#10054: Misleading error message from unsaturated type family application -------------------------------------+------------------------------------- Reporter: sherman | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler (Type | Version: 7.8.3 checker) | Keywords: type Resolution: fixed | families, constraint kinds Operating System: MacOS X | Architecture: x86_64 Type of failure: Incorrect | (amd64) warning at compile-time | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Changes (by rwbarton): * status: new => closed * resolution: => fixed Comment: Thanks for the report. As you correctly diagnosed, the program is invalid due to the undersaturated type family application in `Foo (Ap Show) a`. The fact that GHC does not report this error (and subsequently gets confused) was a regression introduced somewhere in the 7.8 line, but was fixed in 7.8.4. {{{ rwbarton@morphism:/tmp$ ghc-7.8.1 Test [1 of 1] Compiling Main ( Test.hs, Test.o ) Test.hs:12:9: Type synonym ‘Ap’ should have 2 arguments, but has been given 1 In the type signature for ‘test’: test :: Ap Show a => Foo (Ap Show) a rwbarton@morphism:/tmp$ ghc-7.8.3 Test [1 of 1] Compiling Main ( Test.hs, Test.o ) 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 rwbarton@morphism:/tmp$ ghc-7.8.4 Test [1 of 1] Compiling Main ( Test.hs, Test.o ) Test.hs:12:9: Type synonym ‘Ap’ should have 2 arguments, but has been given 1 In the type signature for ‘test’: test :: Ap Show a => Foo (Ap Show) a }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10054#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler