[GHC] #13482: PartialTypeSignatures, AllowAmbiguousTypes and ScopedTypeVariables don't play nicely together

25 Mar 2017

      #13482: PartialTypeSignatures, AllowAmbiguousTypes and ScopedTypeVariables don't
play nicely together
-------------------------------------+-------------------------------------
           Reporter:  dramforever    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.2
           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:
-------------------------------------+-------------------------------------
 **Motivation**: I was playing with [http://lpaste.net/353915 something
 like this] when I found this great confusion.

 I was able to produce a minimal example. In each declaration/definition,
 the function body is the same, but the signature varies.

 I expect every single one of them to compile, resulting in the same type
 as `minimal4`.

 The actual results are shown in-line.

 I think this sufficiently shows that this behavior is a bug.

 (The line numbers in the following code aren't useful; I pasted them in
 after compiling)

 (The question marks are there because CMD wasn't able to display bullets,
 I think.)

 {{{#!hs
 {-# LANGUAGE PartialTypeSignatures #-}
 {-# LANGUAGE AllowAmbiguousTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE KindSignatures #-}

 minimal1_noksig :: forall m. _ => Bool
 minimal1_noksig = (mempty :: m) == (mempty :: m)
 {-
 ambi.hs:17:30: error:
     ? Expected a type, but ‘m’ has kind ‘k0’
     ? In an expression type signature: m
       In the first argument of ‘(==)’, namely ‘(mempty :: m)’
       In the expression: (mempty :: m) == (mempty :: m)
 -}

 minimal1 :: forall (m :: *). _ => Bool
 minimal1 = (mempty :: m) == (mempty :: m)
 {-
 ambi.hs:11:1: error:
     ? Ambiguous type variable ‘m0’
       prevents the constraint ‘(Monoid m0)’ from being solved.
     ? When checking that the inferred type
         minimal1 :: forall m. (Monoid m, Eq m) => Bool
       is as general as its (partial) signature
         minimal1 :: Bool
 -}

 minimal2 :: forall m. (Eq m, _) => Bool
 minimal2 = (mempty :: m) == (mempty :: m)
 {-
 ambi.hs:14:1: error:
     ? Could not deduce (Monoid m1)
       from the context: (Eq m, Monoid m)
         bound by the inferred type for ‘minimal2’:
                    (Eq m, Monoid m) => Bool
         at ambi.hs:14:1-33
       The type variable ‘m1’ is ambiguous
     ? When checking that the inferred type
         minimal2 :: forall m. (Monoid m, Eq m) => Bool
       is as general as its (partial) signature
         minimal2 :: forall m. (Eq m, Monoid m) => Bool
 -}

 minimal3 :: forall m. (Monoid m, _) => Bool
 minimal3 = (mempty :: m) == (mempty :: m)
 -- Compiles

 minimal4 :: forall m. (Monoid m, Eq m) => Bool
 minimal4 = (mempty :: m) == (mempty :: m)
 -- Compiles
 }}}

 The code was run in GHCi. GHC version is 8.0.2.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13482
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