X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2FbasicTypes%2FBasicTypes.lhs;h=0dbd0f6c0991e9fd0d41948e5f5475d5a5bf84f8;hb=a97f155c04a7ff981da3e589158ecf70db72d1cd;hp=6b662bd6a64c18c590d9dc541187fe88e13d1c79;hpb=0065d5ab628975892cea1ec7303f968c3338cbe1;p=ghc-hetmet.git

diff --git a/compiler/basicTypes/BasicTypes.lhs b/compiler/basicTypes/BasicTypes.lhs
index 6b662bd..0dbd0f6 100644
--- a/compiler/basicTypes/BasicTypes.lhs
+++ b/compiler/basicTypes/BasicTypes.lhs
@@ -31,6 +31,8 @@ module BasicTypes(
 
 	TopLevelFlag(..), isTopLevel, isNotTopLevel,
 
+	OverlapFlag(..), 
+
 	Boxity(..), isBoxed, 
 
 	TupCon(..), tupleParens,
@@ -217,7 +219,7 @@ instance Outputable TopLevelFlag where
 
 %************************************************************************
 %*									*
-\subsection[Top-level/local]{Top-level/not-top level flag}
+		Top-level/not-top level flag
 %*									*
 %************************************************************************
 
@@ -235,7 +237,7 @@ isBoxed Unboxed = False
 
 %************************************************************************
 %*									*
-\subsection[Recursive/Non-Recursive]{Recursive/Non-Recursive flag}
+		Recursive/Non-Recursive flag
 %*									*
 %************************************************************************
 
@@ -263,6 +265,47 @@ instance Outputable RecFlag where
 
 %************************************************************************
 %*									*
+		Instance overlap flag
+%*									*
+%************************************************************************
+
+\begin{code}
+data OverlapFlag
+  = NoOverlap	-- This instance must not overlap another
+
+  | OverlapOk	-- Silently ignore this instance if you find a 
+		-- more specific one that matches the constraint
+		-- you are trying to resolve
+		--
+		-- Example: constraint (Foo [Int])
+		-- 	    instances  (Foo [Int])
+		--		       (Foo [a])	OverlapOk
+		-- Since the second instance has the OverlapOk flag,
+		-- the first instance will be chosen (otherwise 
+		-- its ambiguous which to choose)
+
+  | Incoherent	-- Like OverlapOk, but also ignore this instance 
+		-- if it doesn't match the constraint you are
+		-- trying to resolve, but could match if the type variables
+		-- in the constraint were instantiated
+		--
+		-- Example: constraint (Foo [b])
+		--	    instances  (Foo [Int])	Incoherent
+		--		       (Foo [a])
+		-- Without the Incoherent flag, we'd complain that
+		-- instantiating 'b' would change which instance 
+		-- was chosen
+  deriving( Eq )
+
+instance Outputable OverlapFlag where
+   ppr NoOverlap  = empty
+   ppr OverlapOk  = ptext SLIT("[overlap ok]")
+   ppr Incoherent = ptext SLIT("[incoherent]")
+
+\end{code}
+
+%************************************************************************
+%*									*
 		Tuples
 %*									*
 %************************************************************************