2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 This module contains a unifier and a matcher, both of which
7 use an explicit substitution
10 module Unify ( unifyTysX, unifyTyListsX,
11 match, matchTy, matchTys
14 import Type ( Type(..), funTyCon, typeKind, tyVarsOfType,
18 import Var ( TyVar, tyVarKind )
20 import VarEnv ( TyVarSubstEnv, emptySubstEnv, lookupSubstEnv, extendSubstEnv,
24 import Unique ( Uniquable(..) )
25 import Outputable( panic )
26 import Util ( snocView )
29 %************************************************************************
31 \subsection{Unification wih a explicit substitution}
33 %************************************************************************
35 Unify types with an explicit substitution and no monad.
39 = (TyVarSet, -- Set of template tyvars
40 TyVarSubstEnv) -- Not necessarily idempotent
42 unifyTysX :: TyVarSet -- Template tyvars
45 -> Maybe TyVarSubstEnv
46 unifyTysX tmpl_tyvars ty1 ty2
47 = uTysX ty1 ty2 (\(_,s) -> Just s) (tmpl_tyvars, emptySubstEnv)
49 unifyTyListsX :: TyVarSet -> [Type] -> [Type]
50 -> Maybe TyVarSubstEnv
51 unifyTyListsX tmpl_tyvars tys1 tys2
52 = uTyListsX tys1 tys2 (\(_,s) -> Just s) (tmpl_tyvars, emptySubstEnv)
57 -> (MySubst -> Maybe result)
61 uTysX (NoteTy _ ty1) ty2 k subst = uTysX ty1 ty2 k subst
62 uTysX ty1 (NoteTy _ ty2) k subst = uTysX ty1 ty2 k subst
64 -- Variables; go for uVar
65 uTysX (TyVarTy tyvar1) (TyVarTy tyvar2) k subst
68 uTysX (TyVarTy tyvar1) ty2 k subst@(tmpls,_)
69 | tyvar1 `elemVarSet` tmpls
70 = uVarX tyvar1 ty2 k subst
71 uTysX ty1 (TyVarTy tyvar2) k subst@(tmpls,_)
72 | tyvar2 `elemVarSet` tmpls
73 = uVarX tyvar2 ty1 k subst
75 -- Functions; just check the two parts
76 uTysX (FunTy fun1 arg1) (FunTy fun2 arg2) k subst
77 = uTysX fun1 fun2 (uTysX arg1 arg2 k) subst
79 -- Type constructors must match
80 uTysX (TyConApp con1 tys1) (TyConApp con2 tys2) k subst
81 | (con1 == con2 && length tys1 == length tys2)
82 = uTyListsX tys1 tys2 k subst
84 -- Applications need a bit of care!
85 -- They can match FunTy and TyConApp, so use splitAppTy_maybe
86 -- NB: we've already dealt with type variables and Notes,
87 -- so if one type is an App the other one jolly well better be too
88 uTysX (AppTy s1 t1) ty2 k subst
89 = case splitAppTy_maybe ty2 of
90 Just (s2, t2) -> uTysX s1 s2 (uTysX t1 t2 k) subst
91 Nothing -> Nothing -- Fail
93 uTysX ty1 (AppTy s2 t2) k subst
94 = case splitAppTy_maybe ty1 of
95 Just (s1, t1) -> uTysX s1 s2 (uTysX t1 t2 k) subst
96 Nothing -> Nothing -- Fail
98 -- Not expecting for-alls in unification
100 uTysX (ForAllTy _ _) ty2 k subst = panic "Unify.uTysX subst:ForAllTy (1st arg)"
101 uTysX ty1 (ForAllTy _ _) k subst = panic "Unify.uTysX subst:ForAllTy (2nd arg)"
104 -- Anything else fails
105 uTysX ty1 ty2 k subst = Nothing
108 uTyListsX [] [] k subst = k subst
109 uTyListsX (ty1:tys1) (ty2:tys2) k subst = uTysX ty1 ty2 (uTyListsX tys1 tys2 k) subst
110 uTyListsX tys1 tys2 k subst = Nothing -- Fail if the lists are different lengths
114 -- Invariant: tv1 is a unifiable variable
115 uVarX tv1 ty2 k subst@(tmpls, env)
116 = case lookupSubstEnv env tv1 of
117 Just (DoneTy ty1) -> -- Already bound
118 uTysX ty1 ty2 k subst
120 Nothing -- Not already bound
121 | typeKind ty2 == tyVarKind tv1
122 && occur_check_ok ty2
123 -> -- No kind mismatch nor occur check
124 k (tmpls, extendSubstEnv env tv1 (DoneTy ty2))
126 | otherwise -> Nothing -- Fail if kind mis-match or occur check
128 occur_check_ok ty = all occur_check_ok_tv (varSetElems (tyVarsOfType ty))
129 occur_check_ok_tv tv | tv1 == tv = False
130 | otherwise = case lookupSubstEnv env tv of
132 Just (DoneTy ty) -> occur_check_ok ty
137 %************************************************************************
139 \subsection{Matching on types}
141 %************************************************************************
143 Matching is a {\em unidirectional} process, matching a type against a
144 template (which is just a type with type variables in it). The
145 matcher assumes that there are no repeated type variables in the
146 template, so that it simply returns a mapping of type variables to
147 types. It also fails on nested foralls.
149 @matchTys@ matches corresponding elements of a list of templates and
153 matchTy :: TyVarSet -- Template tyvars
155 -> Type -- Proposed instance of template
156 -> Maybe TyVarSubstEnv -- Matching substitution
159 matchTys :: TyVarSet -- Template tyvars
160 -> [Type] -- Templates
161 -> [Type] -- Proposed instance of template
162 -> Maybe (TyVarSubstEnv, -- Matching substitution
163 [Type]) -- Left over instance types
165 matchTy tmpls ty1 ty2 = match ty1 ty2 tmpls (\ senv -> Just senv) emptySubstEnv
167 matchTys tmpls tys1 tys2 = match_list tys1 tys2 tmpls
168 (\ (senv,tys) -> Just (senv,tys))
172 @match@ is the main function.
175 match :: Type -> Type -- Current match pair
176 -> TyVarSet -- Template vars
177 -> (TyVarSubstEnv -> Maybe result) -- Continuation
178 -> TyVarSubstEnv -- Current subst
181 -- When matching against a type variable, see if the variable
182 -- has already been bound. If so, check that what it's bound to
183 -- is the same as ty; if not, bind it and carry on.
185 match (TyVarTy v) ty tmpls k senv
186 | v `elemVarSet` tmpls
187 = -- v is a template variable
188 case lookupSubstEnv senv v of
189 Nothing -> k (extendSubstEnv senv v (DoneTy ty))
190 Just (DoneTy ty') | ty' == ty -> k senv -- Succeeds
191 | otherwise -> Nothing -- Fails
194 = -- v is not a template variable; ty had better match
195 -- Can't use (==) because types differ
197 TyVarTy v' | v == v' -> k senv -- Success
198 other -> Nothing -- Failure
200 match (FunTy arg1 res1) (FunTy arg2 res2) tmpls k senv
201 = match arg1 arg2 tmpls (match res1 res2 tmpls k) senv
203 match (AppTy fun1 arg1) ty2 tmpls k senv
204 = case splitAppTy_maybe ty2 of
205 Just (fun2,arg2) -> match fun1 fun2 tmpls (match arg1 arg2 tmpls k) senv
206 Nothing -> Nothing -- Fail
208 match (TyConApp tc1 tys1) (TyConApp tc2 tys2) tmpls k senv
210 = match_list tys1 tys2 tmpls k' senv
212 k' (senv', tys2') | null tys2' = k senv' -- Succeed
213 | otherwise = Nothing -- Fail
215 -- With type synonyms, we have to be careful for the exact
216 -- same reasons as in the unifier. Please see the
217 -- considerable commentary there before changing anything
219 match (NoteTy _ ty1) ty2 tmpls k senv = match ty1 ty2 tmpls k senv
220 match ty1 (NoteTy _ ty2) tmpls k senv = match ty1 ty2 tmpls k senv
223 match _ _ _ _ _ = Nothing
225 match_list [] tys2 tmpls k senv = k (senv, tys2)
226 match_list (ty1:tys1) [] tmpls k senv = Nothing -- Not enough arg tys => failure
227 match_list (ty1:tys1) (ty2:tys2) tmpls k senv = match ty1 ty2 tmpls (match_list tys1 tys2 tmpls k) senv