[GHC] #11141: Better error message when instance signature is incorrect

27 Nov 2015

      #11141: Better error message when instance signature is incorrect
-------------------------------------+-------------------------------------
           Reporter:  ezyang         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.2
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Documentation
  Unknown/Multiple                   |  bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 I was recently trying to solve an ambiguity error in a type class instance
 I was writing, and ended up with some code that looked like this (the
 ambiguity error has been eliminated for simplicity):

 {{{
 {-# LANGUAGE FunctionalDependencies #-}
 {-# LANGUAGE InstanceSigs #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 module A where

 data F a = F a
 instance Show a => Show (F a) where
     show :: forall a. Show a => F a -> String
     show (F x) = show x
 }}}

 This instance signature is incorrect: it's not necessary to add a
 universal quantifier to scope over type variables defined in the instance
 head; they are automatically in scope.  But GHC unhelpfully reports:

 {{{
 ezyang@sabre:~$ Dev/ghc-7.10.2/usr/bin/ghci A.hs
 GHCi, version 7.10.2: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling A                ( A.hs, interpreted )

 A.hs:8:13:
     Could not deduce (Show a0)
     from the context (Show a)
       bound by the type signature for show :: Show a => F a -> String
       at A.hs:8:13-45
     The type variable ‘a0’ is ambiguous
     When checking that:
         forall a. Show a => forall a1. Show a1 => F a1 -> String
       is more polymorphic than: forall a. Show a => F a -> String
     When checking that instance signature for ‘show’
       is more general than its signature in the class
       Instance sig: forall a.
                     Show a =>
                     forall a1. Show a1 => F a1 -> String
          Class sig: forall a. Show a => F a -> String
     In the instance declaration for ‘Show (F a)’
 Failed, modules loaded: none.
 }}}

 Why did I get the wrong idea about how instance signatures work? Well, GHC
 doesn't warn if you write this:

 {{{
 instance Show a => Show (F a) where
     show :: Show a => F a -> String
     show (F x) = show x
 }}}

 Because this turns into a full type signature 'Show a, Show a => F a ->
 String' (the first a and the second a end up getting unified.) I'm not
 sure if it should warn; it's an easy mistake to make if you are treating
 instance methods like plain function definitions (of course you have to
 write the full type signature...)

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11141
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler