#14824: automatically select instance -------------------------------------+------------------------------------- Reporter: zaoqi | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by AntC): See the Haskell 2010 Report section 4.3.4 **Ambiguous types, ...** The type for your expression `m [X1] :: forall a0. C X1 a0 => a0`. You've put the expression at the GHCi `*Main>` prompt, so there's an implicit `print` around it, which gives type {{{ forall a0. (C X1 a0, Show a0) => String }}} Then type var `a0` is ambiguous in the sense in section 4.3.4. That's what the error message is saying. What type do you expect GHC to infer for `a0`? What are the rules of inference that will give that type? Please reference the Haskell 2010 Report and/or GHC User Guide. Explain how the rules will work for expression `m [X0]` -- given the instances for class `C` as above. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14824#comment:11 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler