[project @ 2002-05-27 15:28:07 by simonpj]

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

diff --git a/ghc/compiler/basicTypes/BasicTypes.lhs b/ghc/compiler/basicTypes/BasicTypes.lhs
index 42e5e9f..62a68a9 100644 (file)
--- a/ghc/compiler/basicTypes/BasicTypes.lhs
+++ b/ghc/compiler/basicTypes/BasicTypes.lhs
@@ -21,7 +21,9 @@ module BasicTypes(
        Unused, unused,
 
        Fixity(..), FixityDirection(..),
-       defaultFixity, maxPrecedence, negateFixity, negatePrecedence,
+       defaultFixity, maxPrecedence, 
+       arrowFixity, negateFixity, negatePrecedence,
+       compareFixity,
 
        IPName(..), ipNameName, mapIPName,
 
@@ -156,10 +158,39 @@ defaultFixity = Fixity maxPrecedence InfixL
 negateFixity :: Fixity
 negateFixity     = Fixity negatePrecedence InfixL      -- Precedence of unary negate is wired in as infixl 6!
 
+arrowFixity :: Fixity  -- Fixity of '->' in types
+arrowFixity = Fixity 0 InfixR
+
 negatePrecedence :: Int
 negatePrecedence = 6
 \end{code}
 
+Consider
+
+\begin{verbatim}
+       a `op1` b `op2` c
+\end{verbatim}
+@(compareFixity op1 op2)@ tells which way to arrange appication, or
+whether there's an error.
+
+\begin{code}
+compareFixity :: Fixity -> Fixity
+             -> (Bool,         -- Error please
+                 Bool)         -- Associate to the right: a op1 (b op2 c)
+compareFixity (Fixity prec1 dir1) (Fixity prec2 dir2)
+  = case prec1 `compare` prec2 of
+       GT -> left
+       LT -> right
+       EQ -> case (dir1, dir2) of
+                       (InfixR, InfixR) -> right
+                       (InfixL, InfixL) -> left
+                       _                -> error_please
+  where
+    right       = (False, True)
+    left         = (False, False)
+    error_please = (True,  False)
+\end{code}
+
 
 %************************************************************************
 %*                                                                     *