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I recently sketched out a solution to #13327. Here is the type signature that I wanted to write:

dataCast1 :: forall (c :: Type -> Type) (t :: forall (k :: Type). k -> Type).
             Typeable t
          => (forall d. Data d => c (t d))
          -> Maybe (c a)

But this doesn't typecheck:

    • Could not deduce (Typeable (t k0))
      from the context: (Data a, Typeable (t k))
        bound by the type signature for:
                   dataCast1 :: forall a.
                                Data a =>
                                forall k (c :: * -> *) (t :: forall k1. k1 -> *).
                                Typeable (t k) =>
                                (forall d. Data d => c (t * d)) -> Maybe (c a)
        at NewData.hs:(170,3)-(173,26)
      The type variable ‘k0’ is ambiguous
    • In the ambiguity check for ‘dataCast1’
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      When checking the class method:
        dataCast1 :: forall a.
                     Data a =>
                     forall k (c :: * -> *) (t :: forall k1. k1 -> *).
                     Typeable (t k) =>
                     (forall d. Data d => c (t * d)) -> Maybe (c a)
      In the class declaration for ‘Data’
    |
170 |   dataCast1 :: forall (c :: Type -> Type) (t :: forall (k :: Type). k -> Type).
    |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...

This makes sense, since GHC has no way to conclude that the k in t's kind is also Typeable. I tried to convince GHC of that fact:

dataCast1 :: forall (c :: Type -> Type) (t :: forall (k :: Type). Typeable k => k -> Type).
             Typeable t
          => (forall d. Data d => c (t d))
          -> Maybe (c a)

But this also doesn't work:

NewData.hs:171:25: error:
    • Illegal constraint in a type: Typeable k0
    • In the first argument of ‘Typeable’, namely ‘t’
      In the type signature:
        dataCast1 :: forall (c :: Type -> Type)
                            (t :: forall (k :: Type). Typeable k => k -> Type).
                     Typeable t => (forall d. Data d => c (t d)) -> Maybe (c a)
      In the class declaration for ‘Data’
    |
171 |                Typeable t
    |                         ^

NewData.hs:172:40: error:
    • Illegal constraint in a type: Typeable k0
    • In the first argument of ‘c’, namely ‘(t d)’
      In the type signature:
        dataCast1 :: forall (c :: Type -> Type)
                            (t :: forall (k :: Type). Typeable k => k -> Type).
                     Typeable t => (forall d. Data d => c (t d)) -> Maybe (c a)
      In the class declaration for ‘Data’
    |
172 |             => (forall d. Data d => c (t d))
    |                                        ^^^

At this point, I'm stuck, since I have no idea how to work around this Illegal constraint in a type error. This error message appears to have originated as a part of the TypeInType patch, since there's even a test case checking for this behavior.

But is this a fundamental limitation of kind equalities? Or would it be possible to lift this restriction?

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
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Operating system
Architecture