Massive patch for the first months work adding System FC to GHC #1

[ghc-hetmet.git] / compiler / basicTypes / BasicTypes.lhs

diff --git a/compiler/basicTypes/BasicTypes.lhs b/compiler/basicTypes/BasicTypes.lhs
index 6b662bd..ab6d463 100644 (file)
--- 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 +265,46 @@ 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
+
+instance Outputable OverlapFlag where
+   ppr NoOverlap  = empty
+   ppr OverlapOk  = ptext SLIT("[overlap ok]")
+   ppr Incoherent = ptext SLIT("[incoherent]")
+
+\end{code}
+
+%************************************************************************
+%*                                                                     *
                Tuples
 %*                                                                     *
 %************************************************************************