GHC (7.6.3) GADTs bug ? Output is GHC output when GADTs is enabled

output

ghc -Wall -ferror-spans -fforce-recomp -c /home/yorick/Desktop/process.hs

/home/yorick/Desktop/process.hs:34:9-34:
    Couldn't match type `a1' with `a2'
      because type variable `a2' would escape its scope
    This (rigid, skolem) type variable is bound by
      a type expected by the context:
        m a2 -> (a2 -> Process m a) -> Process m a -> r
    The following variables have types that mention a1
      awaiting :: m a1 -> (a1 -> Process m a) -> Process m a -> r
        (bound at /home/yorick/Desktop/process.hs:30:13)
    Expected type: m a2 -> (a2 -> Process m a) -> Process m a -> r
      Actual type: m a1 -> (a1 -> Process m a) -> Process m a -> r
    In the second argument of `k', namely `awaiting'
    In the expression: k yielding awaiting onHalt
    In the expression:
      let
        yielding [] next = let ... in action onYield onAwait onHalt
        yielding (x : xs) next = let ... in action onYield onAwait onHalt
        awaiting req recv fb = let ... in action onYield onAwait onHalt
      in k yielding awaiting onHalt

/home/yorick/Desktop/process.hs:41:5-28:
    Couldn't match type `a0' with `a'
      because type variable `a' would escape its scope
    This (rigid, skolem) type variable is bound by
      a type expected by the context:
        m a -> (a -> Process m o) -> Process m o -> m [o]
    The following variables have types that mention a0
      onAwait :: m a0 -> (a0 -> Process m o) -> Process m o -> m [o]
        (bound at /home/yorick/Desktop/process.hs:39:9)
    Expected type: m a -> (a -> Process m o) -> Process m o -> m [o]
      Actual type: m a0 -> (a0 -> Process m o) -> Process m o -> m [o]
    In the second argument of `k', namely `onAwait'
    In the expression: k onYield onAwait onHalt
    In the expression:
      let
        onYield xs next = liftM (xs ++) (collectProcess next)
        onAwait req recv _ = collectProcess . recv =<< req
        onHalt = return []
      in k onYield onAwait onHalt

process.hs

{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE RankNTypes                #-}
-- {-# LANGUAGE GADTs                     #-} when enabled, the code doesn't compile 
module Data.Process where

import Control.Monad (liftM)

newtype Process m o = Process
    { unProcess ::
           forall r.
           ([o] -> Process m o -> r) ->
           (forall a. m a -> (a -> Process m o) -> Process m o -> r) ->
           r ->
           r }

instance Monad (Process m) where
    return a = Process $ \onYield _ _ -> onYield [a] halt

    Process k >>= f = Process $ \onYield onAwait onHalt ->
        let yielding [] next =
                let (Process action) = next >>= f in
                action onYield onAwait onHalt
            yielding (x:xs) next =
                let (Process action) = append (f x) (rest >>= f)
                    rest =
                        if null xs
                        then next
                        else yieldAllWith xs next in
                action onYield onAwait onHalt
            awaiting req recv fb =
                let (Process action) =
                        awaitWith req ((f =<<) . recv) (fb >>= f) in
                action onYield onAwait onHalt in
        k yielding awaiting onHalt

collectProcess :: Monad m => Process m o -> m [o]
collectProcess (Process k) =
    let onYield xs next     = liftM (xs ++) (collectProcess next)
        onAwait req recv _  = collectProcess . recv =<< req
        onHalt              = return [] in
    k onYield onAwait onHalt

halt :: Process m o
halt = Process $ \_ _ h -> h

awaitWith :: m a -> (a -> Process m o) -> Process m o -> Process m o
awaitWith req k fb = Process $ \_ onAwait _ -> onAwait req k fb

yieldAllWith :: [o] -> Process m o -> Process m o
yieldAllWith xs next = Process $ \onYield _ _ -> onYield xs next

append :: Process m o -> Process m o -> Process m o
append (Process kl) p2@(Process kr) = Process $ \onYield onAwait onHalt ->
    kl (\xs next -> onYield xs (append next p2))
    (\r k fb -> onAwait r (\a -> append (k a) p2) (append fb p2))
    (kr onYield onAwait onHalt)

[14/02/22-20:11:28] if you add a {-# LANGUAGE NoMonoLocalBinds #-} then it works even with GADTs

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