[GHC] #12220: TypeApplications and DefaultSignatures - problems deducing type variables.

22 Jun 2016

      #12220: TypeApplications and DefaultSignatures - problems deducing type variables.
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           Reporter:  mkloczko       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Linux
  DefaultSignatures,                 |
  TypeApplications                   |
       Architecture:  x86_64         |   Type of failure:  None/Unknown
  (amd64)                            |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The following example code throws an error.

 The example code:
 {{{#!hs
 {-#LANGUAGE TypeApplications#-}
 {-#LANGUAGE MultiParamTypeClasses #-}
 {-#LANGUAGE AllowAmbiguousTypes #-}
 {-#LANGUAGE FlexibleInstances #-}
 {-#LANGUAGE ScopedTypeVariables #-}
 {-#LANGUAGE DefaultSignatures #-}
 module Test1 where

 -- | Type a is only used for
 -- type application.
 class ToUse a where
     toUse :: Int -> Int

 -- | The type used for
 -- type application
 data Default

 -- | The instance using Default as type application.
 -- To call use:
 -- > toUse @Default
 instance ToUse Default where
     toUse a = 3*a

 -- | Typeclass whose methods work
 -- only with type application.
 class (ToUse a) => Uses a b where
     uses :: b -> [b]
     -- | Default Signature, which generates the problem.
     -- It is the same as the normal one
     -- Comment it to 'fix' the bug.
     default uses :: b -> [b]
     uses v = [v]

 -- | Normal instances, nothing special
 instance Uses Default Int where
     uses v = take (toUse @Default 3) $ repeat v
 -- | Another normal one
 instance Uses Default String where
     uses v = take (toUse @Default 2) $ repeat v

 -- | This one works nicely
 instance (ToUse t, Uses t a, Uses t b) => Uses t (a,b) where
     uses (vl,vr) = zip ls rs
         where ls = uses @t vl
               rs = uses @t vr

 -- | But this one doesn't.
 -- Unless you comment the default signature.
 instance (ToUse t, Uses t a, Uses t b, Uses t c) => Uses t (a,b,c)
 }}}

 The error:
 {{{
     • Could not deduce (Uses a0 a)
         arising from a use of ‘Test1.$dmuses’
       from the context: (ToUse t, Uses t a, Uses t b, Uses t c)
         bound by the instance declaration at Test1.hs:47:10-66
       The type variable ‘a0’ is ambiguous
       Relevant bindings include
         uses :: (a, b, c) -> [(a, b, c)] (bound at Test1.hs:47:10)
     • In the expression: Test1.$dmuses
       In an equation for ‘uses’: uses = Test1.$dmuses
       In the instance declaration for ‘Uses t (a, b, c)’
 }}}

 Commenting out the default signature fixes the problem.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12220
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