Major refactoring of the way that UNPACK pragmas are handled

The situation was pretty dire.  The way in which data constructors
were handled, notably the mapping between their *source* argument types
and their *representation* argument types (after seq'ing and unpacking)
was scattered in three different places, and hard to keep in sync.

Now it is all in one place:

 * The dcRep field of a DataCon gives its representation,
   specified by a DataConRep

 * As well as having the wrapper, the DataConRep has a "boxer"
   of type DataConBoxer (defined in MkId for loopy reasons).
   The boxer used at a pattern match to reconstruct the source-level
   arguments from the rep-level bindings in the pattern match.

 * The unboxing in the wrapper and the boxing in the boxer are dual,
   and are now constructed together, by MkId.mkDataConRep. This is
   the key function of this change.

 * All the computeBoxingStrategy code in TcTyClsDcls disappears.

Much nicer.

There is a little bit of refactoring left to do; the strange
deepSplitProductType functions are now called only in WwLib, so
I moved them there, and I think they could be tidied up further.

2 files changed, 11 insertions, 7 deletions

diff --git a/compiler/deSugar/DsMeta.hs b/compiler/deSugar/DsMeta.hs
index 405b7687a5..f25039c8a9 100644
--- a/compiler/deSugar/DsMeta.hs
+++ b/compiler/deSugar/DsMeta.hs
@@ -519,9 +519,9 @@ repBangTy ty= do
   rep2 strictTypeName [s, t]
   where
     (str, ty') = case ty of
-		   L _ (HsBangTy HsUnpack ty) -> (unpackedName,  ty)
-		   L _ (HsBangTy _ ty)        -> (isStrictName,  ty)
-		   _                          -> (notStrictName, ty)
+		   L _ (HsBangTy (HsBang True) ty) -> (unpackedName,  ty)
+		   L _ (HsBangTy _ ty)             -> (isStrictName,  ty)
+		   _                               -> (notStrictName, ty)
 
 -------------------------------------------------------
 -- 			Deriving clause
diff --git a/compiler/deSugar/DsUtils.lhs b/compiler/deSugar/DsUtils.lhs
index 609041ba24..504a76dc86 100644
--- a/compiler/deSugar/DsUtils.lhs
+++ b/compiler/deSugar/DsUtils.lhs
@@ -316,10 +316,14 @@ mkCoAlgCaseMatchResult dflags var ty match_alts
     mk_case fail = do alts <- mapM (mk_alt fail) sorted_alts
                       return (mkWildCase (Var var) (idType var) ty (mk_default fail ++ alts))
 
-    mk_alt fail (con, args, MatchResult _ body_fn) = do
-          body <- body_fn fail
-          us <- newUniqueSupply
-          return (mkReboxingAlt (uniqsFromSupply us) con args body)
+    mk_alt fail (con, args, MatchResult _ body_fn)
+      = do { body <- body_fn fail
+           ; case dataConBoxer con of {
+                Nothing -> return (DataAlt con, args, body) ;
+                Just (DCB boxer) -> 
+        do { us <- newUniqueSupply
+           ; let (rep_ids, binds) = initUs_ us (boxer ty_args args)
+           ; return (DataAlt con, rep_ids, mkLets binds body) } } }
 
     mk_default fail | exhaustive_case = []
 		    | otherwise       = [(DEFAULT, [], fail)]