getTvSubstEnv, setTvSubstEnv, zapTvSubstEnv, getTvInScope,
extendTvInScope, extendTvInScopeList,
extendTvSubst, extendTvSubstList, isInScope, composeTvSubst, zipTyEnv,
- isEmptyTvSubst,
+ isEmptyTvSubst, unionTvSubst,
-- ** Performing substitution on types
substTy, substTys, substTyWith, substTysWith, substTheta,
extendTvSubstList (TvSubst in_scope env) tvs tys
= TvSubst in_scope (extendVarEnvList env (tvs `zip` tys))
+unionTvSubst :: TvSubst -> TvSubst -> TvSubst
+-- Works when the ranges are disjoint
+unionTvSubst (TvSubst in_scope1 env1) (TvSubst in_scope2 env2)
+ = ASSERT( not (env1 `intersectsVarEnv` env2) )
+ TvSubst (in_scope1 `unionInScope` in_scope2)
+ (env1 `plusVarEnv` env2)
+
-- mkOpenTvSubst and zipOpenTvSubst generate the in-scope set from
-- the types given; but it's just a thunk so with a bit of luck
-- it'll never be evaluated