE OverlappingInstances #-}
{-# LANGUAGE UndecidableInstances #-}
import Data.Data

class Test a where
  foo :: Monad m => m a

instance Num a => Test a where
  foo = return 1

instance Test Int where
  foo = return 2

test constr = fromConstrM foo constr

but when I compile I get:

    Overlapping instances for Test d
      arising from a use of `foo' at test.hs:15:26-28
    Matching instances:
      instance [overlap ok] (Num a) => Test a
        -- Defined at test.hs:9:9-23
      instance [overlap ok] Test Int -- Defined at test.hs:12:9-16
    (The choice depends on the instantiation of `d'
     To pick the first instance above, use -XIncoherentInstances
     when compiling the other instance declarations)
    In the first argument of `fromConstrM', namely `foo'
    In the expression: fromConstrM foo constr
    In the definition of `test': test constr = fromConstrM foo constr
Failed, modules loaded: none.

Is there a way out? Right now I use a "case (typeOf x) of" kind of
construct, but it gets pretty messy and I even have to unsafeCoerce at one

- Timo
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