2 % (c) The GRASP/AQUA Project, Glasgow University, 2000
4 \section[FunDeps]{FunDeps - functional dependencies}
6 It's better to read it as: "if we know these, then we're going to know these"
10 Equation, pprEquation, pprEquationDoc,
11 oclose, grow, improve, checkInstFDs, checkClsFD, pprFundeps
14 #include "HsVersions.h"
16 import Name ( getSrcLoc )
17 import Var ( Id, TyVar )
18 import Class ( Class, FunDep, classTvsFds )
19 import Subst ( mkSubst, emptyInScopeSet, substTy )
20 import TcType ( Type, ThetaType, PredType(..),
21 predTyUnique, mkClassPred, tyVarsOfTypes, tyVarsOfPred,
22 unifyTyListsX, unifyExtendTysX, tcEqType
28 import Maybes ( maybeToBool )
29 import ListSetOps ( equivClassesByUniq )
33 %************************************************************************
35 \subsection{Close type variables}
37 %************************************************************************
39 (oclose preds tvs) closes the set of type variables tvs,
40 wrt functional dependencies in preds. The result is a superset
41 of the argument set. For example, if we have
42 class C a b | a->b where ...
44 oclose [C (x,y) z, C (x,p) q] {x,y} = {x,y,z}
45 because if we know x and y then that fixes z.
51 a) When determining ambiguity. The type
52 forall a,b. C a b => a
53 is not ambiguous (given the above class decl for C) because
56 b) When generalising a type T. Usually we take FV(T) \ FV(Env),
59 where the '+' is the oclosure operation. Notice that we do not
60 take FV(T)+. This puzzled me for a bit. Consider
64 and suppose e have that E :: C a b => a, and suppose that b is
65 free in the environment. Then we quantify over 'a' only, giving
66 the type forall a. C a b => a. Since a->b but we don't have b->a,
67 we might have instance decls like
68 instance C Bool Int where ...
69 instance C Char Int where ...
70 so knowing that b=Int doesn't fix 'a'; so we quantify over it.
75 If we have class C a b => D a b where ....
76 class D a b | a -> b where ...
77 and the preds are [C (x,y) z], then we want to see the fd in D,
78 even though it is not explicit in C, giving [({x,y},{z})]
80 Similarly for instance decls? E.g. Suppose we have
81 instance C a b => Eq (T a b) where ...
82 and we infer a type t with constraints Eq (T a b) for a particular
83 expression, and suppose that 'a' is free in the environment.
84 We could generalise to
85 forall b. Eq (T a b) => t
86 but if we reduced the constraint, to C a b, we'd see that 'a' determines
87 b, so that a better type might be
88 t (with free constraint C a b)
89 Perhaps it doesn't matter, because we'll still force b to be a
90 particular type at the call sites. Generalising over too many
91 variables (provided we don't shadow anything by quantifying over a
92 variable that is actually free in the envt) may postpone errors; it
93 won't hide them altogether.
97 oclose :: [PredType] -> TyVarSet -> TyVarSet
98 oclose preds fixed_tvs
99 | null tv_fds = fixed_tvs -- Fast escape hatch for common case
100 | otherwise = loop fixed_tvs
103 | new_fixed_tvs `subVarSet` fixed_tvs = fixed_tvs
104 | otherwise = loop new_fixed_tvs
106 new_fixed_tvs = foldl extend fixed_tvs tv_fds
108 extend fixed_tvs (ls,rs) | ls `subVarSet` fixed_tvs = fixed_tvs `unionVarSet` rs
109 | otherwise = fixed_tvs
111 tv_fds :: [(TyVarSet,TyVarSet)]
112 -- In our example, tv_fds will be [ ({x,y}, {z}), ({x,p},{q}) ]
113 -- Meaning "knowing x,y fixes z, knowing x,p fixes q"
114 tv_fds = [ (tyVarsOfTypes xs, tyVarsOfTypes ys)
115 | ClassP cls tys <- preds, -- Ignore implicit params
116 let (cls_tvs, cls_fds) = classTvsFds cls,
118 let (xs,ys) = instFD fd cls_tvs tys
123 grow :: [PredType] -> TyVarSet -> TyVarSet
125 | null preds = fixed_tvs
126 | otherwise = loop fixed_tvs
129 | new_fixed_tvs `subVarSet` fixed_tvs = fixed_tvs
130 | otherwise = loop new_fixed_tvs
132 new_fixed_tvs = foldl extend fixed_tvs pred_sets
134 extend fixed_tvs pred_tvs
135 | fixed_tvs `intersectsVarSet` pred_tvs = fixed_tvs `unionVarSet` pred_tvs
136 | otherwise = fixed_tvs
138 pred_sets = [tyVarsOfPred pred | pred <- preds]
141 %************************************************************************
143 \subsection{Generate equations from functional dependencies}
145 %************************************************************************
150 type Equation = (TyVarSet, Type, Type) -- These two types should be equal, for some
151 -- substitution of the tyvars in the tyvar set
152 -- To "execute" the equation, make fresh type variable for each tyvar in the set,
153 -- instantiate the two types with these fresh variables, and then unify.
155 -- For example, ({a,b}, (a,Int,b), (Int,z,Bool))
156 -- We unify z with Int, but since a and b are quantified we do nothing to them
157 -- We usually act on an equation by instantiating the quantified type varaibles
158 -- to fresh type variables, and then calling the standard unifier.
160 -- INVARIANT: they aren't already equal
164 pprEquationDoc (eqn, doc) = vcat [pprEquation eqn, nest 2 doc]
166 pprEquation (qtvs, t1, t2) = ptext SLIT("forall") <+> braces (pprWithCommas ppr (varSetElems qtvs))
167 <+> ppr t1 <+> ptext SLIT(":=:") <+> ppr t2
170 improve :: InstEnv Id -- Gives instances for given class
171 -> [(PredType,SDoc)] -- Current constraints; doc says where they come from
172 -> [(Equation,SDoc)] -- Derived equalities that must also hold
173 -- (NB the above INVARIANT for type Equation)
174 -- The SDoc explains why the equation holds (for error messages)
176 type InstEnv a = Class -> [(TyVarSet, [Type], a)]
177 -- This is a bit clumsy, because InstEnv is really
178 -- defined in module InstEnv. However, we don't want
179 -- to define it here because InstEnv
180 -- is their home. Nor do we want to make a recursive
181 -- module group (InstEnv imports stuff from FunDeps).
184 Given a bunch of predicates that must hold, such as
186 C Int t1, C Int t2, C Bool t3, ?x::t4, ?x::t5
188 improve figures out what extra equations must hold.
189 For example, if we have
191 class C a b | a->b where ...
193 then improve will return
199 * improve does not iterate. It's possible that when we make
200 t1=t2, for example, that will in turn trigger a new equation.
201 This would happen if we also had
203 If t1=t2, we also get t7=t8.
205 improve does *not* do this extra step. It relies on the caller
208 * The equations unify types that are not already equal. So there
209 is no effect iff the result of improve is empty
214 improve inst_env preds
215 = [ eqn | group <- equivClassesByUniq (predTyUnique . fst) preds,
216 eqn <- checkGroup inst_env group ]
219 checkGroup :: InstEnv Id -> [(PredType,SDoc)] -> [(Equation, SDoc)]
220 -- The preds are all for the same class or implicit param
222 checkGroup inst_env (p1@(IParam _ ty, _) : ips)
223 = -- For implicit parameters, all the types must match
224 [ ((emptyVarSet, ty, ty'), mkEqnMsg p1 p2)
225 | p2@(IParam _ ty', _) <- ips, not (ty `tcEqType` ty')]
227 checkGroup inst_env clss@((ClassP cls _, _) : _)
228 = -- For classes life is more complicated
229 -- Suppose the class is like
230 -- classs C as | (l1 -> r1), (l2 -> r2), ... where ...
231 -- Then FOR EACH PAIR (ClassP c tys1, ClassP c tys2) in the list clss
233 -- U l1[tys1/as] = U l2[tys2/as]
234 -- (where U is a unifier)
236 -- If so, we return the pair
237 -- U r1[tys1/as] = U l2[tys2/as]
239 -- We need to do something very similar comparing each predicate
240 -- with relevant instance decls
241 pairwise_eqns ++ instance_eqns
244 (cls_tvs, cls_fds) = classTvsFds cls
245 cls_inst_env = inst_env cls
247 -- NOTE that we iterate over the fds first; they are typically
248 -- empty, which aborts the rest of the loop.
249 pairwise_eqns :: [(Equation,SDoc)]
250 pairwise_eqns -- This group comes from pairwise comparison
251 = [ (eqn, mkEqnMsg p1 p2)
253 p1@(ClassP _ tys1, _) : rest <- tails clss,
254 p2@(ClassP _ tys2, _) <- rest,
255 eqn <- checkClsFD emptyVarSet fd cls_tvs tys1 tys2
258 instance_eqns :: [(Equation,SDoc)]
259 instance_eqns -- This group comes from comparing with instance decls
260 = [ (eqn, mkEqnMsg p1 p2)
262 (qtvs, tys1, dfun_id) <- cls_inst_env,
263 let p1 = (mkClassPred cls tys1,
264 ptext SLIT("arising from the instance declaration at") <+> ppr (getSrcLoc dfun_id)),
265 p2@(ClassP _ tys2, _) <- clss,
266 eqn <- checkClsFD qtvs fd cls_tvs tys1 tys2
269 mkEqnMsg (pred1,from1) (pred2,from2)
270 = vcat [ptext SLIT("When using functional dependencies to combine"),
271 nest 2 (sep [ppr pred1 <> comma, nest 2 from1]),
272 nest 2 (sep [ppr pred2 <> comma, nest 2 from2])]
275 checkClsFD :: TyVarSet -- Quantified type variables; see note below
276 -> FunDep TyVar -> [TyVar] -- One functional dependency from the class
280 checkClsFD qtvs fd clas_tvs tys1 tys2
281 -- 'qtvs' are the quantified type variables, the ones which an be instantiated
282 -- to make the types match. For example, given
283 -- class C a b | a->b where ...
284 -- instance C (Maybe x) (Tree x) where ..
286 -- and an Inst of form (C (Maybe t1) t2),
287 -- then we will call checkClsFD with
289 -- qtvs = {x}, tys1 = [Maybe x, Tree x]
290 -- tys2 = [Maybe t1, t2]
292 -- We can instantiate x to t1, and then we want to force
293 -- (Tree x) [t1/x] :=: t2
295 -- We use 'unify' even though we are often only matching
296 -- unifyTyListsX will only bind variables in qtvs, so it's OK!
297 = case unifyTyListsX qtvs ls1 ls2 of
299 Just unif -> -- pprTrace "checkFD" (vcat [ppr_fd fd,
300 -- ppr (varSetElems qtvs) <+> (ppr ls1 $$ ppr ls2),
302 [ (qtvs', substTy full_unif r1, substTy full_unif r2)
303 | (r1,r2) <- rs1 `zip` rs2,
304 not (maybeToBool (unifyExtendTysX qtvs unif r1 r2))]
305 -- Don't include any equations that already hold
306 -- taking account of the fact that any qtvs that aren't
307 -- already instantiated can be instantiated to anything at all
308 -- NB: qtvs, not qtvs' because unifyExtendTysX only tries to
309 -- look template tyvars up in the substitution
311 full_unif = mkSubst emptyInScopeSet unif
312 -- No for-alls in sight; hmm
314 qtvs' = filterVarSet (\v -> not (v `elemSubstEnv` unif)) qtvs
315 -- qtvs' are the quantified type variables
316 -- that have not been substituted out
318 -- Eg. class C a b | a -> b
319 -- instance C Int [y]
320 -- Given constraint C Int z
321 -- we generate the equation
324 (ls1, rs1) = instFD fd clas_tvs tys1
325 (ls2, rs2) = instFD fd clas_tvs tys2
327 instFD :: FunDep TyVar -> [TyVar] -> [Type] -> FunDep Type
328 instFD (ls,rs) tvs tys
329 = (map lookup ls, map lookup rs)
331 env = zipVarEnv tvs tys
332 lookup tv = lookupVarEnv_NF env tv
336 checkInstFDs :: ThetaType -> Class -> [Type] -> Bool
337 -- Check that functional dependencies are obeyed in an instance decl
338 -- For example, if we have
339 -- class theta => C a b | a -> b
341 -- Then we require fv(t2) `subset` oclose(fv(t1), theta)
343 checkInstFDs theta clas inst_taus
346 (tyvars, fds) = classTvsFds clas
347 fundep_ok fd = tyVarsOfTypes rs `subVarSet` oclose theta (tyVarsOfTypes ls)
349 (ls,rs) = instFD fd tyvars inst_taus
352 %************************************************************************
354 \subsection{Miscellaneous}
356 %************************************************************************
359 pprFundeps :: Outputable a => [FunDep a] -> SDoc
360 pprFundeps [] = empty
361 pprFundeps fds = hsep (ptext SLIT("|") : punctuate comma (map ppr_fd fds))
363 ppr_fd (us, vs) = hsep [interppSP us, ptext SLIT("->"), interppSP vs]