Revert "Implement goEff directly"

This reverts commit 08112da.

Author: garyb

src/Control/Monad/Free.purs

@@ -81,6 +81,12 @@ resumeGosub (Gosub f) = f (\a g ->
     Gosub h -> Right (h (\b i -> b unit >>= (\x -> i x >>= g)))
   )
 
+unsafeLeft :: forall a b. Either a b -> a
+unsafeLeft (Left x) = x
+
+unsafeRight :: forall a b. Either a b -> b
+unsafeRight (Right x) = x
+
 resume :: forall f a. (Functor f) => Free f a -> Either (f (Free f a)) a
 resume f = case f of
   Pure x -> Right x
@@ -94,10 +100,26 @@ go fn f = case resume f of
   Left l -> go fn (fn l)
   Right r -> r
 
+foreign import goEffImpl
+  "function goEffImpl(resume, isRight, fromLeft, fromRight, fn, value) {\
+  \  return function(){\
+  \    while (true) {\
+  \      var r = resume(value);\
+  \      if (isRight(r)) return fromRight(r);\
+  \      value = fn(fromLeft(r))();\
+  \    }\
+  \  };\
+  \}" :: forall e f a. Fn6
+         (Free f a -> Either (f (Free f a)) a)
+         (Either (f (Free f a)) a -> Boolean)
+         (Either (f (Free f a)) a -> (f (Free f a)))
+         (Either (f (Free f a)) a -> a)
+         (f (Free f a) -> Eff e (Free f a))
+         (Free f a)
+         (Eff e a)
+
 goEff :: forall e f a. (Functor f) => (f (Free f a) -> Eff e (Free f a)) -> Free f a -> Eff e a
-goEff fn f = case resume f of
-  Left l -> fn l >>= goEff fn
-  Right r -> return r
+goEff fn f = runFn6 goEffImpl resume isRight unsafeLeft unsafeRight fn f
 
 -- Note: can blow the stack!
 goMC :: forall f m a. (Monad m) => Natural f m -> FreeC f a -> m a