2 % (c) The University of Glasgow 2006
7 -- The above warning supression flag is a temporary kludge.
8 -- While working on this module you are encouraged to remove it and fix
9 -- any warnings in the module. See
10 -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
15 -- the "tc" prefix indicates that matching always
16 -- respects newtypes (rather than looking through them)
17 tcMatchTy, tcMatchTys, tcMatchTyX,
18 ruleMatchTyX, tcMatchPreds, MatchEnv(..),
23 #include "HsVersions.h"
39 %************************************************************************
43 %************************************************************************
46 Matching is much tricker than you might think.
48 1. The substitution we generate binds the *template type variables*
49 which are given to us explicitly.
51 2. We want to match in the presence of foralls;
52 e.g (forall a. t1) ~ (forall b. t2)
54 That is what the RnEnv2 is for; it does the alpha-renaming
55 that makes it as if a and b were the same variable.
56 Initialising the RnEnv2, so that it can generate a fresh
57 binder when necessary, entails knowing the free variables of
60 3. We must be careful not to bind a template type variable to a
61 locally bound variable. E.g.
62 (forall a. x) ~ (forall b. b)
63 where x is the template type variable. Then we do not want to
64 bind x to a/b! This is a kind of occurs check.
65 The necessary locals accumulate in the RnEnv2.
70 = ME { me_tmpls :: VarSet -- Template tyvars
71 , me_env :: RnEnv2 -- Renaming envt for nested foralls
72 } -- In-scope set includes template tyvars
74 tcMatchTy :: TyVarSet -- Template tyvars
77 -> Maybe TvSubst -- One-shot; in principle the template
78 -- variables could be free in the target
80 tcMatchTy tmpls ty1 ty2
81 = case match menv emptyTvSubstEnv ty1 ty2 of
82 Just subst_env -> Just (TvSubst in_scope subst_env)
85 menv = ME { me_tmpls = tmpls, me_env = mkRnEnv2 in_scope }
86 in_scope = mkInScopeSet (tmpls `unionVarSet` tyVarsOfType ty2)
87 -- We're assuming that all the interesting
88 -- tyvars in tys1 are in tmpls
90 tcMatchTys :: TyVarSet -- Template tyvars
93 -> Maybe TvSubst -- One-shot; in principle the template
94 -- variables could be free in the target
96 tcMatchTys tmpls tys1 tys2
97 = case match_tys menv emptyTvSubstEnv tys1 tys2 of
98 Just subst_env -> Just (TvSubst in_scope subst_env)
101 menv = ME { me_tmpls = tmpls, me_env = mkRnEnv2 in_scope }
102 in_scope = mkInScopeSet (tmpls `unionVarSet` tyVarsOfTypes tys2)
103 -- We're assuming that all the interesting
104 -- tyvars in tys1 are in tmpls
106 -- This is similar, but extends a substitution
107 tcMatchTyX :: TyVarSet -- Template tyvars
108 -> TvSubst -- Substitution to extend
112 tcMatchTyX tmpls (TvSubst in_scope subst_env) ty1 ty2
113 = case match menv subst_env ty1 ty2 of
114 Just subst_env -> Just (TvSubst in_scope subst_env)
117 menv = ME {me_tmpls = tmpls, me_env = mkRnEnv2 in_scope}
120 :: [TyVar] -- Bind these
121 -> [PredType] -> [PredType]
123 tcMatchPreds tmpls ps1 ps2
124 = match_list (match_pred menv) emptyTvSubstEnv ps1 ps2
126 menv = ME { me_tmpls = mkVarSet tmpls, me_env = mkRnEnv2 in_scope_tyvars }
127 in_scope_tyvars = mkInScopeSet (tyVarsOfTheta ps1 `unionVarSet` tyVarsOfTheta ps2)
129 -- This one is called from the expression matcher, which already has a MatchEnv in hand
130 ruleMatchTyX :: MatchEnv
131 -> TvSubstEnv -- Substitution to extend
136 ruleMatchTyX menv subst ty1 ty2 = match menv subst ty1 ty2 -- Rename for export
139 Now the internals of matching
142 match :: MatchEnv -- For the most part this is pushed downwards
143 -> TvSubstEnv -- Substitution so far:
144 -- Domain is subset of template tyvars
145 -- Free vars of range is subset of
146 -- in-scope set of the RnEnv2
147 -> Type -> Type -- Template and target respectively
149 -- This matcher works on source types; that is,
150 -- it respects NewTypes and PredType
152 match menv subst ty1 ty2 | Just ty1' <- tcView ty1 = match menv subst ty1' ty2
153 | Just ty2' <- tcView ty2 = match menv subst ty1 ty2'
155 match menv subst (TyVarTy tv1) ty2
156 | tv1' `elemVarSet` me_tmpls menv
157 = case lookupVarEnv subst tv1' of
158 Nothing -- No existing binding
159 | any (inRnEnvR rn_env) (varSetElems (tyVarsOfType ty2))
160 -> Nothing -- Occurs check
162 -> do { subst1 <- match_kind menv subst tv1 ty2
163 ; return (extendVarEnv subst tv1' ty2) }
165 Just ty1' -- There is an existing binding; check whether ty2 matches it
166 | tcEqTypeX (nukeRnEnvL rn_env) ty1' ty2
167 -- ty1 has no locally-bound variables, hence nukeRnEnvL
168 -- Note tcEqType...we are doing source-type matching here
170 | otherwise -> Nothing -- ty2 doesn't match
173 | otherwise -- tv1 is not a template tyvar
175 TyVarTy tv2 | tv1' == rnOccR rn_env tv2 -> Just subst
179 tv1' = rnOccL rn_env tv1
181 match menv subst (ForAllTy tv1 ty1) (ForAllTy tv2 ty2)
182 = match menv' subst ty1 ty2
183 where -- Use the magic of rnBndr2 to go under the binders
184 menv' = menv { me_env = rnBndr2 (me_env menv) tv1 tv2 }
186 match menv subst (PredTy p1) (PredTy p2)
187 = match_pred menv subst p1 p2
188 match menv subst (TyConApp tc1 tys1) (TyConApp tc2 tys2)
189 | tc1 == tc2 = match_tys menv subst tys1 tys2
190 match menv subst (FunTy ty1a ty1b) (FunTy ty2a ty2b)
191 = do { subst' <- match menv subst ty1a ty2a
192 ; match menv subst' ty1b ty2b }
193 match menv subst (AppTy ty1a ty1b) ty2
194 | Just (ty2a, ty2b) <- repSplitAppTy_maybe ty2
195 -- 'repSplit' used because the tcView stuff is done above
196 = do { subst' <- match menv subst ty1a ty2a
197 ; match menv subst' ty1b ty2b }
199 match menv subst ty1 ty2
203 match_kind :: MatchEnv -> TvSubstEnv -> TyVar -> Type -> Maybe TvSubstEnv
204 -- Match the kind of the template tyvar with the kind of Type
205 -- Note [Matching kinds]
206 match_kind menv subst tv ty
207 | isCoVar tv = do { let (ty1,ty2) = splitCoercionKind (tyVarKind tv)
208 (ty3,ty4) = coercionKind ty
209 ; subst1 <- match menv subst ty1 ty3
210 ; match menv subst1 ty2 ty4 }
211 | otherwise = if typeKind ty `isSubKind` tyVarKind tv
215 -- Note [Matching kinds]
216 -- ~~~~~~~~~~~~~~~~~~~~~
217 -- For ordinary type variables, we don't want (m a) to match (n b)
218 -- if say (a::*) and (b::*->*). This is just a yes/no issue.
220 -- For coercion kinds matters are more complicated. If we have a
221 -- coercion template variable co::a~[b], where a,b are presumably also
222 -- template type variables, then we must match co's kind against the
223 -- kind of the actual argument, so as to give bindings to a,b.
225 -- In fact I have no example in mind that *requires* this kind-matching
226 -- to instantiate template type variables, but it seems like the right
227 -- thing to do. C.f. Note [Matching variable types] in Rules.lhs
230 match_tys menv subst tys1 tys2 = match_list (match menv) subst tys1 tys2
233 match_list :: (TvSubstEnv -> a -> a -> Maybe TvSubstEnv)
234 -> TvSubstEnv -> [a] -> [a] -> Maybe TvSubstEnv
235 match_list fn subst [] [] = Just subst
236 match_list fn subst (ty1:tys1) (ty2:tys2) = do { subst' <- fn subst ty1 ty2
237 ; match_list fn subst' tys1 tys2 }
238 match_list fn subst tys1 tys2 = Nothing
241 match_pred menv subst (ClassP c1 tys1) (ClassP c2 tys2)
242 | c1 == c2 = match_tys menv subst tys1 tys2
243 match_pred menv subst (IParam n1 t1) (IParam n2 t2)
244 | n1 == n2 = match menv subst t1 t2
245 match_pred menv subst p1 p2 = Nothing
249 %************************************************************************
253 %************************************************************************
255 Note [Pruning dead case alternatives]
256 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
257 Consider data T a where
265 type instance F Bool = Int
267 Now consider case x of { T1 -> e1; T2 -> e2 }
269 The question before the house is this: if I know something about the type
270 of x, can I prune away the T1 alternative?
272 Suppose x::T Char. It's impossible to construct a (T Char) using T1,
273 Answer = YES (clearly)
275 Suppose x::T (F a), where 'a' is in scope. Then 'a' might be instantiated
276 to 'Bool', in which case x::T Int, so
277 ANSWER = NO (clearly)
279 Suppose x::T X. Then *in Haskell* it's impossible to construct a (non-bottom)
280 value of type (T X) using T1. But *in FC* it's quite possible. The newtype
283 So (T CoX) :: T X ~ T Int; hence (T1 `cast` sym (T CoX)) is a non-bottom value
284 of type (T X) constructed with T1. Hence
285 ANSWER = NO (surprisingly)
287 Furthermore, this can even happen; see Trac #1251. GHC's newtype-deriving
288 mechanism uses a cast, just as above, to move from one dictionary to another,
289 in effect giving the programmer access to CoX.
291 Finally, suppose x::T Y. Then *even in FC* we can't construct a
292 non-bottom value of type (T Y) using T1. That's because we can get
293 from Y to Char, but not to Int.
296 Here's a related question. data Eq a b where EQ :: Eq a a
298 case x of { EQ -> ... }
300 Suppose x::Eq Int Char. Is the alternative dead? Clearly yes.
302 What about x::Eq Int a, in a context where we have evidence that a~Char.
303 Then again the alternative is dead.
308 We are really doing a test for unsatisfiability of the type
309 constraints implied by the match. And that is clearly, in general, a
312 However, since we are simply dropping dead code, a conservative test
313 suffices. There is a continuum of tests, ranging from easy to hard, that
314 drop more and more dead code.
316 For now we implement a very simple test: type variables match
317 anything, type functions (incl newtypes) match anything, and only
318 distinct data types fail to match. We can elaborate later.
321 dataConCannotMatch :: [Type] -> DataCon -> Bool
322 -- Returns True iff the data con *definitely cannot* match a
323 -- scrutinee of type (T tys)
324 -- where T is the type constructor for the data con
326 dataConCannotMatch tys con
327 | null eq_spec = False -- Common
328 | all isTyVarTy tys = False -- Also common
330 = cant_match_s (map (substTyVar subst . fst) eq_spec)
333 dc_tvs = dataConUnivTyVars con
334 eq_spec = dataConEqSpec con
335 subst = zipTopTvSubst dc_tvs tys
337 cant_match_s :: [Type] -> [Type] -> Bool
338 cant_match_s tys1 tys2 = ASSERT( equalLength tys1 tys2 )
339 or (zipWith cant_match tys1 tys2)
341 cant_match :: Type -> Type -> Bool
343 | Just t1' <- coreView t1 = cant_match t1' t2
344 | Just t2' <- coreView t2 = cant_match t1 t2'
346 cant_match (FunTy a1 r1) (FunTy a2 r2)
347 = cant_match a1 a2 || cant_match r1 r2
349 cant_match (TyConApp tc1 tys1) (TyConApp tc2 tys2)
350 | isDataTyCon tc1 && isDataTyCon tc2
351 = tc1 /= tc2 || cant_match_s tys1 tys2
353 cant_match (FunTy {}) (TyConApp tc _) = isDataTyCon tc
354 cant_match (TyConApp tc _) (FunTy {}) = isDataTyCon tc
355 -- tc can't be FunTyCon by invariant
357 cant_match (AppTy f1 a1) ty2
358 | Just (f2, a2) <- repSplitAppTy_maybe ty2
359 = cant_match f1 f2 || cant_match a1 a2
360 cant_match ty1 (AppTy f2 a2)
361 | Just (f1, a1) <- repSplitAppTy_maybe ty1
362 = cant_match f1 f2 || cant_match a1 a2
364 cant_match ty1 ty2 = False -- Safe!
366 -- Things we could add;
368 -- look through newtypes
369 -- take account of tyvar bindings (EQ example above)