Straightened out implicit coercions for indexed types

[ghc-hetmet.git] / compiler / basicTypes / MkId.lhs

diff --git a/compiler/basicTypes/MkId.lhs b/compiler/basicTypes/MkId.lhs
index 0ad0bc6..54bbae9 100644 (file)
--- a/compiler/basicTypes/MkId.lhs
+++ b/compiler/basicTypes/MkId.lhs
@@ -64,7 +64,7 @@ import TyCon          ( TyCon, isNewTyCon, tyConDataCons, FieldLabel,
                           tyConStupidTheta, isProductTyCon, isDataTyCon,
                           isRecursiveTyCon, isFamInstTyCon,
                           tyConFamInst_maybe, tyConFamilyCoercion_maybe,
-                          newTyConCo )
+                          newTyConCo_maybe )
 import Class           ( Class, classTyCon, classSelIds )
 import Var             ( Id, TyVar, Var, setIdType )
 import VarSet          ( isEmptyVarSet, subVarSet, varSetElems )
@@ -220,14 +220,14 @@ This coercion is conditionally applied by wrapFamInstBody.
 mkDataConIds :: Name -> Name -> DataCon -> DataConIds
 mkDataConIds wrap_name wkr_name data_con
   | isNewTyCon tycon
-  = DCIds Nothing nt_work_id                    -- Newtype, only has a worker
+  = DCIds Nothing nt_work_id                 -- Newtype, only has a worker
 
-  | any isMarkedStrict all_strict_marks                -- Algebraic, needs wrapper
-    || not (null eq_spec)
-    || isFamInstTyCon tycon
+  | any isMarkedStrict all_strict_marks             -- Algebraic, needs wrapper
+    || not (null eq_spec)                   -- NB: LoadIface.ifaceDeclSubBndrs
+    || isFamInstTyCon tycon                 --     depends on this test
   = DCIds (Just alg_wrap_id) wrk_id
 
-  | otherwise                                  -- Algebraic, no wrapper
+  | otherwise                               -- Algebraic, no wrapper
   = DCIds Nothing wrk_id
   where
     (univ_tvs, ex_tvs, eq_spec, 
@@ -394,6 +394,15 @@ wrapFamInstBody tycon args result_expr
   | otherwise
   = result_expr
 
+-- Apply the coercion in the opposite direction.
+--
+unwrapFamInstBody :: TyCon -> [Type] -> CoreExpr -> CoreExpr
+unwrapFamInstBody tycon args result_expr
+  | Just co_con <- tyConFamilyCoercion_maybe tycon
+  = mkCoerce (mkTyConApp co_con args) result_expr
+  | otherwise
+  = result_expr
+
 \end{code}
 
 
@@ -842,15 +851,28 @@ wrapNewTypeBody :: TyCon -> [Type] -> CoreExpr -> CoreExpr
 -- If a coercion constructor is prodivided in the newtype, then we use
 -- it, otherwise the wrap/unwrap are both no-ops 
 --
+-- If the we are dealing with a newtype instance, we have a second coercion
+-- identifying the family instance with the constructor of the newtype
+-- instance.  This coercion is applied in any case (ie, composed with the
+-- coercion constructor of the newtype or applied by itself).
+--
 wrapNewTypeBody tycon args result_expr
-  | Just co_con <- newTyConCo tycon
-  = mkCoerce (mkSymCoercion (mkTyConApp co_con args)) result_expr
-  | otherwise
-  = result_expr
+  = wrapFamInstBody tycon args inner
+  where
+    inner
+      | Just co_con <- newTyConCo_maybe tycon
+      = mkCoerce (mkSymCoercion (mkTyConApp co_con args)) result_expr
+      | otherwise
+      = result_expr
 
+-- When unwrapping, we do *not* apply any family coercion, because this will
+-- be done via a CoPat by the type checker.  We have to do it this way as
+-- computing the right type arguments for the coercion requires more than just
+-- a spliting operation (cf, TcPat.tcConPat).
+--
 unwrapNewTypeBody :: TyCon -> [Type] -> CoreExpr -> CoreExpr
 unwrapNewTypeBody tycon args result_expr
-  | Just co_con <- newTyConCo tycon
+  | Just co_con <- newTyConCo_maybe tycon
   = mkCoerce (mkTyConApp co_con args) result_expr
   | otherwise
   = result_expr