\begin{code}
module FunDeps (
- oclose,
- instantiateFdClassTys,
- pprFundeps
+ oclose, grow, improve, checkInstFDs, checkClsFD, pprFundeps
) where
#include "HsVersions.h"
import Var ( TyVar )
import Class ( Class, FunDep, classTvsFds )
-import Type ( Type, tyVarsOfTypes )
+import Type ( Type, PredType(..), predTyUnique, tyVarsOfTypes, tyVarsOfPred )
+import Subst ( mkSubst, emptyInScopeSet, substTy )
+import Unify ( unifyTyListsX )
import Outputable ( Outputable, SDoc, interppSP, ptext, empty, hsep, punctuate, comma )
-import UniqSet
import VarSet
import VarEnv
-import Util ( zipEqual )
+import List ( tails )
+import ListSetOps ( equivClassesByUniq )
\end{code}
+%************************************************************************
+%* *
+\subsection{Close type variables}
+%* *
+%************************************************************************
+
+(oclose preds tvs) closes the set of type variables tvs,
+wrt functional dependencies in preds. The result is a superset
+of the argument set. For example, if we have
+ class C a b | a->b where ...
+then
+ oclose [C (x,y) z, C (x,p) q] {x,y} = {x,y,z}
+because if we know x and y then that fixes z.
+
+Using oclose
+~~~~~~~~~~~~
+oclose is used
+
+a) When determining ambiguity. The type
+ forall a,b. C a b => a
+is not ambiguous (given the above class decl for C) because
+a determines b.
+
+b) When generalising a type T. Usually we take FV(T) \ FV(Env),
+but in fact we need
+ FV(T) \ (FV(Env)+)
+where the '+' is the oclosure operation. Notice that we do not
+take FV(T)+. This puzzled me for a bit. Consider
+
+ f = E
+
+and suppose e have that E :: C a b => a, and suppose that b is
+free in the environment. Then we quantify over 'a' only, giving
+the type forall a. C a b => a. Since a->b but we don't have b->a,
+we might have instance decls like
+ instance C Bool Int where ...
+ instance C Char Int where ...
+so knowing that b=Int doesn't fix 'a'; so we quantify over it.
+
+ ---------------
+ A WORRY: ToDo!
+ ---------------
+If we have class C a b => D a b where ....
+ class D a b | a -> b where ...
+and the preds are [C (x,y) z], then we want to see the fd in D,
+even though it is not explicit in C, giving [({x,y},{z})]
+
+Similarly for instance decls? E.g. Suppose we have
+ instance C a b => Eq (T a b) where ...
+and we infer a type t with constraints Eq (T a b) for a particular
+expression, and suppose that 'a' is free in the environment.
+We could generalise to
+ forall b. Eq (T a b) => t
+but if we reduced the constraint, to C a b, we'd see that 'a' determines
+b, so that a better type might be
+ t (with free constraint C a b)
+Perhaps it doesn't matter, because we'll still force b to be a
+particular type at the call sites. Generalising over too many
+variables (provided we don't shadow anything by quantifying over a
+variable that is actually free in the envt) may postpone errors; it
+won't hide them altogether.
+
+
+\begin{code}
+oclose :: [PredType] -> TyVarSet -> TyVarSet
+oclose preds fixed_tvs
+ | null tv_fds = fixed_tvs -- Fast escape hatch for common case
+ | otherwise = loop fixed_tvs
+ where
+ loop fixed_tvs
+ | new_fixed_tvs `subVarSet` fixed_tvs = fixed_tvs
+ | otherwise = loop new_fixed_tvs
+ where
+ new_fixed_tvs = foldl extend fixed_tvs tv_fds
+
+ extend fixed_tvs (ls,rs) | ls `subVarSet` fixed_tvs = fixed_tvs `unionVarSet` rs
+ | otherwise = fixed_tvs
+
+ tv_fds :: [(TyVarSet,TyVarSet)]
+ -- In our example, tv_fds will be [ ({x,y}, {z}), ({x,p},{q}) ]
+ -- Meaning "knowing x,y fixes z, knowing x,p fixes q"
+ tv_fds = [ (tyVarsOfTypes xs, tyVarsOfTypes ys)
+ | Class cls tys <- preds, -- Ignore implicit params
+ let (cls_tvs, cls_fds) = classTvsFds cls,
+ fd <- cls_fds,
+ let (xs,ys) = instFD fd cls_tvs tys
+ ]
+\end{code}
+
+\begin{code}
+grow :: [PredType] -> TyVarSet -> TyVarSet
+grow preds fixed_tvs
+ | null pred_sets = fixed_tvs
+ | otherwise = loop fixed_tvs
+ where
+ loop fixed_tvs
+ | new_fixed_tvs `subVarSet` fixed_tvs = fixed_tvs
+ | otherwise = loop new_fixed_tvs
+ where
+ new_fixed_tvs = foldl extend fixed_tvs pred_sets
+
+ extend fixed_tvs pred_tvs
+ | fixed_tvs `intersectsVarSet` pred_tvs = fixed_tvs `unionVarSet` pred_tvs
+ | otherwise = fixed_tvs
+
+ pred_sets = [tyVarsOfPred pred | pred <- preds]
+\end{code}
+
+%************************************************************************
+%* *
+\subsection{Generate equations from functional dependencies}
+%* *
+%************************************************************************
+
+
+\begin{code}
+----------
+type Equation = (Type,Type) -- These two types should be equal
+ -- INVARIANT: they aren't already equal
+
+----------
+improve :: InstEnv a -- Gives instances for given class
+ -> [PredType] -- Current constraints
+ -> [Equation] -- Derived equalities that must also hold
+ -- (NB the above INVARIANT for type Equation)
+
+type InstEnv a = Class -> [(TyVarSet, [Type], a)]
+-- This is a bit clumsy, because InstEnv is really
+-- defined in module InstEnv. However, we don't want
+-- to define it (and ClsInstEnv) here because InstEnv
+-- is their home. Nor do we want to make a recursive
+-- module group (InstEnv imports stuff from FunDeps).
+\end{code}
+
+Given a bunch of predicates that must hold, such as
+
+ C Int t1, C Int t2, C Bool t3, ?x::t4, ?x::t5
+
+improve figures out what extra equations must hold.
+For example, if we have
+
+ class C a b | a->b where ...
+
+then improve will return
+
+ [(t1,t2), (t4,t5)]
+
+NOTA BENE:
+
+ * improve does not iterate. It's possible that when we make
+ t1=t2, for example, that will in turn trigger a new equation.
+ This would happen if we also had
+ C t1 t7, C t2 t8
+ If t1=t2, we also get t7=t8.
+
+ improve does *not* do this extra step. It relies on the caller
+ doing so.
+
+ * The equations unify types that are not already equal. So there
+ is no effect iff the result of improve is empty
+
+
+
\begin{code}
-oclose :: [FunDep Type] -> TyVarSet -> TyVarSet
--- (oclose fds tvs) closes the set of type variables tvs,
--- wrt the functional dependencies fds. The result is a superset
--- of the argument set.
---
--- In fact the functional dependencies are *instantiated*, so we
--- first have to extract the free vars.
---
--- For example,
--- oclose [a -> b] {a} = {a,b}
--- oclose [a b -> c] {a} = {a}
--- oclose [a b -> c] {a,b} = {a,b,c}
--- If all of the things on the left of an arrow are in the set, add
--- the things on the right of that arrow.
-
-oclose fds vs
- = go vs
+improve inst_env preds
+ = [ eqn | group <- equivClassesByUniq predTyUnique preds,
+ eqn <- checkGroup inst_env group ]
+
+----------
+checkGroup :: InstEnv a -> [PredType] -> [Equation]
+ -- The preds are all for the same class or implicit param
+
+checkGroup inst_env (IParam _ ty : ips)
+ = -- For implicit parameters, all the types must match
+ [(ty, ty') | IParam _ ty' <- ips, ty /= ty']
+
+checkGroup inst_env clss@(Class cls tys : _)
+ = -- For classes life is more complicated
+ -- Suppose the class is like
+ -- classs C as | (l1 -> r1), (l2 -> r2), ... where ...
+ -- Then FOR EACH PAIR (Class c tys1, Class c tys2) in the list clss
+ -- we check whether
+ -- U l1[tys1/as] = U l2[tys2/as]
+ -- (where U is a unifier)
+ --
+ -- If so, we return the pair
+ -- U r1[tys1/as] = U l2[tys2/as]
+ --
+ -- We need to do something very similar comparing each predicate
+ -- with relevant instance decls
+ pairwise_eqns ++ instance_eqns
+
where
- go vs = case oclose1 tv_fds vs of
- (vs', False) -> vs'
- (vs', True) -> go vs'
-
- tv_fds :: [FunDep TyVar]
- tv_fds = [(get_tvs xs, get_tvs ys) | (xs, ys) <- fds]
- get_tvs = varSetElems . tyVarsOfTypes
-
-oclose1 [] vs = (vs, False)
-oclose1 (fd@(ls, rs):fds) vs =
- if osubset ls vs then
- (vs'', b1 || b2)
- else
- vs'b1
- where
- vs'b1@(vs', b1) = oclose1 fds vs
- (vs'', b2) = ounion rs vs'
-
-osubset [] vs = True
-osubset (u:us) vs = if u `elementOfUniqSet` vs then osubset us vs else False
-
-ounion [] ys = (ys, False)
-ounion (x:xs) ys
- | x `elementOfUniqSet` ys = (ys', b)
- | otherwise = (addOneToUniqSet ys' x, True)
- where
- (ys', b) = ounion xs ys
-
-instantiateFdClassTys :: Class -> [Type] -> [FunDep Type]
--- Get the FDs of the class, and instantiate them
-instantiateFdClassTys clas tys
- = [(map lookup us, map lookup vs) | (us,vs) <- fundeps]
+ (cls_tvs, cls_fds) = classTvsFds cls
+ cls_inst_env = inst_env cls
+
+ -- NOTE that we iterate over the fds first; they are typically
+ -- empty, which aborts the rest of the loop.
+ pairwise_eqns :: [(Type,Type)]
+ pairwise_eqns -- This group comes from pairwise comparison
+ = [ eqn | fd <- cls_fds,
+ Class _ tys1 : rest <- tails clss,
+ Class _ tys2 <- rest,
+ eqn <- checkClsFD emptyVarSet fd cls_tvs tys1 tys2
+ ]
+
+ instance_eqns :: [(Type,Type)]
+ instance_eqns -- This group comes from comparing with instance decls
+ = [ eqn | fd <- cls_fds,
+ (qtvs, tys1, _) <- cls_inst_env,
+ Class _ tys2 <- clss,
+ eqn <- checkClsFD qtvs fd cls_tvs tys1 tys2
+ ]
+
+
+----------
+checkClsFD :: TyVarSet
+ -> FunDep TyVar -> [TyVar] -- One functional dependency from the class
+ -> [Type] -> [Type]
+ -> [Equation]
+
+checkClsFD qtvs fd clas_tvs tys1 tys2
+-- We use 'unify' even though we are often only matching
+-- unifyTyListsX will only bind variables in qtvs, so it's OK!
+ = case unifyTyListsX qtvs ls1 ls2 of
+ Nothing -> []
+ Just unif -> [(sr1, sr2) | (r1,r2) <- rs1 `zip` rs2,
+ let sr1 = substTy full_unif r1,
+ let sr2 = substTy full_unif r2,
+ sr1 /= sr2]
+ where
+ full_unif = mkSubst emptyInScopeSet unif
+ -- No for-alls in sight; hmm
+ where
+ (ls1, rs1) = instFD fd clas_tvs tys1
+ (ls2, rs2) = instFD fd clas_tvs tys2
+
+instFD :: FunDep TyVar -> [TyVar] -> [Type] -> FunDep Type
+instFD (ls,rs) tvs tys
+ = (map lookup ls, map lookup rs)
where
- (tyvars, fundeps) = classTvsFds clas
- env = mkVarEnv (zipEqual "instantiateFdClassTys" tyvars tys)
+ env = zipVarEnv tvs tys
lookup tv = lookupVarEnv_NF env tv
+\end{code}
+\begin{code}
+checkInstFDs :: Class -> [Type] -> Bool
+-- Check that functional dependencies are obeyed in an instance decl
+-- For example, if we have
+-- class C a b | a -> b
+-- instance C t1 t2
+-- Then we require fv(t2) `subset` fv(t1)
+checkInstFDs clas inst_taus
+ = all fundep_ok fds
+ where
+ (tyvars, fds) = classTvsFds clas
+ fundep_ok fd = tyVarsOfTypes rs `subVarSet` tyVarsOfTypes ls
+ where
+ (ls,rs) = instFD fd tyvars inst_taus
+\end{code}
+
+%************************************************************************
+%* *
+\subsection{Miscellaneous}
+%* *
+%************************************************************************
+
+\begin{code}
pprFundeps :: Outputable a => [FunDep a] -> SDoc
pprFundeps [] = empty
pprFundeps fds = hsep (ptext SLIT("|") : punctuate comma (map ppr_fd fds))
projects / ghc-hetmet.git / blobdiff