Re: [GHC] #8359: ConstraintKinds require UndecidableInstances when it doesn't need it

26 Sep 2013

      #8359: ConstraintKinds require UndecidableInstances when it doesn't need it
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        Reporter:  thomaseding                |            Owner:
            Type:  bug                        |           Status:  new
        Priority:  normal                     |        Milestone:
       Component:  Compiler                   |          Version:  7.4.2
      Resolution:                             |         Keywords:
Operating System:  Unknown/Multiple           |     Architecture:
 Type of failure:  GHC rejects valid program  |  Unknown/Multiple
       Test Case:                             |       Difficulty:  Unknown
        Blocking:                             |       Blocked By:
                                              |  Related Tickets:
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Description changed by simonpj:

Old description:
...
Using ConstraintKinds to alias a bunch of class constraints fails to
compile without UndecidableInstances. The same code that manually spells
out class constraints without ConstraintKinds compiles just fine though.
Test case below
-------------------------------------
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE MultiParamTypeClasses #-}
class DifferentTypes a b
type DifferentTypes3 a b c = (DifferentTypes a b, DifferentTypes b c,
DifferentTypes a c)
class Foo a
class Bar a
-- Buggy instance requires UndecidableInstances to compile
instance (DifferentTypes3 a b c, Bar a, Bar b, Bar c) => Foo (a, b, c)
-- Equivalent instance compiles when manually expanding constraint type
instance (DifferentTypes a b, DifferentTypes b c, DifferentTypes a c, Bar
a, Bar b, Bar c) => Foo (a, b, c)
New description:

 Using `ConstraintKinds` to alias a bunch of class constraints fails to
 compile without `UndecidableInstances`. The same code that manually spells
 out class constraints without `ConstraintKinds` compiles just fine though.

 Test case below
 {{{
 -------------------------------------

 {-# LANGUAGE ConstraintKinds #-}

 {-# LANGUAGE MultiParamTypeClasses #-}

 class DifferentTypes a b

 type DifferentTypes3 a b c = (DifferentTypes a b, DifferentTypes b c,
 DifferentTypes a c)

 class Foo a

 class Bar a

 -- Buggy instance requires UndecidableInstances to compile

 instance (DifferentTypes3 a b c, Bar a, Bar b, Bar c) => Foo (a, b, c)

 -- Equivalent instance compiles when manually expanding constraint type

 instance (DifferentTypes a b, DifferentTypes b c, DifferentTypes a c, Bar
 a, Bar b, Bar c) => Foo (a, b, c)
 }}}

--

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8359#comment:1
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