[GHC] #10338: GHC Forgets Constraints

21 Apr 2015

      #10338: GHC Forgets Constraints
-------------------------------------+-------------------------------------
              Reporter:  crockeea    |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.8.3
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------
 This is possibly fixed by #10195, but I don't have a convenient means of
 testing it. At any rate, this testcase is considerably simpler than the
 one in #10195.

 {{{#!hs
 {-# LANGUAGE TypeFamilies, FlexibleContexts, GADTs, MultiParamTypeClasses
 #-}

 type family F r

 class (Functor t) => T t r where
   fromScalar :: r -> t r

 data Foo t r where
   Foo :: t (F r) -> Foo t r
   Scalar :: r -> Foo t r

 toF :: r -> F r
 toF = undefined

 convert :: (T t (F r))
   => Foo t r -> Foo t r
 convert (Scalar c) =
   let fromScalar' = fromScalar
   in Foo $ fromScalar' $ toF c
 }}}

 This code compiles with GHC 7.8.4.

 When I add a generic instance for `T` (which requires
 `FlexibleInstances`): `instance (Functor t, Num r) => T t r`

 GHC complains:

 {{{#!hs
 Could not deduce (Num (F r)) arising from a use of ‘fromScalar’
         from the context (T t (F r))
           bound by the type signature for
                      convert :: (T t (F r)) => Foo t r -> Foo t r
           at Main.hs:(17,12)-(18,23)
         In the expression: fromScalar
         In an equation for ‘fromScalar'’: fromScalar' = fromScalar
         In the expression:
           let fromScalar' = fromScalar in Foo $ fromScalar' $ toF c
 }}}

 Of course the problem can be fixed by adding a type signature to
 `fromScalar` and adding `ScopedTypeVariables`:

 {{{#!hs
     convert :: forall t r . (T t (F r))
       => Foo t r -> Foo t r
     convert (Scalar c) =
       let fromScalar' = fromScalar :: F r -> t (F r)
       in Foo $ fromScalar' $ toF c
 }}}

 Like #10195, this is triggered when a generic instance is in scope. The
 (main) problem of course is that GHC tries to match the generic instance
 instead of using the `T (F r)` constraint supplied by `convert`.

 A secondary issue is that I think the example should have the same
 behavior pre- and post-instance, i.e either both should compile or both
 should not compile. I'm not sure if a monomorphism restriction is actually
 being triggered here or if that's just a red herring.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10338
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler