untouchable type inside the constraints

The following code is taken and simplified from the test-suite of accelerate-fourier:

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE Rank2Types #-}
module UntouchableType (tests) where

import Test.QuickCheck (Testable, Arbitrary, arbitrary, quickCheck, )


newtype Sign a = Sign a
   deriving (Show)

instance Arbitrary (Sign a) where
   arbitrary = undefined

quickCheckWithSign :: (Testable prop) => (Sign Double -> prop) -> IO ()
quickCheckWithSign = quickCheck


data Array sh a = Array

type family FullShape sh :: *

data SubTransform a =
        SubTransform (forall sh. (FullShape sh ~ sh) => Array sh a)

transform2d :: SubTransform Double -> Bool
transform2d = undefined

transformChirp2 :: Sign a -> Array sh a
transformChirp2 = undefined


tests :: IO ()
tests =
   quickCheck $ \sign ->
      transform2d (SubTransform (transformChirp2 (sign::Sign Double)))

GHC-8.2.2 says about it:

[1 of 1] Compiling UntouchableType  ( UntouchableType.hs, interpreted )

UntouchableType.hs:34:4: error:
    • Ambiguous type variable ‘p0’ arising from a use of ‘quickCheck’
      prevents the constraint ‘(Arbitrary p0)’ from being solved.
      Probable fix: use a type annotation to specify what ‘p0’ should be.
      These potential instances exist:
        instance (Arbitrary a, Arbitrary b) => Arbitrary (Either a b)
          -- Defined in ‘Test.QuickCheck.Arbitrary’
        instance Arbitrary Ordering
          -- Defined in ‘Test.QuickCheck.Arbitrary’
        instance Arbitrary Integer
          -- Defined in ‘Test.QuickCheck.Arbitrary’
        ...plus 20 others
        ...plus 62 instances involving out-of-scope types
        (use -fprint-potential-instances to see them all)
    • In the expression:
        quickCheck
          $ \ sign
              -> transform2d
                   (SubTransform (transformChirp2 (sign :: Sign Double)))
      In an equation for ‘tests’:
          tests
            = quickCheck
                $ \ sign
                    -> transform2d
                         (SubTransform (transformChirp2 (sign :: Sign Double)))
   |
34 |    quickCheck $ \sign ->
   |    ^^^^^^^^^^^^^^^^^^^^^...

UntouchableType.hs:35:51: error:
    • Couldn't match expected type ‘Sign Double’ with actual type ‘p0’
        ‘p0’ is untouchable
          inside the constraints: FullShape sh ~ sh
          bound by a type expected by the context:
                     forall sh. FullShape sh ~ sh => Array sh Double
          at UntouchableType.hs:35:20-69
    • In the first argument of ‘transformChirp2’, namely
        ‘(sign :: Sign Double)’
      In the first argument of ‘SubTransform’, namely
        ‘(transformChirp2 (sign :: Sign Double))’
      In the first argument of ‘transform2d’, namely
        ‘(SubTransform (transformChirp2 (sign :: Sign Double)))’
    • Relevant bindings include
        sign :: p0 (bound at UntouchableType.hs:34:18)
   |
35 |       transform2d (SubTransform (transformChirp2 (sign::Sign Double)))
   |                                                   ^^^^
Failed, no modules loaded.

I have tested GHC versions back to GHC-7.4.2, all of them report essentially the same type error.

I do not really understand the type error message, but here are my observations that I find strange: The type annotation sign :: Sign Double does not prevent the type error, but replacing quickCheck by quickCheckWithSign does. Replacing the constraint FullShape sh ~ sh by, e.g. Show sh, let the error disappear. Generalizing transform2d to SubTransform a causes another type error although I expected that SubTransform Double can be infered from Sign Double.

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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