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 core types; that is, it ignores PredTypes
150 -- Watch out if newtypes become transparent agin!
151 -- this matcher must respect newtypes
153 match menv subst ty1 ty2 | Just ty1' <- coreView ty1 = match menv subst ty1' ty2
154 | Just ty2' <- coreView ty2 = match menv subst ty1 ty2'
156 match menv subst (TyVarTy tv1) ty2
157 | tv1' `elemVarSet` me_tmpls menv
158 = case lookupVarEnv subst tv1' of
159 Nothing -- No existing binding
160 | any (inRnEnvR rn_env) (varSetElems (tyVarsOfType ty2))
161 -> Nothing -- Occurs check
163 -> do { subst1 <- match_kind menv subst tv1 ty2
164 ; return (extendVarEnv subst tv1' ty2) }
166 Just ty1' -- There is an existing binding; check whether ty2 matches it
167 | tcEqTypeX (nukeRnEnvL rn_env) ty1' ty2
168 -- ty1 has no locally-bound variables, hence nukeRnEnvL
169 -- Note tcEqType...we are doing source-type matching here
171 | otherwise -> Nothing -- ty2 doesn't match
174 | otherwise -- tv1 is not a template tyvar
176 TyVarTy tv2 | tv1' == rnOccR rn_env tv2 -> Just subst
180 tv1' = rnOccL rn_env tv1
182 match menv subst (ForAllTy tv1 ty1) (ForAllTy tv2 ty2)
183 = match menv' subst ty1 ty2
184 where -- Use the magic of rnBndr2 to go under the binders
185 menv' = menv { me_env = rnBndr2 (me_env menv) tv1 tv2 }
187 match menv subst (PredTy p1) (PredTy p2)
188 = match_pred menv subst p1 p2
189 match menv subst (TyConApp tc1 tys1) (TyConApp tc2 tys2)
190 | tc1 == tc2 = match_tys menv subst tys1 tys2
191 match menv subst (FunTy ty1a ty1b) (FunTy ty2a ty2b)
192 = do { subst' <- match menv subst ty1a ty2a
193 ; match menv subst' ty1b ty2b }
194 match menv subst (AppTy ty1a ty1b) ty2
195 | Just (ty2a, ty2b) <- repSplitAppTy_maybe ty2
196 -- 'repSplit' used because the tcView stuff is done above
197 = do { subst' <- match menv subst ty1a ty2a
198 ; match menv subst' ty1b ty2b }
200 match menv subst ty1 ty2
204 match_kind :: MatchEnv -> TvSubstEnv -> TyVar -> Type -> Maybe TvSubstEnv
205 -- Match the kind of the template tyvar with the kind of Type
206 -- Note [Matching kinds]
207 match_kind menv subst tv ty
208 | isCoVar tv = do { let (ty1,ty2) = splitCoercionKind (tyVarKind tv)
209 (ty3,ty4) = coercionKind ty
210 ; subst1 <- match menv subst ty1 ty3
211 ; match menv subst1 ty2 ty4 }
212 | otherwise = if typeKind ty `isSubKind` tyVarKind tv
216 -- Note [Matching kinds]
217 -- ~~~~~~~~~~~~~~~~~~~~~
218 -- For ordinary type variables, we don't want (m a) to match (n b)
219 -- if say (a::*) and (b::*->*). This is just a yes/no issue.
221 -- For coercion kinds matters are more complicated. If we have a
222 -- coercion template variable co::a~[b], where a,b are presumably also
223 -- template type variables, then we must match co's kind against the
224 -- kind of the actual argument, so as to give bindings to a,b.
226 -- In fact I have no example in mind that *requires* this kind-matching
227 -- to instantiate template type variables, but it seems like the right
228 -- thing to do. C.f. Note [Matching variable types] in Rules.lhs
231 match_tys menv subst tys1 tys2 = match_list (match menv) subst tys1 tys2
234 match_list :: (TvSubstEnv -> a -> a -> Maybe TvSubstEnv)
235 -> TvSubstEnv -> [a] -> [a] -> Maybe TvSubstEnv
236 match_list fn subst [] [] = Just subst
237 match_list fn subst (ty1:tys1) (ty2:tys2) = do { subst' <- fn subst ty1 ty2
238 ; match_list fn subst' tys1 tys2 }
239 match_list fn subst tys1 tys2 = Nothing
242 match_pred menv subst (ClassP c1 tys1) (ClassP c2 tys2)
243 | c1 == c2 = match_tys menv subst tys1 tys2
244 match_pred menv subst (IParam n1 t1) (IParam n2 t2)
245 | n1 == n2 = match menv subst t1 t2
246 match_pred menv subst p1 p2 = Nothing
250 %************************************************************************
254 %************************************************************************
256 Note [Pruning dead case alternatives]
257 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
258 Consider data T a where
266 type instance F Bool = Int
268 Now consider case x of { T1 -> e1; T2 -> e2 }
270 The question before the house is this: if I know something about the type
271 of x, can I prune away the T1 alternative?
273 Suppose x::T Char. It's impossible to construct a (T Char) using T1,
274 Answer = YES (clearly)
276 Suppose x::T (F a), where 'a' is in scope. Then 'a' might be instantiated
277 to 'Bool', in which case x::T Int, so
278 ANSWER = NO (clearly)
280 Suppose x::T X. Then *in Haskell* it's impossible to construct a (non-bottom)
281 value of type (T X) using T1. But *in FC* it's quite possible. The newtype
284 So (T CoX) :: T X ~ T Int; hence (T1 `cast` sym (T CoX)) is a non-bottom value
285 of type (T X) constructed with T1. Hence
286 ANSWER = NO (surprisingly)
288 Furthermore, this can even happen; see Trac #1251. GHC's newtype-deriving
289 mechanism uses a cast, just as above, to move from one dictionary to another,
290 in effect giving the programmer access to CoX.
292 Finally, suppose x::T Y. Then *even in FC* we can't construct a
293 non-bottom value of type (T Y) using T1. That's because we can get
294 from Y to Char, but not to Int.
297 Here's a related question. data Eq a b where EQ :: Eq a a
299 case x of { EQ -> ... }
301 Suppose x::Eq Int Char. Is the alternative dead? Clearly yes.
303 What about x::Eq Int a, in a context where we have evidence that a~Char.
304 Then again the alternative is dead.
309 We are really doing a test for unsatisfiability of the type
310 constraints implied by the match. And that is clearly, in general, a
313 However, since we are simply dropping dead code, a conservative test
314 suffices. There is a continuum of tests, ranging from easy to hard, that
315 drop more and more dead code.
317 For now we implement a very simple test: type variables match
318 anything, type functions (incl newtypes) match anything, and only
319 distinct data types fail to match. We can elaborate later.
322 dataConCannotMatch :: [Type] -> DataCon -> Bool
323 -- Returns True iff the data con *definitely cannot* match a
324 -- scrutinee of type (T tys)
325 -- where T is the type constructor for the data con
327 dataConCannotMatch tys con
328 | null eq_spec = False -- Common
329 | all isTyVarTy tys = False -- Also common
331 = cant_match_s (map (substTyVar subst . fst) eq_spec)
334 dc_tvs = dataConUnivTyVars con
335 eq_spec = dataConEqSpec con
336 subst = zipTopTvSubst dc_tvs tys
338 cant_match_s :: [Type] -> [Type] -> Bool
339 cant_match_s tys1 tys2 = ASSERT( equalLength tys1 tys2 )
340 or (zipWith cant_match tys1 tys2)
342 cant_match :: Type -> Type -> Bool
344 | Just t1' <- coreView t1 = cant_match t1' t2
345 | Just t2' <- coreView t2 = cant_match t1 t2'
347 cant_match (FunTy a1 r1) (FunTy a2 r2)
348 = cant_match a1 a2 || cant_match r1 r2
350 cant_match (TyConApp tc1 tys1) (TyConApp tc2 tys2)
351 | isDataTyCon tc1 && isDataTyCon tc2
352 = tc1 /= tc2 || cant_match_s tys1 tys2
354 cant_match (FunTy {}) (TyConApp tc _) = isDataTyCon tc
355 cant_match (TyConApp tc _) (FunTy {}) = isDataTyCon tc
356 -- tc can't be FunTyCon by invariant
358 cant_match (AppTy f1 a1) ty2
359 | Just (f2, a2) <- repSplitAppTy_maybe ty2
360 = cant_match f1 f2 || cant_match a1 a2
361 cant_match ty1 (AppTy f2 a2)
362 | Just (f1, a1) <- repSplitAppTy_maybe ty1
363 = cant_match f1 f2 || cant_match a1 a2
365 cant_match ty1 ty2 = False -- Safe!
367 -- Things we could add;
369 -- look through newtypes
370 -- take account of tyvar bindings (EQ example above)