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

15 Jun 2015

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

Old description:
...
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.
New description:

 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 error 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#comment:1
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