Compilation error when trying to use existential types in Haskell

Question

I am getting an error message like so:

Linear.hs:215:27:
    Couldn't match expected type `forall v1.
                                  Ident v1 =>
                                  SubstT v1 (GenericLL (a v1 c)) n x'
                with actual type `forall v1. Ident v1 => SubstT v1 a0 m0 b0'
    Expected type: (forall v1.
                    Ident v1 =>
                    SubstT v1 (GenericLL (a v1 c)) n x)
                   -> n x
      Actual type: (forall v1. Ident v1 => SubstT v1 a0 m0 b0) -> m0 b0
    In the first argument of `flattenImpl', namely `runSubstT'
    In the expression: flattenImpl runSubstT

The actual type seems more general than the expected type. What are then possible causes for this kind of error message? Is the message misleading or am I reading it incorrectly?

What I am trying to do is to pass an existentially quantified function runSubstT, whose type is:

runSubstT :: (Monad m) => (forall v. (Ident v) => SubstT v a m b) -> m b

I will also settle with some good description of how the GHC compiler performs type matching on existentially quantified types.

Answer 1

I have a guess, but I cannot be sure until you post enough code to reproduce the error. My guess, based on looking at this:

runSubstT :: (Monad m) => (forall v. (Ident v) => SubstT v a m b) -> m b

is that making the second type 'a' dependent on the first existential type 'v' may be a problem. The expected type:

forall v1. Ident v1 => SubstT v1 (GenericLL (a v1 c)) n x'

replaces the second type with (GenericLL (a v1 c)) which depends on the first existential type 'v1'. This could be the root of the conflict, but without the code I cannot trace the login of the type checker.