#14539: untouchable type inside the constraints -------------------------------------+------------------------------------- Reporter: Lemming | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- 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`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14539 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler