Refactor, improve, and document the deriving mechanism

[ghc-hetmet.git] / compiler / types / Type.lhs

diff --git a/compiler/types/Type.lhs b/compiler/types/Type.lhs
index 4146fa7..aa3cd07 100644 (file)
--- a/compiler/types/Type.lhs
+++ b/compiler/types/Type.lhs
@@ -10,7 +10,7 @@ Type - public interface
 -- The above warning supression flag is a temporary kludge.
 -- While working on this module you are encouraged to remove it and fix
 -- any warnings in the module. See
---     http://hackage.haskell.org/trac/ghc/wiki/CodingStyle#Warnings
+--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
 -- for details
 
 module Type (
@@ -415,8 +415,14 @@ splitNewTyConApp_maybe (FunTy arg res)   = Just (funTyCon, [arg,res])
 splitNewTyConApp_maybe other         = Nothing
 
 newTyConInstRhs :: TyCon -> [Type] -> Type
-newTyConInstRhs tycon tys =
-    let (tvs, ty) = newTyConRhs tycon in substTyWith tvs tys ty
+-- Unwrap one 'layer' of newtype
+-- Use the eta'd version if possible
+newTyConInstRhs tycon tys 
+    = ASSERT2( equalLength tvs tys1, ppr tycon $$ ppr tys $$ ppr tvs )
+      mkAppTys (substTyWith tvs tys1 ty) tys2
+  where
+    (tvs, ty)    = newTyConEtadRhs tycon
+    (tys1, tys2) = splitAtList tvs tys
 \end{code}