I have the following data type:
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE ExtendedDefaultRules #-}
class ToString a where
data M = forall a. (ToString a) => B a
In GHCI I can do the following with no problems:
let bs = [B, B]
But if I try to do this in a compiled file, I get the following error:
No instance for (ToString a0) arising from a use of ‘B’ The type variable ‘a0’ is ambiguous Relevant bindings include bs :: [a0 -> M] (bound at src/My/Module:7:1)
Which extension(s) am I missing that would allow me to create the list of B as shown? Or what am I missing that GHCI is adding?
This is because GHCi doesn't turn on the monomorphism restriction, and GHC (by default) does. To witness, the following file typechecks:
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE NoMonomorphismRestriction #-}
class ToString a where
data M = forall a. (ToString a) => B a
bs = [B, B]
The inferred type of bs is fairly useless, of course:
*Main> :t bs
bs :: ToString a => [a -> M]
If you don't want to turn off the monomorphism restriction, you can just add a type signature to the definition of bs:
{-# LANGUAGE ExistentialQuantification #-}
class ToString a where
data M = forall a. (ToString a) => B a
bs :: (ToString a) => [a -> M]
bs = [B, B]