[GHC] #13662: Type inference fails in connection with TypeFamilies, 'let', type equalities ...

8 May 2017


      #13662: Type inference fails in connection with TypeFamilies, 'let', type
equalities ...
-------------------------------------+-------------------------------------
           Reporter:  Lemming        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1-rc1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 The following example is a pretty involved use of type equality.
 It was accepted by GHC-8.0 and before, but GHC-8.2.0.20170505 cannot infer
 a type.
 {{{
 $ cat MonomorphismEquality.hs
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 module MonomorphismEquality (run) where

 newtype Value a = Value a

 type family Repr (f :: * -> *) a :: *
 type instance Repr f Int = f Int

 class (Repr Value i ~ Value ir) => Native i ir where

 instance Native Int Int where


 fromInt :: (Native i ir) => i -> a
 fromInt = undefined

 apply :: (Int -> a -> a) -> a -> a
 apply weight = id

 run :: Float -> Float
 run =
    let weight = \clip v -> fromInt clip * v
    in  apply weight

 $ ghc-8.0.2 -Wall MonomorphismEquality.hs
 [1 of 1] Compiling MonomorphismEquality ( MonomorphismEquality.hs,
 MonomorphismEquality.o )

 MonomorphismEquality.hs:6:19: warning: [-Wunused-top-binds]
     Defined but not used: data constructor ‘Value’

 MonomorphismEquality.hs:20:7: warning: [-Wunused-matches]
     Defined but not used: ‘weight’

 $ ghc-8.2.0.20170505 -Wall MonomorphismEquality.hs
 [1 of 1] Compiling MonomorphismEquality ( MonomorphismEquality.hs,
 MonomorphismEquality.o )

 MonomorphismEquality.hs:24:28: error:
     • Ambiguous type variable ‘ir0’ arising from a use of ‘fromInt’
       prevents the constraint ‘(Native Int ir0)’ from being solved.
       Probable fix: use a type annotation to specify what ‘ir0’ should be.
       These potential instance exist:
         instance Native Int Int -- Defined at
 MonomorphismEquality.hs:13:10
     • In the first argument of ‘(*)’, namely ‘fromInt clip’
       In the expression: fromInt clip * v
       In the expression: \ clip v -> fromInt clip * v
    |
 24 |    let weight = \clip v -> fromInt clip * v
    |                            ^^^^^^^^^^^^
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13662
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