2 % (c) The University of Glasgow 2006
8 -- the "tc" prefix indicates that matching always
9 -- respects newtypes (rather than looking through them)
10 tcMatchTy, tcMatchTys, tcMatchTyX,
11 ruleMatchTyX, tcMatchPreds, MatchEnv(..),
16 #include "HsVersions.h"
31 %************************************************************************
35 %************************************************************************
38 Matching is much tricker than you might think.
40 1. The substitution we generate binds the *template type variables*
41 which are given to us explicitly.
43 2. We want to match in the presence of foralls;
44 e.g (forall a. t1) ~ (forall b. t2)
46 That is what the RnEnv2 is for; it does the alpha-renaming
47 that makes it as if a and b were the same variable.
48 Initialising the RnEnv2, so that it can generate a fresh
49 binder when necessary, entails knowing the free variables of
52 3. We must be careful not to bind a template type variable to a
53 locally bound variable. E.g.
54 (forall a. x) ~ (forall b. b)
55 where x is the template type variable. Then we do not want to
56 bind x to a/b! This is a kind of occurs check.
57 The necessary locals accumulate in the RnEnv2.
62 = ME { me_tmpls :: VarSet -- Template tyvars
63 , me_env :: RnEnv2 -- Renaming envt for nested foralls
64 } -- In-scope set includes template tyvars
66 tcMatchTy :: TyVarSet -- Template tyvars
69 -> Maybe TvSubst -- One-shot; in principle the template
70 -- variables could be free in the target
72 tcMatchTy tmpls ty1 ty2
73 = case match menv emptyTvSubstEnv ty1 ty2 of
74 Just subst_env -> Just (TvSubst in_scope subst_env)
77 menv = ME { me_tmpls = tmpls, me_env = mkRnEnv2 in_scope }
78 in_scope = mkInScopeSet (tmpls `unionVarSet` tyVarsOfType ty2)
79 -- We're assuming that all the interesting
80 -- tyvars in tys1 are in tmpls
82 tcMatchTys :: TyVarSet -- Template tyvars
85 -> Maybe TvSubst -- One-shot; in principle the template
86 -- variables could be free in the target
88 tcMatchTys tmpls tys1 tys2
89 = case match_tys menv emptyTvSubstEnv tys1 tys2 of
90 Just subst_env -> Just (TvSubst in_scope subst_env)
93 menv = ME { me_tmpls = tmpls, me_env = mkRnEnv2 in_scope }
94 in_scope = mkInScopeSet (tmpls `unionVarSet` tyVarsOfTypes tys2)
95 -- We're assuming that all the interesting
96 -- tyvars in tys1 are in tmpls
98 -- This is similar, but extends a substitution
99 tcMatchTyX :: TyVarSet -- Template tyvars
100 -> TvSubst -- Substitution to extend
104 tcMatchTyX tmpls (TvSubst in_scope subst_env) ty1 ty2
105 = case match menv subst_env ty1 ty2 of
106 Just subst_env -> Just (TvSubst in_scope subst_env)
109 menv = ME {me_tmpls = tmpls, me_env = mkRnEnv2 in_scope}
112 :: [TyVar] -- Bind these
113 -> [PredType] -> [PredType]
115 tcMatchPreds tmpls ps1 ps2
116 = match_list (match_pred menv) emptyTvSubstEnv ps1 ps2
118 menv = ME { me_tmpls = mkVarSet tmpls, me_env = mkRnEnv2 in_scope_tyvars }
119 in_scope_tyvars = mkInScopeSet (tyVarsOfTheta ps1 `unionVarSet` tyVarsOfTheta ps2)
121 -- This one is called from the expression matcher, which already has a MatchEnv in hand
122 ruleMatchTyX :: MatchEnv
123 -> TvSubstEnv -- Substitution to extend
128 ruleMatchTyX menv subst ty1 ty2 = match menv subst ty1 ty2 -- Rename for export
131 Now the internals of matching
134 match :: MatchEnv -- For the most part this is pushed downwards
135 -> TvSubstEnv -- Substitution so far:
136 -- Domain is subset of template tyvars
137 -- Free vars of range is subset of
138 -- in-scope set of the RnEnv2
139 -> Type -> Type -- Template and target respectively
141 -- This matcher works on source types; that is,
142 -- it respects NewTypes and PredType
144 match menv subst ty1 ty2 | Just ty1' <- tcView ty1 = match menv subst ty1' ty2
145 | Just ty2' <- tcView ty2 = match menv subst ty1 ty2'
147 match menv subst (TyVarTy tv1) ty2
148 | tv1' `elemVarSet` me_tmpls menv
149 = case lookupVarEnv subst tv1' of
150 Nothing -- No existing binding
151 | any (inRnEnvR rn_env) (varSetElems (tyVarsOfType ty2))
152 -> Nothing -- Occurs check
153 | not (typeKind ty2 `isSubKind` tyVarKind tv1)
154 -> Nothing -- Kind mis-match
156 -> Just (extendVarEnv subst tv1' ty2)
158 Just ty1' -- There is an existing binding; check whether ty2 matches it
159 | tcEqTypeX (nukeRnEnvL rn_env) ty1' ty2
160 -- ty1 has no locally-bound variables, hence nukeRnEnvL
161 -- Note tcEqType...we are doing source-type matching here
163 | otherwise -> Nothing -- ty2 doesn't match
166 | otherwise -- tv1 is not a template tyvar
168 TyVarTy tv2 | tv1' == rnOccR rn_env tv2 -> Just subst
172 tv1' = rnOccL rn_env tv1
174 match menv subst (ForAllTy tv1 ty1) (ForAllTy tv2 ty2)
175 = match menv' subst ty1 ty2
176 where -- Use the magic of rnBndr2 to go under the binders
177 menv' = menv { me_env = rnBndr2 (me_env menv) tv1 tv2 }
179 match menv subst (PredTy p1) (PredTy p2)
180 = match_pred menv subst p1 p2
181 match menv subst (TyConApp tc1 tys1) (TyConApp tc2 tys2)
182 | tc1 == tc2 = match_tys menv subst tys1 tys2
183 match menv subst (FunTy ty1a ty1b) (FunTy ty2a ty2b)
184 = do { subst' <- match menv subst ty1a ty2a
185 ; match menv subst' ty1b ty2b }
186 match menv subst (AppTy ty1a ty1b) ty2
187 | Just (ty2a, ty2b) <- repSplitAppTy_maybe ty2
188 -- 'repSplit' used because the tcView stuff is done above
189 = do { subst' <- match menv subst ty1a ty2a
190 ; match menv subst' ty1b ty2b }
192 match menv subst ty1 ty2
196 match_tys menv subst tys1 tys2 = match_list (match menv) subst tys1 tys2
199 match_list :: (TvSubstEnv -> a -> a -> Maybe TvSubstEnv)
200 -> TvSubstEnv -> [a] -> [a] -> Maybe TvSubstEnv
201 match_list fn subst [] [] = Just subst
202 match_list fn subst (ty1:tys1) (ty2:tys2) = do { subst' <- fn subst ty1 ty2
203 ; match_list fn subst' tys1 tys2 }
204 match_list fn subst tys1 tys2 = Nothing
207 match_pred menv subst (ClassP c1 tys1) (ClassP c2 tys2)
208 | c1 == c2 = match_tys menv subst tys1 tys2
209 match_pred menv subst (IParam n1 t1) (IParam n2 t2)
210 | n1 == n2 = match menv subst t1 t2
211 match_pred menv subst p1 p2 = Nothing
215 %************************************************************************
219 %************************************************************************
221 Note [Pruning dead case alternatives]
222 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
223 Consider data T a where
231 type instance F Bool = Int
233 Now consider case x of { T1 -> e1; T2 -> e2 }
235 The question before the house is this: if I know something about the type
236 of x, can I prune away the T1 alternative?
238 Suppose x::T Char. It's impossible to construct a (T Char) using T1,
239 Answer = YES (clearly)
241 Suppose x::T (F a), where 'a' is in scope. Then 'a' might be instantiated
242 to 'Bool', in which case x::T Int, so
243 ANSWER = NO (clearly)
245 Suppose x::T X. Then *in Haskell* it's impossible to construct a (non-bottom)
246 value of type (T X) using T1. But *in FC* it's quite possible. The newtype
249 So (T CoX) :: T X ~ T Int; hence (T1 `cast` sym (T CoX)) is a non-bottom value
250 of type (T X) constructed with T1. Hence
251 ANSWER = NO (surprisingly)
253 Furthermore, this can even happen; see Trac #1251. GHC's newtype-deriving
254 mechanism uses a cast, just as above, to move from one dictionary to another,
255 in effect giving the programmer access to CoX.
257 Finally, suppose x::T Y. Then *even in FC* we can't construct a
258 non-bottom value of type (T Y) using T1. That's because we can get
259 from Y to Char, but not to Int.
262 Here's a related question. data Eq a b where EQ :: Eq a a
264 case x of { EQ -> ... }
266 Suppose x::Eq Int Char. Is the alternative dead? Clearly yes.
268 What about x::Eq Int a, in a context where we have evidence that a~Char.
269 Then again the alternative is dead.
274 We are really doing a test for unsatisfiability of the type
275 constraints implied by the match. And that is clearly, in general, a
278 However, since we are simply dropping dead code, a conservative test
279 suffices. There is a continuum of tests, ranging from easy to hard, that
280 drop more and more dead code.
282 For now we implement a very simple test: type variables match
283 anything, type functions (incl newtypes) match anything, and only
284 distinct data types fail to match. We can elaborate later.
287 dataConCannotMatch :: [Type] -> DataCon -> Bool
288 -- Returns True iff the data con *definitely cannot* match a
289 -- scrutinee of type (T tys)
290 -- where T is the type constructor for the data con
292 dataConCannotMatch tys con
293 | null eq_spec = False -- Common
294 | all isTyVarTy tys = False -- Also common
296 = cant_match_s (map (substTyVar subst . fst) eq_spec)
299 dc_tvs = dataConUnivTyVars con
300 eq_spec = dataConEqSpec con
301 subst = zipTopTvSubst dc_tvs tys
303 cant_match_s :: [Type] -> [Type] -> Bool
304 cant_match_s tys1 tys2 = ASSERT( equalLength tys1 tys2 )
305 or (zipWith cant_match tys1 tys2)
307 cant_match :: Type -> Type -> Bool
309 | Just t1' <- coreView t1 = cant_match t1' t2
310 | Just t2' <- coreView t2 = cant_match t1 t2'
312 cant_match (FunTy a1 r1) (FunTy a2 r2)
313 = cant_match a1 a2 || cant_match r1 r2
315 cant_match (TyConApp tc1 tys1) (TyConApp tc2 tys2)
316 | isDataTyCon tc1 && isDataTyCon tc2
317 = tc1 /= tc2 || cant_match_s tys1 tys2
319 cant_match (FunTy {}) (TyConApp tc _) = isDataTyCon tc
320 cant_match (TyConApp tc _) (FunTy {}) = isDataTyCon tc
321 -- tc can't be FunTyCon by invariant
323 cant_match (AppTy f1 a1) ty2
324 | Just (f2, a2) <- repSplitAppTy_maybe ty2
325 = cant_match f1 f2 || cant_match a1 a2
326 cant_match ty1 (AppTy f2 a2)
327 | Just (f1, a1) <- repSplitAppTy_maybe ty1
328 = cant_match f1 f2 || cant_match a1 a2
330 cant_match ty1 ty2 = False -- Safe!
332 -- Things we could add;
334 -- look through newtypes
335 -- take account of tyvar bindings (EQ example above)