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haskelltypesrepa

How to allow unification of a type variable with different types?

I have a function with a following type signature

{-# LANGUAGE FlexibleContexts #-}
dataLat :: Load r DIM1 Double
        => (Array r DIM1 Double -> Array U DIM1 Double, Array U DIM1 Double)

Array, U and DIM1 come from Repa library. dataLat creates data that is later passed to other functions as a tuple. At one point r type variable gets unified with type D (this is again from Repa), but at later point r should also unify with type L (this is my type). The problem is that it has already been unified with D and cannot be therefore unified with L. I end up with Couldn't match expected type error. I think this should be solved by some form of higher rank types, but I am unable to figure out how this should be written. Can anyone give me a hand?

Solution 
  • Try {-# LANGUAGE NoMonomorphismRestriction #-}

    http://www.haskell.org/ghc/docs/7.6.1/html/users_guide/monomorphism.html