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"
38 %************************************************************************
42 %************************************************************************
45 Matching is much tricker than you might think.
47 1. The substitution we generate binds the *template type variables*
48 which are given to us explicitly.
50 2. We want to match in the presence of foralls;
51 e.g (forall a. t1) ~ (forall b. t2)
53 That is what the RnEnv2 is for; it does the alpha-renaming
54 that makes it as if a and b were the same variable.
55 Initialising the RnEnv2, so that it can generate a fresh
56 binder when necessary, entails knowing the free variables of
59 3. We must be careful not to bind a template type variable to a
60 locally bound variable. E.g.
61 (forall a. x) ~ (forall b. b)
62 where x is the template type variable. Then we do not want to
63 bind x to a/b! This is a kind of occurs check.
64 The necessary locals accumulate in the RnEnv2.
69 = ME { me_tmpls :: VarSet -- Template tyvars
70 , me_env :: RnEnv2 -- Renaming envt for nested foralls
71 } -- In-scope set includes template tyvars
73 tcMatchTy :: TyVarSet -- Template tyvars
76 -> Maybe TvSubst -- One-shot; in principle the template
77 -- variables could be free in the target
79 tcMatchTy tmpls ty1 ty2
80 = case match menv emptyTvSubstEnv ty1 ty2 of
81 Just subst_env -> Just (TvSubst in_scope subst_env)
84 menv = ME { me_tmpls = tmpls, me_env = mkRnEnv2 in_scope }
85 in_scope = mkInScopeSet (tmpls `unionVarSet` tyVarsOfType ty2)
86 -- We're assuming that all the interesting
87 -- tyvars in tys1 are in tmpls
89 tcMatchTys :: TyVarSet -- Template tyvars
92 -> Maybe TvSubst -- One-shot; in principle the template
93 -- variables could be free in the target
95 tcMatchTys tmpls tys1 tys2
96 = case match_tys menv emptyTvSubstEnv tys1 tys2 of
97 Just subst_env -> Just (TvSubst in_scope subst_env)
100 menv = ME { me_tmpls = tmpls, me_env = mkRnEnv2 in_scope }
101 in_scope = mkInScopeSet (tmpls `unionVarSet` tyVarsOfTypes tys2)
102 -- We're assuming that all the interesting
103 -- tyvars in tys1 are in tmpls
105 -- This is similar, but extends a substitution
106 tcMatchTyX :: TyVarSet -- Template tyvars
107 -> TvSubst -- Substitution to extend
111 tcMatchTyX tmpls (TvSubst in_scope subst_env) ty1 ty2
112 = case match menv subst_env ty1 ty2 of
113 Just subst_env -> Just (TvSubst in_scope subst_env)
116 menv = ME {me_tmpls = tmpls, me_env = mkRnEnv2 in_scope}
119 :: [TyVar] -- Bind these
120 -> [PredType] -> [PredType]
122 tcMatchPreds tmpls ps1 ps2
123 = match_list (match_pred menv) emptyTvSubstEnv ps1 ps2
125 menv = ME { me_tmpls = mkVarSet tmpls, me_env = mkRnEnv2 in_scope_tyvars }
126 in_scope_tyvars = mkInScopeSet (tyVarsOfTheta ps1 `unionVarSet` tyVarsOfTheta ps2)
128 -- This one is called from the expression matcher, which already has a MatchEnv in hand
129 ruleMatchTyX :: MatchEnv
130 -> TvSubstEnv -- Substitution to extend
135 ruleMatchTyX menv subst ty1 ty2 = match menv subst ty1 ty2 -- Rename for export
138 Now the internals of matching
141 match :: MatchEnv -- For the most part this is pushed downwards
142 -> TvSubstEnv -- Substitution so far:
143 -- Domain is subset of template tyvars
144 -- Free vars of range is subset of
145 -- in-scope set of the RnEnv2
146 -> Type -> Type -- Template and target respectively
148 -- This matcher works on source types; that is,
149 -- it respects NewTypes and PredType
151 match menv subst ty1 ty2 | Just ty1' <- tcView ty1 = match menv subst ty1' ty2
152 | Just ty2' <- tcView ty2 = match menv subst ty1 ty2'
154 match menv subst (TyVarTy tv1) ty2
155 | tv1' `elemVarSet` me_tmpls menv
156 = case lookupVarEnv subst tv1' of
157 Nothing -- No existing binding
158 | any (inRnEnvR rn_env) (varSetElems (tyVarsOfType ty2))
159 -> Nothing -- Occurs check
160 | not (typeKind ty2 `isSubKind` tyVarKind tv1)
161 -> Nothing -- Kind mis-match
163 -> Just (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_tys menv subst tys1 tys2 = match_list (match menv) subst tys1 tys2
206 match_list :: (TvSubstEnv -> a -> a -> Maybe TvSubstEnv)
207 -> TvSubstEnv -> [a] -> [a] -> Maybe TvSubstEnv
208 match_list fn subst [] [] = Just subst
209 match_list fn subst (ty1:tys1) (ty2:tys2) = do { subst' <- fn subst ty1 ty2
210 ; match_list fn subst' tys1 tys2 }
211 match_list fn subst tys1 tys2 = Nothing
214 match_pred menv subst (ClassP c1 tys1) (ClassP c2 tys2)
215 | c1 == c2 = match_tys menv subst tys1 tys2
216 match_pred menv subst (IParam n1 t1) (IParam n2 t2)
217 | n1 == n2 = match menv subst t1 t2
218 match_pred menv subst p1 p2 = Nothing
222 %************************************************************************
226 %************************************************************************
228 Note [Pruning dead case alternatives]
229 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
230 Consider data T a where
238 type instance F Bool = Int
240 Now consider case x of { T1 -> e1; T2 -> e2 }
242 The question before the house is this: if I know something about the type
243 of x, can I prune away the T1 alternative?
245 Suppose x::T Char. It's impossible to construct a (T Char) using T1,
246 Answer = YES (clearly)
248 Suppose x::T (F a), where 'a' is in scope. Then 'a' might be instantiated
249 to 'Bool', in which case x::T Int, so
250 ANSWER = NO (clearly)
252 Suppose x::T X. Then *in Haskell* it's impossible to construct a (non-bottom)
253 value of type (T X) using T1. But *in FC* it's quite possible. The newtype
256 So (T CoX) :: T X ~ T Int; hence (T1 `cast` sym (T CoX)) is a non-bottom value
257 of type (T X) constructed with T1. Hence
258 ANSWER = NO (surprisingly)
260 Furthermore, this can even happen; see Trac #1251. GHC's newtype-deriving
261 mechanism uses a cast, just as above, to move from one dictionary to another,
262 in effect giving the programmer access to CoX.
264 Finally, suppose x::T Y. Then *even in FC* we can't construct a
265 non-bottom value of type (T Y) using T1. That's because we can get
266 from Y to Char, but not to Int.
269 Here's a related question. data Eq a b where EQ :: Eq a a
271 case x of { EQ -> ... }
273 Suppose x::Eq Int Char. Is the alternative dead? Clearly yes.
275 What about x::Eq Int a, in a context where we have evidence that a~Char.
276 Then again the alternative is dead.
281 We are really doing a test for unsatisfiability of the type
282 constraints implied by the match. And that is clearly, in general, a
285 However, since we are simply dropping dead code, a conservative test
286 suffices. There is a continuum of tests, ranging from easy to hard, that
287 drop more and more dead code.
289 For now we implement a very simple test: type variables match
290 anything, type functions (incl newtypes) match anything, and only
291 distinct data types fail to match. We can elaborate later.
294 dataConCannotMatch :: [Type] -> DataCon -> Bool
295 -- Returns True iff the data con *definitely cannot* match a
296 -- scrutinee of type (T tys)
297 -- where T is the type constructor for the data con
299 dataConCannotMatch tys con
300 | null eq_spec = False -- Common
301 | all isTyVarTy tys = False -- Also common
303 = cant_match_s (map (substTyVar subst . fst) eq_spec)
306 dc_tvs = dataConUnivTyVars con
307 eq_spec = dataConEqSpec con
308 subst = zipTopTvSubst dc_tvs tys
310 cant_match_s :: [Type] -> [Type] -> Bool
311 cant_match_s tys1 tys2 = ASSERT( equalLength tys1 tys2 )
312 or (zipWith cant_match tys1 tys2)
314 cant_match :: Type -> Type -> Bool
316 | Just t1' <- coreView t1 = cant_match t1' t2
317 | Just t2' <- coreView t2 = cant_match t1 t2'
319 cant_match (FunTy a1 r1) (FunTy a2 r2)
320 = cant_match a1 a2 || cant_match r1 r2
322 cant_match (TyConApp tc1 tys1) (TyConApp tc2 tys2)
323 | isDataTyCon tc1 && isDataTyCon tc2
324 = tc1 /= tc2 || cant_match_s tys1 tys2
326 cant_match (FunTy {}) (TyConApp tc _) = isDataTyCon tc
327 cant_match (TyConApp tc _) (FunTy {}) = isDataTyCon tc
328 -- tc can't be FunTyCon by invariant
330 cant_match (AppTy f1 a1) ty2
331 | Just (f2, a2) <- repSplitAppTy_maybe ty2
332 = cant_match f1 f2 || cant_match a1 a2
333 cant_match ty1 (AppTy f2 a2)
334 | Just (f1, a1) <- repSplitAppTy_maybe ty1
335 = cant_match f1 f2 || cant_match a1 a2
337 cant_match ty1 ty2 = False -- Safe!
339 -- Things we could add;
341 -- look through newtypes
342 -- take account of tyvar bindings (EQ example above)