From: simonpj@microsoft.com <unknown>
Date: Mon, 20 Jul 2009 06:12:26 +0000 (+0000)
Subject: Fix Trac #3382: desugaring of NPats
X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=commitdiff_plain;h=a62da487378d873399d2dedb85fc0d546fa911d8

Fix Trac #3382: desugaring of NPats

Max spotted that the short-cut rules for desugaring NPats (where
we compare against a literal) were wrong now that we have overloaded
strings.
---

diff --git a/compiler/deSugar/MatchLit.lhs b/compiler/deSugar/MatchLit.lhs
index 9d3ff49..be8ea23 100644
--- a/compiler/deSugar/MatchLit.lhs
+++ b/compiler/deSugar/MatchLit.lhs
@@ -154,39 +154,44 @@ tidyLitPat lit = LitPat lit
 ----------------
 tidyNPat :: HsOverLit Id -> Maybe (SyntaxExpr Id) -> SyntaxExpr Id -> Pat Id
 tidyNPat (OverLit val False _ ty) mb_neg _
-	-- Take short cuts only if the literal is not using rebindable syntax
-  | isIntTy    ty = mk_con_pat intDataCon    (HsIntPrim int_val)
-  | isWordTy   ty = mk_con_pat wordDataCon   (HsWordPrim int_val)
-  | isFloatTy  ty = mk_con_pat floatDataCon  (HsFloatPrim  rat_val)
-  | isDoubleTy ty = mk_con_pat doubleDataCon (HsDoublePrim rat_val)
---  | isStringTy lit_ty = mk_con_pat stringDataCon (HsStringPrim str_val)
+	-- False: Take short cuts only if the literal is not using rebindable syntax
+	-- 
+	-- Once that is settled, look for cases where the type of the 
+	-- entire overloaded literal matches the type of the underlying literal,
+	-- and in that case take the short cut
+	-- NB: Watch out for wierd cases like Trac #3382
+	-- 	  f :: Int -> Int
+	--	  f "blah" = 4
+	--     which might be ok if we hvae 'instance IsString Int'
+	--    
+
+  | isIntTy ty,    Just int_lit <- mb_int_lit = mk_con_pat intDataCon    (HsIntPrim    int_lit)
+  | isWordTy ty,   Just int_lit <- mb_int_lit = mk_con_pat wordDataCon   (HsWordPrim   int_lit)
+  | isFloatTy ty,  Just rat_lit <- mb_rat_lit = mk_con_pat floatDataCon  (HsFloatPrim  rat_lit)
+  | isDoubleTy ty, Just rat_lit <- mb_rat_lit = mk_con_pat doubleDataCon (HsDoublePrim rat_lit)
+  | isStringTy ty, Just str_lit <- mb_str_lit = tidyLitPat (HsString str_lit)
   where
     mk_con_pat :: DataCon -> HsLit -> Pat Id
     mk_con_pat con lit = unLoc (mkPrefixConPat con [noLoc $ LitPat lit] ty)
 
-    neg_val = case (mb_neg, val) of
-		(Nothing, 	       _) -> val
-		(Just _,  HsIntegral   i) -> HsIntegral   (-i)
-		(Just _,  HsFractional f) -> HsFractional (-f)
-		(Just _,  HsIsString _)	  -> panic "tidyNPat"
-			     
-    int_val :: Integer
-    int_val = case neg_val of
-		HsIntegral i -> i
-		_ -> panic "tidyNPat"
+    mb_int_lit :: Maybe Integer
+    mb_int_lit = case (mb_neg, val) of
+		   (Nothing, HsIntegral i) -> Just i
+		   (Just _,  HsIntegral i) -> Just (-i)
+		   _ -> Nothing
 	
-    rat_val :: Rational
-    rat_val = case neg_val of
-		HsIntegral   i -> fromInteger i
-		HsFractional f -> f
-		_ -> panic "tidyNPat"
+    mb_rat_lit :: Maybe Rational
+    mb_rat_lit = case (mb_neg, val) of
+		   (Nothing, HsIntegral   i) -> Just (fromInteger i)
+		   (Just _,  HsIntegral   i) -> Just (fromInteger (-i))
+		   (Nothing, HsFractional f) -> Just f
+		   (Just _, HsFractional f)  -> Just (-f)
+		   _ -> Nothing
 	
-{-
-    str_val :: FastString
-    str_val = case val of
-		HsIsString s -> s
-		_ -> panic "tidyNPat"
--}
+    mb_str_lit :: Maybe FastString
+    mb_str_lit = case (mb_neg, val) of
+		   (Nothing, HsIsString s) -> Just s
+		   _ -> Nothing
 
 tidyNPat over_lit mb_neg eq 
   = NPat over_lit mb_neg eq