On 2015-12-20 08:01 PM, Noon Silk wrote:

{-# LANGUAGE NoMonomorphismRestriction #-}

data Status = Foo | Bar

data Rec m a = Rec {
             get    :: m a
           , status :: Status
           }

defRec :: (Monad m) => Rec m a
defRec = undefined

myRec :: (Monad m) => Rec m a
myRec = Rec x y
  where
      Rec x y = defRec

Why this is an ambiguous-type error is a really long story. But a factor is analogous to "show . read". Another factor is that since you turn off the monomorphism restriction, there is a type generalization step, and the generalizing of y's type is separate from the generalization of x's type.

I have found this solution, it works by connecting types to suppress the generalize step:

{-# LANGUAGE NoMonomorphismRestriction, ScopedTypeVariables #-}
...
myRec :: forall m a. (Monad m) => Rec m a
myRec = Rec x y
  where
      Rec x y = defRec :: Rec m a

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