#10054: Misleading error message from unsaturated type family application -------------------------------------+------------------------------------- Reporter: sherman | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 (Type checker) | Operating System: MacOS X Keywords: type | Type of failure: Incorrect families, constraint kinds | warning at compile-time Architecture: x86_64 | Blocked By: (amd64) | Related Tickets: Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- This program {{{#!hs {-# LANGUAGE GADTs, ConstraintKinds, KindSignatures #-} {-# LANGUAGE TypeFamilies #-} import GHC.Exts (Constraint) data Foo :: (* -> Constraint) -> * -> * where MkFoo :: c a => Foo c a type family Ap (f :: * -> Constraint) (x :: *) :: Constraint where Ap f x = f x test :: Ap Show a => Foo (Ap Show) a test = MkFoo }}} yields the following error message: {{{ [1 of 1] Compiling Main ( Test.hs, interpreted ) Test.hs:13:8: Could not deduce (Ap Show a) arising from a use of ‘MkFoo’ from the context (Ap Show a) bound by the type signature for test :: (Ap Show a) => Foo (Ap Show) a at Test.hs:12:9-36 Relevant bindings include test :: Foo (Ap Show) a (bound at Test.hs:13:1) In the expression: MkFoo In an equation for ‘test’: test = MkFoo Failed, modules loaded: none. }}} I have a hypothesis as to why this error occurs. Because the application of {{{Ap}}} in {{{Ap Show a}}} is fully applied in the context, it reduces to the constraint {{{Show a}}}. However, when error messages describe the typeclass context, they don't reduce type synonyms or type families. However, the {{{Ap Show a}}} that cannot be deduced, despite appearing like a saturated application of {{{Ap}}}, must not be due to the fact that the {{{Ap Show}}} in the argument to the {{{Foo}}} type constructor is partially applied. If the definition of {{{Ap}}} is eta-reduced, the program in fact successfully typechecks. Or, if the result type of {{{test}}} is instead specified as {{{Foo Show a}}}, the program successfully typechecks. If the type variable {{{a}}} is specialized to a concrete type with a {{{Show}}} instance such as {{{Int}}}, the misleading error message still occurs, with {{{Int}}} substituted for {{{a}}}. I am wondering if there might be some way to report the error in a way that brings attention to the fact that the compiler cannot figure out what {{{Ap Show a}}} ''is'' as the constraint that cannot be deduced. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10054 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler