[GHC] #10532: Unnecessary constraints when using overlapping instances

15 Jun 2015

      #10532: Unnecessary constraints when using overlapping instances
-------------------------------------+-------------------------------------
              Reporter:              |             Owner:
  MikeIzbicki                        |            Status:  new
                  Type:  bug         |         Milestone:
              Priority:  normal      |           Version:  7.10.1
             Component:  Compiler    |  Operating System:  Unknown/Multiple
  (Type checker)                     |   Type of failure:  None/Unknown
              Keywords:              |        Blocked By:
          Architecture:              |   Related Tickets:
  Unknown/Multiple                   |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------
 The following code compiles just fine:

 {{{
 class Foo a where foo :: a -> Int
 instance Foo a where foo _ = 5

 f :: a -> Int
 f = foo
 }}}

 But if we modify the code to use overlapping instances like so:

 {{{
 class Foo a where foo :: a -> Int
 instance {-# OVERLAPS #-} Foo Int where foo = id
 instance {-# OVERLAPPABLE #-} Foo a where foo _ = 5

 f:: a -> Int
 f = foo
 }}}

 We get the following compiler message:
 {{{
     Overlapping instances for Foo a arising from a use of ‘foo’
     Matching instances:
       instance [overlappable] Foo a
         -- Defined at ...
       instance [overlap ok] Foo Int
         -- Defined at ...
     (The choice depends on the instantiation of ‘a’
      To pick the first instance above, use IncoherentInstances
      when compiling the other instance declarations)
 }}}

 Including the constraint in the type signature makes the problem go away,
 but the constraint shouldn't be needed.  In most use cases this wouldn't
 be a big deal, but it's making my type signatures much messier looking
 than they actually are.

 Adding IncoherentInstances as the error message suggests gives the
 instance I don't want.  Weirdly, I get different instances depending on
 whether the constraint is in the type signature or not.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10532
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler