[GHC] #14231: Core lint error "in result of Static argument"

14 Sep 2017


      #14231: Core lint error "in result of Static argument"
-------------------------------------+-------------------------------------
           Reporter:  mpickering     |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           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:
-------------------------------------+-------------------------------------
 Whilst investigating #14211 I encountered a core lint error.

 {{{
 {-# LANGUAGE CPP                       #-}
 {-# LANGUAGE RankNTypes                #-}

 module Async where

 data AsyncT m a =
     AsyncT {
         runAsyncT :: forall r.
                Maybe Int                          -- state
             -> m r                                          -- stop
             -> (a -> Maybe Int -> Maybe (AsyncT m a) -> m r)  -- yield
             -> m r
     }

 ------------------------------------------------------------------------------
 -- Monad
 ------------------------------------------------------------------------------

 {-# INLINE bindWith #-}
 bindWith
     :: (forall c. AsyncT m c -> AsyncT m c -> AsyncT m c)
     -> AsyncT m a
     -> (a -> AsyncT m b)
     -> AsyncT m b
 bindWith k (AsyncT m) f = AsyncT $ \_ stp yld ->
 m Nothing stp (\a _ m -> (\x -> (runAsyncT x) Nothing stp yld) $ maybe (f
 a) (\r -> f a `k` (bindWith k r f)) m )
 }}}

 Compile with `ghc -O2 -fno-worker-wrapper -fstatic-argument-transformation
 -dcore-lint`.

 Error:
 {{{
 *** Core Lint errors : in result of Static argument ***
 <no location info>: warning:
 In the expression: bindWith @ m_aV5 @ a_aV6 @ b_aV7 k_aSU x_aX3 f_aSW
     Mismatch in type between binder and occurrence
     Var: bindWith_rpi
     Binder type: forall (m1 :: * -> *) a1 b1 .

     (forall c .   AsyncT m_aV5 c -> AsyncT m_aV5 c -> AsyncT m_aV5 c)
     -> AsyncT m_aV5 a_aV6 -> (a_aV6 -> AsyncT m_aV5 b_aV7) -> AsyncT m_aV5
 b_aV7
     Occurrence type: forall (m :: * -> *) a b .

     (forall c .   AsyncT m c -> AsyncT m c -> AsyncT m c)
     -> AsyncT m a -> (a -> AsyncT m b) -> AsyncT m b
       Before subst: forall (m :: * -> *) a b .

     (forall c .   AsyncT m c -> AsyncT m c -> AsyncT m c)
     -> AsyncT m a -> (a -> AsyncT m b) -> AsyncT m b
 *** Offending Program ***
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14231
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