Arrow.hs: fix loopl/loopr transposition

[ghc-base.git] / GHC / HetMet / Arrow.hs

diff --git a/GHC/HetMet/Arrow.hs b/GHC/HetMet/Arrow.hs
index 5bead2c..f40ed5b 100644 (file)
--- a/GHC/HetMet/Arrow.hs
+++ b/GHC/HetMet/Arrow.hs
@@ -1,4 +1,4 @@
-{-# OPTIONS -XRankNTypes -XMultiParamTypeClasses -XNoMonomorphismRestriction -XTypeOperators -XFlexibleInstances -XFunctionalDependencies #-}
+{-# OPTIONS -XRankNTypes -XMultiParamTypeClasses -XNoMonomorphismRestriction -XTypeOperators -XFlexibleInstances -XFunctionalDependencies -XEmptyDataDecls #-}
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  GHC.HetMet.Arrow
@@ -48,15 +48,52 @@ instance Arrow a => GArrowReify a (,) () x y x y where
   ga_reify     =  arr
 
 instance ArrowLoop a => GArrowLoop a (,) () where
-  ga_loopl     =  loop
-  ga_loopr  f  =  loop (ga_swap >>> f >>> ga_swap)
+  ga_loopr     =  loop
+  ga_loopl  f  =  loop (ga_swap >>> f >>> ga_swap)
 
 instance ArrowApply a => GArrowApply a (,) () a where
   ga_applyl    = ga_swap >>> app
   ga_applyr    = app
 
-
-
+instance Arrow a => GArrowProd a (,) () where
+
+-- The uninhabited type
+data Void
+
+-- In Coq we could simply prove that these cases are impossible; in Haskell we need to have some faith.
+voidImpossible :: Void -> a
+voidImpossible = error "this is impossible; you have a bug in your compiler"
+
+instance ArrowChoice a => GArrow a Either Void where
+  ga_first     =  left
+  ga_second    =  right
+  ga_uncancell =  arr Right
+  ga_uncancelr =  arr Left
+  ga_cancell   =  arr unVoidLeft
+                   where
+                    unVoidLeft  (Left  v) = voidImpossible v
+                    unVoidRight (Right x) = x
+  ga_cancelr   =  arr unVoidRight
+                   where
+                    unVoidRight (Left  x) = x
+                    unVoidRight (Right v) = voidImpossible v
+  ga_assoc     =  arr eitherAssoc
+                   where
+                    eitherAssoc (Left (Left  x)) = Left         x
+                    eitherAssoc (Left (Right y)) = Right (Left  y)
+                    eitherAssoc (Right       z ) = Right (Right z)
+  ga_unassoc   =  arr eitherUnAssoc
+                   where
+                    eitherUnAssoc (Left         x ) = Left  (Left  x)
+                    eitherUnAssoc (Right (Left  y)) = Left  (Right y)
+                    eitherUnAssoc (Right (Right z)) = Right        z
+
+instance ArrowChoice a => GArrowSum a Either Void where
+  ga_never = arr voidImpossible
+  ga_merge = arr merge
+              where
+               merge (Left  x) = x
+               merge (Right x) = x