[Haskell-cafe] NoMonomorphismRestriction forces odd situation when using contexts

[Albert Y. C. Lai]

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