[project @ 2005-11-24 09:46:01 by simonpj]

A patch to the already-somewhat-delicate machinery that deals with
pattern-matching on unboxed tuples.  This patch deals with pattern
matches that can fail, e.g.
	case f x of
	  (# Just x, Nothing #) -> ...

The fix is in desugaring of HsCase (DsExpr.lhs).

The test is dsrun013

diff --git a/ghc/compiler/deSugar/DsExpr.lhs b/ghc/compiler/deSugar/DsExpr.lhs
index ce5a9d8..04511ce 100644 (file)
--- a/ghc/compiler/deSugar/DsExpr.lhs
+++ b/ghc/compiler/deSugar/DsExpr.lhs
@@ -257,14 +257,29 @@ dsExpr (HsCoreAnn fs expr)
     returnDs (Note (CoreNote $ unpackFS fs) core_expr)
 
 -- Special case to handle unboxed tuple patterns; they can't appear nested
+-- The idea is that 
+--     case e of (# p1, p2 #) -> rhs
+-- should desugar to
+--     case e of (# x1, x2 #) -> ... match p1, p2 ...
+-- NOT
+--     let x = e in case x of ....
+--
+-- But there may be a big 
+--     let fail = ... in case e of ...
+-- wrapping the whole case, which complicates matters slightly
+-- It all seems a bit fragile.  Test is dsrun013.
+
 dsExpr (HsCase discrim matches@(MatchGroup _ ty))
  | isUnboxedTupleType (funArgTy ty)
  =  dsLExpr discrim                    `thenDs` \ core_discrim ->
     matchWrapper CaseAlt matches       `thenDs` \ ([discrim_var], matching_code) ->
-    case matching_code of
-       Case (Var x) bndr ty alts | x == discrim_var -> 
-               returnDs (Case core_discrim bndr ty alts)
-       _ -> panic ("dsLExpr: tuple pattern:\n" ++ showSDoc (ppr matching_code))
+    let
+       scrungle (Case (Var x) bndr ty alts) 
+               | x == discrim_var = Case core_discrim bndr ty alts
+       scrungle (Let binds body)  = Let binds (scrungle body)
+       scrungle other = panic ("dsLExpr: tuple pattern:\n" ++ showSDoc (ppr other))
+    in
+    returnDs (scrungle matching_code)
 
 dsExpr (HsCase discrim matches)
   = dsLExpr discrim                    `thenDs` \ core_discrim ->