[GHC] #12431: Type checker rejects valid program

25 Jul 2016

      #12431: Type checker rejects valid program
-------------------------------------+-------------------------------------
           Reporter:  bgamari        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Since at least 6e280c2c5b2903ae38f4da15a41ea94793907407 GHC fails to
 compile `resourcet` due to a likely erroneous type error.

 Here is a minimal case that reproduces the error,

 {{{#!hs
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE FlexibleContexts #-}

 module Hi where

 import Control.Monad (liftM, ap)
 data Allocated a = Allocated a

 newtype Acquire a = Acquire ((forall b. IO b -> IO b) -> IO (Allocated a))

 instance Functor Acquire where
     fmap = liftM

 instance Applicative Acquire where
     pure a = Acquire (\_ -> return (Allocated a))
     (<*>) = ap

 instance Monad Acquire where
     return = pure
     Acquire f >>= g' = Acquire $ \restore -> do
         Allocated x <- f restore
         let Acquire g = g' x
         Allocated y <- g restore
         return $! Allocated y
 }}}

 This fails with,
 {{{
 $ ghc Hi.hs
 [1 of 1] Compiling Hi               ( Hi.hs, Hi.o )

 Hi.hs:22:21: error:
     • Couldn't match expected type ‘t’
                   with actual type ‘(forall b1. IO b1 -> IO b1) -> IO
 (Allocated b)’
       ‘t’ is a rigid type variable bound by
         the inferred type of g :: t at Hi.hs:22:13-28
     • In the pattern: Acquire g
       In a pattern binding: Acquire g = g' x
       In the expression:
         do { Allocated x <- f restore;
              let Acquire g = g' x;
              Allocated y <- g restore;
              return $! Allocated y }
     • Relevant bindings include
         g' :: a -> Acquire b (bound at Hi.hs:20:19)
         (>>=) :: Acquire a -> (a -> Acquire b) -> Acquire b
           (bound at Hi.hs:20:15)
 }}}

 Despite compiling with 8.0.1 and earlier versions.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12431
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