[GHC] #13061: Incorrect constraints given single flexible undecidable instance

3 Jan 2017


      #13061: Incorrect constraints given single flexible undecidable instance
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           Reporter:  JCarr          |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           Keywords:  Constraint,    |  Operating System:  Unknown/Multiple
  UndecidableInstances,              |
  FlexibleInstances                  |
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 If a class has an instance in the form F a => G a in its file, and no
 other instances, then the functions in G a will have the constraint F a,
 rather than G a.

 Example file
 {{{#!hs
 {-# LANGUAGE FlexibleInstances, UndecidableInstances #-}
 class A a where
     f :: a -> a
 instance {-# OVERLAPPABLE #-} Eq a => A a where
     f = id
 }}}

 f will have type Eq a => a -> a -> Bool

 If we add: {{{#!hs
 instance A Int where
     f = id
 }}}
 Then f will have the type A a => a -> a -> Bool.
 A function of type A a => ... will always be rejected unless Eq is also
 included

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13061
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