[project @ 2003-07-29 10:14:57 by simonpj]

[ghc-hetmet.git] / ghc / compiler / coreSyn / CoreSyn.lhs

diff --git a/ghc/compiler/coreSyn/CoreSyn.lhs b/ghc/compiler/coreSyn/CoreSyn.lhs
index 4c70bb3..a074499 100644 (file)
--- a/ghc/compiler/coreSyn/CoreSyn.lhs
+++ b/ghc/compiler/coreSyn/CoreSyn.lhs
@@ -79,12 +79,27 @@ data Expr b -- "b" for the type of binders,
   | Lam   b (Expr b)
   | Let   (Bind b) (Expr b)
   | Case  (Expr b) b [Alt b]   -- Binder gets bound to value of scrutinee
-       -- Invariant: the list of alternatives is ALWAYS EXHAUSTIVE
-       -- Invariant: the DEFAULT case must be *first*, if it occurs at all
+       -- Invariant: The list of alternatives is ALWAYS EXHAUSTIVE,
+       --            meaning that it covers all cases that can occur
+       --            See the example below
+       --
+       -- Invariant: The DEFAULT case must be *first*, if it occurs at all
   | Note  Note (Expr b)
   | Type  Type                 -- This should only show up at the top
                                -- level of an Arg
 
+-- An "exhausive" case does not necessarily mention all constructors:
+--     data Foo = Red | Green | Blue
+--
+--     ...case x of 
+--             Red   -> True
+--             other -> f (case x of 
+--                             Green -> ...
+--                             Blue  -> ... )
+-- The inner case does not need a Red alternative, because x can't be Red at
+-- that program point.
+
+
 type Arg b = Expr b            -- Can be a Type
 
 type Alt b = (AltCon, [b], Expr b)     -- (DEFAULT, [], rhs) is the default alternative