#4921: report ambiguous type variables more consistently -------------------------------------+------------------------------------- Reporter: Saizan | Owner: Type: feature | Status: new request | Milestone: 7.10.1 Priority: low | Version: 7.0.1 Component: Compiler | Keywords: (Type checker) | Architecture: Unknown/Multiple Resolution: | Difficulty: Unknown Operating System: | Blocked By: Unknown/Multiple | Related Tickets: Type of failure: | None/Unknown | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Changes (by thomie): * difficulty: => Unknown Comment: This seems fixed, pending a regression test. The error messages for `x` and `y` are no longer completely different. Ghc shows for `y`, since version 7.6.3: `The type variable ‘b0’ is ambiguous`. And since version 7.8.3 it doesn't show anymore: `Possible fix: add an instance declaration for (C Int b)`. Here is the full error message with HEAD: {{{ $ ghc-7.9.20141125 test.hs [1 of 1] Compiling Amb ( test.hs, test.o ) test.hs:10:9: No instance for (C a0 b1) arising from a use of ‘f’ The type variables ‘a0’, ‘b1’ are ambiguous Relevant bindings include x :: a0 (bound at test.hs:10:1) Note: there is a potential instance available: instance C Int Char -- Defined at test.hs:7:10 In the first argument of ‘fst’, namely ‘f’ In the expression: fst f In an equation for ‘x’: x = fst f test.hs:11:9: No instance for (C Int b0) arising from a use of ‘f’ The type variable ‘b0’ is ambiguous Note: there is a potential instance available: instance C Int Char -- Defined at test.hs:7:10 In the first argument of ‘fst’, namely ‘f’ In the expression: fst f :: Int In an equation for ‘y’: y = fst f :: Int }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/4921#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler