#7437: peculiar behaviour with default instances and type variables -------------------------------------+------------------------------------- Reporter: bos | Owner: simonpj Type: bug | Status: infoneeded Priority: normal | Milestone: 7.10.1 Component: Compiler | Version: 7.6.1 (Type checker) | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: GHC | Related Tickets: accepts invalid program | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Changes (by thomie): * status: new => infoneeded * component: Compiler => Compiler (Type checker) Comment: Uncommenting the `FlexibleInstances` pragma does not have an effect on the error message with ghc HEAD (or ghc-7.8.3), and the message also seems much improved: {{{ $ cat test.hs {-# LANGUAGE DefaultSignatures, FlexibleContexts, DeriveGeneric #-} {-# LANGUAGE FlexibleInstances #-} module Whee where ... $ ghc-7.9.20141121 test.hs [1 of 1] Compiling Whee ( test.hs, test.o ) test.hs:12:1: Could not deduce (Put a0) from the context (Put a, Generic t, GPut (Rep t)) bound by the type signature for put :: (Put a, Generic t, GPut (Rep t)) => t -> [()] at test.hs:(12,1)-(17,21) The type variable ‘a0’ is ambiguous In the ambiguity check for the type signature for ‘put’: put :: forall a. Put a => forall t. (Generic t, GPut (Rep t)) => t -> [()] To defer the ambiguity check to use sites, enable AllowAmbiguousTypes When checking the class method: put :: a -> [()] In the class declaration for ‘Put’ }}} Does that means this issue is resolved? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7437#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler