[project @ 2001-01-25 17:54:24 by simonpj]

[ghc-hetmet.git] / ghc / compiler / types / Unify.lhs

diff --git a/ghc/compiler/types/Unify.lhs b/ghc/compiler/types/Unify.lhs
index d576aaa..d535bb0 100644 (file)
--- a/ghc/compiler/types/Unify.lhs
+++ b/ghc/compiler/types/Unify.lhs
@@ -7,14 +7,14 @@ This module contains a unifier and a matcher, both of which
 use an explicit substitution
 
 \begin{code}
-module Unify ( unifyTysX, unifyTyListsX,
-              match, matchTy, matchTys
+module Unify ( unifyTysX, unifyTyListsX, allDistinctTyVars,
+              match, matchTy, matchTys,
   ) where 
 
 #include "HsVersions.h"
 
 import TypeRep ( Type(..) )     -- friend
-import Type    ( typeKind, tyVarsOfType, splitAppTy_maybe,
+import Type    ( typeKind, tyVarsOfType, splitAppTy_maybe, getTyVar_maybe,
                   splitUTy, isUTy, deNoteType
                )
 
@@ -38,6 +38,37 @@ import Outputable
 %*                                                                     *
 %************************************************************************
 
+(allDistinctTyVars tys tvs) = True 
+       iff 
+all the types tys are type variables, 
+distinct from each other and from tvs.
+
+This is useful when checking that unification hasn't unified signature
+type variables.  For example, if the type sig is
+       f :: forall a b. a -> b -> b
+we want to check that 'a' and 'b' havn't 
+       (a) been unified with a non-tyvar type
+       (b) been unified with each other (all distinct)
+       (c) been unified with a variable free in the environment
+
+\begin{code}
+allDistinctTyVars :: [Type] -> TyVarSet -> Bool
+
+allDistinctTyVars []       acc
+  = True
+allDistinctTyVars (ty:tys) acc 
+  = case getTyVar_maybe ty of
+       Nothing                       -> False  -- (a)
+       Just tv | tv `elemVarSet` acc -> False  -- (b) or (c)
+               | otherwise           -> allDistinctTyVars tys (acc `extendVarSet` tv)
+\end{code}    
+
+%************************************************************************
+%*                                                                     *
+\subsection{Unification with an explicit substitution}
+%*                                                                     *
+%************************************************************************
+
 Unify types with an explicit substitution and no monad.
 Ignore usage annotations.