Re: [GHC] #14004: G

22 Jul 2017

      #14004: G
-------------------------------------+-------------------------------------
        Reporter:  zaoqi             |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by zaoqi):

 * status:  infoneeded => new

Old description:
...
{{{#!hs
{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-}
class GEq a b where
    geq :: a -> b -> Bool
instance {-# OVERLAPPING #-} Eq a => GEq a a where
    geq = (==)
instance GEq a b where
    geq _ _ = False
}}}

...
{{{
GHCi, version 7.10.3: http://www.haskell.org/ghc/  :? for help
Prelude> :load GEq.hs
[1 of 1] Compiling Main             ( GEq.hs, interpreted )
Ok, modules loaded: Main.
*Main> geq::a->b->Bool
<interactive>:3:1:
    Overlapping instances for GEq a1 b1 arising from a use of ‘geq’
    Matching instances:
      instance GEq a b -- Defined at GEq.hs:9:10
      instance [overlapping] Eq a => GEq a a -- Defined at GEq.hs:6:30
    (The choice depends on the instantiation of ‘a1, b1’
     To pick the first instance above, use IncoherentInstances
     when compiling the other instance declarations)
    In the expression: geq :: a -> b -> Bool
    In an equation for ‘it’: it = geq :: a -> b -> Bool
*Main>
}}}
In order to
{{{#!hs
{-# LANGUAGE KindSignatures, GADTs #-}
data Dyn :: * where
    Dyn :: a -> Dyn
instance Eq Dyn where
...
}}}
New description:

 {{{#!hs
 {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-}

 class GEq a b where
     geq :: a -> b -> Bool

 instance {-# OVERLAPPING #-} Eq a => GEq a a where
     geq = (==)

 instance GEq a b where
     geq _ _ = False
 }}}

 {{{
 GHCi, version 7.10.3: http://www.haskell.org/ghc/  :? for help
 Prelude> :load GEq.hs
 [1 of 1] Compiling Main             ( GEq.hs, interpreted )
 Ok, modules loaded: Main.
 *Main> geq::a->b->Bool

 <interactive>:3:1:
     Overlapping instances for GEq a1 b1 arising from a use of ‘geq’
     Matching instances:
       instance GEq a b -- Defined at GEq.hs:9:10
       instance [overlapping] Eq a => GEq a a -- Defined at GEq.hs:6:30
     (The choice depends on the instantiation of ‘a1, b1’
      To pick the first instance above, use IncoherentInstances
      when compiling the other instance declarations)
     In the expression: geq :: a -> b -> Bool
     In an equation for ‘it’: it = geq :: a -> b -> Bool
 *Main>
 }}}

 In order to
 {{{#!hs
 {-# LANGUAGE KindSignatures, GADTs #-}
 data Dyn :: * where
     Dyn :: a -> Dyn

 instance Eq Dyn where
     (==) = geq
 }}}

--

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14004#comment:5
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