{- (c) The University of Glasgow 2006 (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 \section[InstEnv]{Utilities for typechecking instance declarations} The bits common to GHC.Tc.TyCl.Instance and GHC.Tc.Deriv. -} {-# LANGUAGE CPP, DeriveDataTypeable #-} module GHC.Core.InstEnv ( DFunId, InstMatch, ClsInstLookupResult, OverlapFlag(..), OverlapMode(..), setOverlapModeMaybe, ClsInst(..), DFunInstType, pprInstance, pprInstanceHdr, pprInstances, instanceHead, instanceSig, mkLocalInstance, mkImportedInstance, instanceDFunId, updateClsInstDFun, instanceRoughTcs, fuzzyClsInstCmp, orphNamesOfClsInst, InstEnvs(..), VisibleOrphanModules, InstEnv, emptyInstEnv, extendInstEnv, deleteFromInstEnv, deleteDFunFromInstEnv, identicalClsInstHead, extendInstEnvList, lookupUniqueInstEnv, lookupInstEnv, instEnvElts, instEnvClasses, memberInstEnv, instIsVisible, classInstances, instanceBindFun, instanceCantMatch, roughMatchTcs, isOverlappable, isOverlapping, isIncoherent ) where #include "HsVersions.h" import GHC.Prelude import GHC.Tc.Utils.TcType -- InstEnv is really part of the type checker, -- and depends on TcType in many ways import GHC.Core ( IsOrphan(..), isOrphan, chooseOrphanAnchor ) import GHC.Unit.Module.Env import GHC.Unit.Types import GHC.Core.Class import GHC.Types.Var import GHC.Types.Var.Set import GHC.Types.Name import GHC.Types.Name.Set import GHC.Types.Unique (getUnique) import GHC.Core.Unify import GHC.Types.Basic import GHC.Types.Unique.DFM import GHC.Types.Id import Data.Data ( Data ) import Data.Maybe ( isJust ) import GHC.Utils.Misc import GHC.Utils.Outputable import GHC.Utils.Panic {- ************************************************************************ * * ClsInst: the data type for type-class instances * * ************************************************************************ -} -- | A type-class instance. Note that there is some tricky laziness at work -- here. See Note [ClsInst laziness and the rough-match fields] for more -- details. data ClsInst = ClsInst { -- Used for "rough matching"; see -- Note [ClsInst laziness and the rough-match fields] -- INVARIANT: is_tcs = roughMatchTcs is_tys is_cls_nm :: Name -- ^ Class name , is_tcs :: [RoughMatchTc] -- ^ Top of type args -- | @is_dfun_name = idName . is_dfun@. -- -- We use 'is_dfun_name' for the visibility check, -- 'instIsVisible', which needs to know the 'Module' which the -- dictionary is defined in. However, we cannot use the 'Module' -- attached to 'is_dfun' since doing so would mean we would -- potentially pull in an entire interface file unnecessarily. -- This was the cause of #12367. , is_dfun_name :: Name -- Used for "proper matching"; see Note [Proper-match fields] , is_tvs :: [TyVar] -- Fresh template tyvars for full match -- See Note [Template tyvars are fresh] , is_cls :: Class -- The real class , is_tys :: [Type] -- Full arg types (mentioning is_tvs) -- INVARIANT: is_dfun Id has type -- forall is_tvs. (...) => is_cls is_tys -- (modulo alpha conversion) , is_dfun :: DFunId -- See Note [Haddock assumptions] , is_flag :: OverlapFlag -- See detailed comments with -- the decl of BasicTypes.OverlapFlag , is_orphan :: IsOrphan } deriving Data -- | A fuzzy comparison function for class instances, intended for sorting -- instances before displaying them to the user. fuzzyClsInstCmp :: ClsInst -> ClsInst -> Ordering fuzzyClsInstCmp x y = stableNameCmp (is_cls_nm x) (is_cls_nm y) `mappend` mconcat (map cmp (zip (is_tcs x) (is_tcs y))) where cmp (OtherTc, OtherTc) = EQ cmp (OtherTc, KnownTc _) = LT cmp (KnownTc _, OtherTc) = GT cmp (KnownTc x, KnownTc y) = stableNameCmp x y isOverlappable, isOverlapping, isIncoherent :: ClsInst -> Bool isOverlappable i = hasOverlappableFlag (overlapMode (is_flag i)) isOverlapping i = hasOverlappingFlag (overlapMode (is_flag i)) isIncoherent i = hasIncoherentFlag (overlapMode (is_flag i)) {- Note [ClsInst laziness and the rough-match fields] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Suppose we load 'instance A.C B.T' from A.hi, but suppose that the type B.T is otherwise unused in the program. Then it's stupid to load B.hi, the data type declaration for B.T -- and perhaps further instance declarations! We avoid this as follows: * is_cls_nm, is_tcs, is_dfun_name are all Names. We can poke them to our heart's content. * Proper-match fields. is_dfun, and its related fields is_tvs, is_cls, is_tys contain TyVars, Class, Type, Class etc, and so are all lazy thunks. When we poke any of these fields we'll typecheck the DFunId declaration, and hence pull in interfaces that it refers to. See Note [Proper-match fields]. * Rough-match fields. During instance lookup, we use the is_cls_nm :: Name and is_tcs :: [RoughMatchTc] fields to perform a "rough match", *without* poking inside the DFunId. The rough-match fields allow us to say "definitely does not match", based only on Names. See GHC.Core.Unify Note [Rough matching in class and family instances] This laziness is very important; see #12367. Try hard to avoid pulling on the structured fields unless you really need the instance. * Another place to watch is InstEnv.instIsVisible, which needs the module to which the ClsInst belongs. We can get this from is_dfun_name. -} {- Note [Template tyvars are fresh] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The is_tvs field of a ClsInst has *completely fresh* tyvars. That is, they are * distinct from any other ClsInst * distinct from any tyvars free in predicates that may be looked up in the class instance environment Reason for freshness: we use unification when checking for overlap etc, and that requires the tyvars to be distinct. The invariant is checked by the ASSERT in lookupInstEnv'. Note [Proper-match fields] ~~~~~~~~~~~~~~~~~~~~~~~~~ The is_tvs, is_cls, is_tys fields are simply cached values, pulled out (lazily) from the dfun id. They are cached here simply so that we don't need to decompose the DFunId each time we want to match it. The hope is that the rough-match fields mean that we often never poke the proper-match fields. However, note that: * is_tvs must be a superset of the free vars of is_tys * is_tvs, is_tys may be alpha-renamed compared to the ones in the dfun Id Note [Haddock assumptions] ~~~~~~~~~~~~~~~~~~~~~~~~~~ For normal user-written instances, Haddock relies on * the SrcSpan of * the Name of * the is_dfun of * an Instance being equal to * the SrcSpan of * the instance head type of * the InstDecl used to construct the Instance. -} instanceDFunId :: ClsInst -> DFunId instanceDFunId = is_dfun updateClsInstDFun :: (DFunId -> DFunId) -> ClsInst -> ClsInst updateClsInstDFun tidy_dfun ispec = ispec { is_dfun = tidy_dfun (is_dfun ispec) } instanceRoughTcs :: ClsInst -> [RoughMatchTc] instanceRoughTcs = is_tcs instance NamedThing ClsInst where getName ispec = getName (is_dfun ispec) instance Outputable ClsInst where ppr = pprInstance pprInstance :: ClsInst -> SDoc -- Prints the ClsInst as an instance declaration pprInstance ispec = hang (pprInstanceHdr ispec) 2 (vcat [ text "--" <+> pprDefinedAt (getName ispec) , whenPprDebug (ppr (is_dfun ispec)) ]) -- * pprInstanceHdr is used in VStudio to populate the ClassView tree pprInstanceHdr :: ClsInst -> SDoc -- Prints the ClsInst as an instance declaration pprInstanceHdr (ClsInst { is_flag = flag, is_dfun = dfun }) = text "instance" <+> ppr flag <+> pprSigmaType (idType dfun) pprInstances :: [ClsInst] -> SDoc pprInstances ispecs = vcat (map pprInstance ispecs) instanceHead :: ClsInst -> ([TyVar], Class, [Type]) -- Returns the head, using the fresh tyvars from the ClsInst instanceHead (ClsInst { is_tvs = tvs, is_tys = tys, is_dfun = dfun }) = (tvs, cls, tys) where (_, _, cls, _) = tcSplitDFunTy (idType dfun) -- | Collects the names of concrete types and type constructors that make -- up the head of a class instance. For instance, given `class Foo a b`: -- -- `instance Foo (Either (Maybe Int) a) Bool` would yield -- [Either, Maybe, Int, Bool] -- -- Used in the implementation of ":info" in GHCi. -- -- The 'tcSplitSigmaTy' is because of -- instance Foo a => Baz T where ... -- The decl is an orphan if Baz and T are both not locally defined, -- even if Foo *is* locally defined orphNamesOfClsInst :: ClsInst -> NameSet orphNamesOfClsInst (ClsInst { is_cls_nm = cls_nm, is_tys = tys }) = orphNamesOfTypes tys `unionNameSet` unitNameSet cls_nm instanceSig :: ClsInst -> ([TyVar], [Type], Class, [Type]) -- Decomposes the DFunId instanceSig ispec = tcSplitDFunTy (idType (is_dfun ispec)) mkLocalInstance :: DFunId -> OverlapFlag -> [TyVar] -> Class -> [Type] -> ClsInst -- Used for local instances, where we can safely pull on the DFunId. -- Consider using newClsInst instead; this will also warn if -- the instance is an orphan. mkLocalInstance dfun oflag tvs cls tys = ClsInst { is_flag = oflag, is_dfun = dfun , is_tvs = tvs , is_dfun_name = dfun_name , is_cls = cls, is_cls_nm = cls_name , is_tys = tys, is_tcs = roughMatchTcs tys , is_orphan = orph } where cls_name = className cls dfun_name = idName dfun this_mod = ASSERT( isExternalName dfun_name ) nameModule dfun_name is_local name = nameIsLocalOrFrom this_mod name -- Compute orphanhood. See Note [Orphans] in GHC.Core.InstEnv (cls_tvs, fds) = classTvsFds cls arg_names = [filterNameSet is_local (orphNamesOfType ty) | ty <- tys] -- See Note [When exactly is an instance decl an orphan?] orph | is_local cls_name = NotOrphan (nameOccName cls_name) | all notOrphan mb_ns = ASSERT( not (null mb_ns) ) head mb_ns | otherwise = IsOrphan notOrphan NotOrphan{} = True notOrphan _ = False mb_ns :: [IsOrphan] -- One for each fundep; a locally-defined name -- that is not in the "determined" arguments mb_ns | null fds = [choose_one arg_names] | otherwise = map do_one fds do_one (_ltvs, rtvs) = choose_one [ns | (tv,ns) <- cls_tvs `zip` arg_names , not (tv `elem` rtvs)] choose_one nss = chooseOrphanAnchor (unionNameSets nss) mkImportedInstance :: Name -- ^ the name of the class -> [RoughMatchTc] -- ^ the types which the class was applied to -> Name -- ^ the 'Name' of the dictionary binding -> DFunId -- ^ the 'Id' of the dictionary. -> OverlapFlag -- ^ may this instance overlap? -> IsOrphan -- ^ is this instance an orphan? -> ClsInst -- Used for imported instances, where we get the rough-match stuff -- from the interface file -- The bound tyvars of the dfun are guaranteed fresh, because -- the dfun has been typechecked out of the same interface file mkImportedInstance cls_nm mb_tcs dfun_name dfun oflag orphan = ClsInst { is_flag = oflag, is_dfun = dfun , is_tvs = tvs, is_tys = tys , is_dfun_name = dfun_name , is_cls_nm = cls_nm, is_cls = cls, is_tcs = mb_tcs , is_orphan = orphan } where (tvs, _, cls, tys) = tcSplitDFunTy (idType dfun) {- Note [When exactly is an instance decl an orphan?] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ (see GHC.Iface.Make.instanceToIfaceInst, which implements this) Roughly speaking, an instance is an orphan if its head (after the =>) mentions nothing defined in this module. Functional dependencies complicate the situation though. Consider module M where { class C a b | a -> b } and suppose we are compiling module X: module X where import M data T = ... instance C Int T where ... This instance is an orphan, because when compiling a third module Y we might get a constraint (C Int v), and we'd want to improve v to T. So we must make sure X's instances are loaded, even if we do not directly use anything from X. More precisely, an instance is an orphan iff If there are no fundeps, then at least of the names in the instance head is locally defined. If there are fundeps, then for every fundep, at least one of the names free in a *non-determined* part of the instance head is defined in this module. (Note that these conditions hold trivially if the class is locally defined.) ************************************************************************ * * InstEnv, ClsInstEnv * * ************************************************************************ A @ClsInstEnv@ all the instances of that class. The @Id@ inside a ClsInstEnv mapping is the dfun for that instance. If class C maps to a list containing the item ([a,b], [t1,t2,t3], dfun), then forall a b, C t1 t2 t3 can be constructed by dfun or, to put it another way, we have instance (...) => C t1 t2 t3, witnessed by dfun -} --------------------------------------------------- {- Note [InstEnv determinism] ~~~~~~~~~~~~~~~~~~~~~~~~~~ We turn InstEnvs into a list in some places that don't directly affect the ABI. That happens when we create output for `:info`. Unfortunately that nondeterminism is nonlocal and it's hard to tell what it affects without following a chain of functions. It's also easy to accidentally make that nondeterminism affect the ABI. Furthermore the envs should be relatively small, so it should be free to use deterministic maps here. Testing with nofib and validate detected no difference between UniqFM and UniqDFM. See also Note [Deterministic UniqFM] -} -- Internally it's safe to indexable this map by -- by @Class@, the classes @Name@, the classes @TyCon@ -- or it's @Unique@. -- This is since: -- getUnique cls == getUnique (className cls) == getUnique (classTyCon cls) -- -- We still use Class as key type as it's both the common case -- and conveys the meaning better. But the implementation of --InstEnv is a bit more lax internally. type InstEnv = UniqDFM Class ClsInstEnv -- Maps Class to instances for that class -- See Note [InstEnv determinism] -- | 'InstEnvs' represents the combination of the global type class instance -- environment, the local type class instance environment, and the set of -- transitively reachable orphan modules (according to what modules have been -- directly imported) used to test orphan instance visibility. data InstEnvs = InstEnvs { ie_global :: InstEnv, -- External-package instances ie_local :: InstEnv, -- Home-package instances ie_visible :: VisibleOrphanModules -- Set of all orphan modules transitively -- reachable from the module being compiled -- See Note [Instance lookup and orphan instances] } -- | Set of visible orphan modules, according to what modules have been directly -- imported. This is based off of the dep_orphs field, which records -- transitively reachable orphan modules (modules that define orphan instances). type VisibleOrphanModules = ModuleSet newtype ClsInstEnv = ClsIE [ClsInst] -- The instances for a particular class, in any order instance Outputable ClsInstEnv where ppr (ClsIE is) = pprInstances is -- INVARIANTS: -- * The is_tvs are distinct in each ClsInst -- of a ClsInstEnv (so we can safely unify them) -- Thus, the @ClassInstEnv@ for @Eq@ might contain the following entry: -- [a] ===> dfun_Eq_List :: forall a. Eq a => Eq [a] -- The "a" in the pattern must be one of the forall'd variables in -- the dfun type. emptyInstEnv :: InstEnv emptyInstEnv = emptyUDFM instEnvElts :: InstEnv -> [ClsInst] instEnvElts ie = [elt | ClsIE elts <- eltsUDFM ie, elt <- elts] -- See Note [InstEnv determinism] instEnvClasses :: InstEnv -> [Class] instEnvClasses ie = [is_cls e | ClsIE (e : _) <- eltsUDFM ie] -- | Test if an instance is visible, by checking that its origin module -- is in 'VisibleOrphanModules'. -- See Note [Instance lookup and orphan instances] instIsVisible :: VisibleOrphanModules -> ClsInst -> Bool instIsVisible vis_mods ispec -- NB: Instances from the interactive package always are visible. We can't -- add interactive modules to the set since we keep creating new ones -- as a GHCi session progresses. = case nameModule_maybe (is_dfun_name ispec) of Nothing -> True Just mod | isInteractiveModule mod -> True | IsOrphan <- is_orphan ispec -> mod `elemModuleSet` vis_mods | otherwise -> True classInstances :: InstEnvs -> Class -> [ClsInst] classInstances (InstEnvs { ie_global = pkg_ie, ie_local = home_ie, ie_visible = vis_mods }) cls = get home_ie ++ get pkg_ie where get env = case lookupUDFM env cls of Just (ClsIE insts) -> filter (instIsVisible vis_mods) insts Nothing -> [] -- | Checks for an exact match of ClsInst in the instance environment. -- We use this when we do signature checking in "GHC.Tc.Module" memberInstEnv :: InstEnv -> ClsInst -> Bool memberInstEnv inst_env ins_item@(ClsInst { is_cls_nm = cls_nm } ) = maybe False (\(ClsIE items) -> any (identicalDFunType ins_item) items) (lookupUDFM_Directly inst_env (getUnique cls_nm)) where identicalDFunType cls1 cls2 = eqType (varType (is_dfun cls1)) (varType (is_dfun cls2)) extendInstEnvList :: InstEnv -> [ClsInst] -> InstEnv extendInstEnvList inst_env ispecs = foldl' extendInstEnv inst_env ispecs extendInstEnv :: InstEnv -> ClsInst -> InstEnv extendInstEnv inst_env ins_item@(ClsInst { is_cls_nm = cls_nm }) = addToUDFM_C_Directly add inst_env (getUnique cls_nm) (ClsIE [ins_item]) where add (ClsIE cur_insts) _ = ClsIE (ins_item : cur_insts) deleteFromInstEnv :: InstEnv -> ClsInst -> InstEnv deleteFromInstEnv inst_env ins_item@(ClsInst { is_cls_nm = cls_nm }) = adjustUDFM_Directly adjust inst_env (getUnique cls_nm) where adjust (ClsIE items) = ClsIE (filterOut (identicalClsInstHead ins_item) items) deleteDFunFromInstEnv :: InstEnv -> DFunId -> InstEnv -- Delete a specific instance fron an InstEnv deleteDFunFromInstEnv inst_env dfun = adjustUDFM adjust inst_env cls where (_, _, cls, _) = tcSplitDFunTy (idType dfun) adjust (ClsIE items) = ClsIE (filterOut same_dfun items) same_dfun (ClsInst { is_dfun = dfun' }) = dfun == dfun' identicalClsInstHead :: ClsInst -> ClsInst -> Bool -- ^ True when when the instance heads are the same -- e.g. both are Eq [(a,b)] -- Used for overriding in GHCi -- Obviously should be insensitive to alpha-renaming identicalClsInstHead (ClsInst { is_cls_nm = cls_nm1, is_tcs = rough1, is_tys = tys1 }) (ClsInst { is_cls_nm = cls_nm2, is_tcs = rough2, is_tys = tys2 }) = cls_nm1 == cls_nm2 && not (instanceCantMatch rough1 rough2) -- Fast check for no match, uses the "rough match" fields && isJust (tcMatchTys tys1 tys2) && isJust (tcMatchTys tys2 tys1) {- ************************************************************************ * * Looking up an instance * * ************************************************************************ @lookupInstEnv@ looks up in a @InstEnv@, using a one-way match. Since the env is kept ordered, the first match must be the only one. The thing we are looking up can have an arbitrary "flexi" part. Note [Instance lookup and orphan instances] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Suppose we are compiling a module M, and we have a zillion packages loaded, and we are looking up an instance for C (T W). If we find a match in module 'X' from package 'p', should be "in scope"; that is, is p:X in the transitive closure of modules imported from M? The difficulty is that the "zillion packages" might include ones loaded through earlier invocations of the GHC API, or earlier module loads in GHCi. They might not be in the dependencies of M itself; and if not, the instances in them should not be visible. #2182, #8427. There are two cases: * If the instance is *not an orphan*, then module X defines C, T, or W. And in order for those types to be involved in typechecking M, it must be that X is in the transitive closure of M's imports. So we can use the instance. * If the instance *is an orphan*, the above reasoning does not apply. So we keep track of the set of orphan modules transitively below M; this is the ie_visible field of InstEnvs, of type VisibleOrphanModules. If module p:X is in this set, then we can use the instance, otherwise we can't. Note [Rules for instance lookup] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ These functions implement the carefully-written rules in the user manual section on "overlapping instances". At risk of duplication, here are the rules. If the rules change, change this text and the user manual simultaneously. The link may be this: http://www.haskell.org/ghc/docs/latest/html/users_guide/glasgow_exts.html#instance-overlap The willingness to be overlapped or incoherent is a property of the instance declaration itself, controlled as follows: * An instance is "incoherent" if it has an INCOHERENT pragma, or if it appears in a module compiled with -XIncoherentInstances. * An instance is "overlappable" if it has an OVERLAPPABLE or OVERLAPS pragma, or if it appears in a module compiled with -XOverlappingInstances, or if the instance is incoherent. * An instance is "overlapping" if it has an OVERLAPPING or OVERLAPS pragma, or if it appears in a module compiled with -XOverlappingInstances, or if the instance is incoherent. compiled with -XOverlappingInstances. Now suppose that, in some client module, we are searching for an instance of the target constraint (C ty1 .. tyn). The search works like this. * Find all instances `I` that *match* the target constraint; that is, the target constraint is a substitution instance of `I`. These instance declarations are the *candidates*. * Eliminate any candidate `IX` for which both of the following hold: - There is another candidate `IY` that is strictly more specific; that is, `IY` is a substitution instance of `IX` but not vice versa. - Either `IX` is *overlappable*, or `IY` is *overlapping*. (This "either/or" design, rather than a "both/and" design, allow a client to deliberately override an instance from a library, without requiring a change to the library.) - If exactly one non-incoherent candidate remains, select it. If all remaining candidates are incoherent, select an arbitrary one. Otherwise the search fails (i.e. when more than one surviving candidate is not incoherent). - If the selected candidate (from the previous step) is incoherent, the search succeeds, returning that candidate. - If not, find all instances that *unify* with the target constraint, but do not *match* it. Such non-candidate instances might match when the target constraint is further instantiated. If all of them are incoherent, the search succeeds, returning the selected candidate; if not, the search fails. Notice that these rules are not influenced by flag settings in the client module, where the instances are *used*. These rules make it possible for a library author to design a library that relies on overlapping instances without the client having to know. Note [Overlapping instances] (NB: these notes are quite old) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Overlap is permitted, but only in such a way that one can make a unique choice when looking up. That is, overlap is only permitted if one template matches the other, or vice versa. So this is ok: [a] [Int] but this is not (Int,a) (b,Int) If overlap is permitted, the list is kept most specific first, so that the first lookup is the right choice. For now we just use association lists. \subsection{Avoiding a problem with overlapping} Consider this little program: \begin{pseudocode} class C a where c :: a class C a => D a where d :: a instance C Int where c = 17 instance D Int where d = 13 instance C a => C [a] where c = [c] instance ({- C [a], -} D a) => D [a] where d = c instance C [Int] where c = [37] main = print (d :: [Int]) \end{pseudocode} What do you think `main' prints (assuming we have overlapping instances, and all that turned on)? Well, the instance for `D' at type `[a]' is defined to be `c' at the same type, and we've got an instance of `C' at `[Int]', so the answer is `[37]', right? (the generic `C [a]' instance shouldn't apply because the `C [Int]' instance is more specific). Ghc-4.04 gives `[37]', while ghc-4.06 gives `[17]', so 4.06 is wrong. That was easy ;-) Let's just consult hugs for good measure. Wait - if I use old hugs (pre-September99), I get `[17]', and stranger yet, if I use hugs98, it doesn't even compile! What's going on!? What hugs complains about is the `D [a]' instance decl. \begin{pseudocode} ERROR "mj.hs" (line 10): Cannot build superclass instance *** Instance : D [a] *** Context supplied : D a *** Required superclass : C [a] \end{pseudocode} You might wonder what hugs is complaining about. It's saying that you need to add `C [a]' to the context of the `D [a]' instance (as appears in comments). But there's that `C [a]' instance decl one line above that says that I can reduce the need for a `C [a]' instance to the need for a `C a' instance, and in this case, I already have the necessary `C a' instance (since we have `D a' explicitly in the context, and `C' is a superclass of `D'). Unfortunately, the above reasoning indicates a premature commitment to the generic `C [a]' instance. I.e., it prematurely rules out the more specific instance `C [Int]'. This is the mistake that ghc-4.06 makes. The fix is to add the context that hugs suggests (uncomment the `C [a]'), effectively deferring the decision about which instance to use. Now, interestingly enough, 4.04 has this same bug, but it's covered up in this case by a little known `optimization' that was disabled in 4.06. Ghc-4.04 silently inserts any missing superclass context into an instance declaration. In this case, it silently inserts the `C [a]', and everything happens to work out. (See `GHC.Types.Id.Make.mkDictFunId' for the code in question. Search for `Mark Jones', although Mark claims no credit for the `optimization' in question, and would rather it stopped being called the `Mark Jones optimization' ;-) So, what's the fix? I think hugs has it right. Here's why. Let's try something else out with ghc-4.04. Let's add the following line: d' :: D a => [a] d' = c Everyone raise their hand who thinks that `d :: [Int]' should give a different answer from `d' :: [Int]'. Well, in ghc-4.04, it does. The `optimization' only applies to instance decls, not to regular bindings, giving inconsistent behavior. Old hugs had this same bug. Here's how we fixed it: like GHC, the list of instances for a given class is ordered, so that more specific instances come before more generic ones. For example, the instance list for C might contain: ..., C Int, ..., C a, ... When we go to look for a `C Int' instance we'll get that one first. But what if we go looking for a `C b' (`b' is unconstrained)? We'll pass the `C Int' instance, and keep going. But if `b' is unconstrained, then we don't know yet if the more specific instance will eventually apply. GHC keeps going, and matches on the generic `C a'. The fix is to, at each step, check to see if there's a reverse match, and if so, abort the search. This prevents hugs from prematurely choosing a generic instance when a more specific one exists. --Jeff BUT NOTE [Nov 2001]: we must actually *unify* not reverse-match in this test. Suppose the instance envt had ..., forall a b. C a a b, ..., forall a b c. C a b c, ... (still most specific first) Now suppose we are looking for (C x y Int), where x and y are unconstrained. C x y Int doesn't match the template {a,b} C a a b but neither does C a a b match the template {x,y} C x y Int But still x and y might subsequently be unified so they *do* match. Simple story: unify, don't match. -} type DFunInstType = Maybe Type -- Just ty => Instantiate with this type -- Nothing => Instantiate with any type of this tyvar's kind -- See Note [DFunInstType: instantiating types] type InstMatch = (ClsInst, [DFunInstType]) type ClsInstLookupResult = ( [InstMatch] -- Successful matches , [ClsInst] -- These don't match but do unify , [InstMatch] ) -- Unsafe overlapped instances under Safe Haskell -- (see Note [Safe Haskell Overlapping Instances] in -- GHC.Tc.Solver). {- Note [DFunInstType: instantiating types] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A successful match is a ClsInst, together with the types at which the dfun_id in the ClsInst should be instantiated The instantiating types are (Either TyVar Type)s because the dfun might have some tyvars that *only* appear in arguments dfun :: forall a b. C a b, Ord b => D [a] When we match this against D [ty], we return the instantiating types [Just ty, Nothing] where the 'Nothing' indicates that 'b' can be freely instantiated. (The caller instantiates it to a flexi type variable, which will presumably later become fixed via functional dependencies.) Note [Infinitary substitution in lookup] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider class C a b instance C c c instance C d (Maybe d) [W] C e (Maybe e) You would think we could just use the second instance, because the first doesn't unify. But that's just ever so slightly wrong. The reason we check for unifiers along with matchers is that we don't want the possibility that a type variable instantiation could cause an instance choice to change. Yet if we have type family M = Maybe M and choose (e |-> M), then both instances match. This is absurd, but we cannot rule it out. Yet, worrying about this case is awfully inconvenient to users, and so we pretend the problem doesn't exist, by considering a lookup that runs into this occurs-check issue to indicate that an instance surely does not apply (i.e. is like the SurelyApart case). In the brief time that we didn't treat infinitary substitutions specially, two tickets were filed: #19044 and #19052, both trying to do Real Work. Why don't we just exclude any instances that are MaybeApart? Because we might have a [W] C e (F e), where F is a type family. The second instance above does not match, but it should be included as a future possibility. Unification will return MaybeApart MARTypeFamily in this case. What can go wrong with this design choice? We might get incoherence -- but not loss of type safety. In particular, if we have [W] C M M (for the M type family above), then GHC might arbitrarily choose either instance, depending on how M reduces (or doesn't). For type families, we can't just ignore the problem (as we essentially do here), because doing so would give us a hole in the type safety proof (as explored in Section 6 of "Closed Type Families with Overlapping Equations", POPL'14). This possibility of an infinitary substitution manifests as closed type families that look like they should reduce, but don't. Users complain: #9082 and #17311. For open type families, we actually can have unsoundness if we don't take infinitary substitutions into account: #8162. But, luckily, for class instances, we just risk coherence -- not great, but it seems better to give users what they likely want. (Also, note that this problem existed for the entire decade of 201x without anyone noticing, so it's manifestly not ruining anyone's day.) -} -- |Look up an instance in the given instance environment. The given class application must match exactly -- one instance and the match may not contain any flexi type variables. If the lookup is unsuccessful, -- yield 'Left errorMessage'. lookupUniqueInstEnv :: InstEnvs -> Class -> [Type] -> Either SDoc (ClsInst, [Type]) lookupUniqueInstEnv instEnv cls tys = case lookupInstEnv False instEnv cls tys of ([(inst, inst_tys)], _, _) | noFlexiVar -> Right (inst, inst_tys') | otherwise -> Left $ text "flexible type variable:" <+> (ppr $ mkTyConApp (classTyCon cls) tys) where inst_tys' = [ty | Just ty <- inst_tys] noFlexiVar = all isJust inst_tys _other -> Left $ text "instance not found" <+> (ppr $ mkTyConApp (classTyCon cls) tys) lookupInstEnv' :: InstEnv -- InstEnv to look in -> VisibleOrphanModules -- But filter against this -> Class -> [Type] -- What we are looking for -> ([InstMatch], -- Successful matches [ClsInst]) -- These don't match but do unify -- (no incoherent ones in here) -- The second component of the result pair happens when we look up -- Foo [a] -- in an InstEnv that has entries for -- Foo [Int] -- Foo [b] -- Then which we choose would depend on the way in which 'a' -- is instantiated. So we report that Foo [b] is a match (mapping b->a) -- but Foo [Int] is a unifier. This gives the caller a better chance of -- giving a suitable error message lookupInstEnv' ie vis_mods cls tys = lookup ie where rough_tcs = roughMatchTcs tys -------------- lookup env = case lookupUDFM env cls of Nothing -> ([],[]) -- No instances for this class Just (ClsIE insts) -> find [] [] insts -------------- find ms us [] = (ms, us) find ms us (item@(ClsInst { is_tcs = mb_tcs, is_tvs = tpl_tvs , is_tys = tpl_tys }) : rest) | not (instIsVisible vis_mods item) = find ms us rest -- See Note [Instance lookup and orphan instances] -- Fast check for no match, uses the "rough match" fields | instanceCantMatch rough_tcs mb_tcs = find ms us rest | Just subst <- tcMatchTys tpl_tys tys = find ((item, map (lookupTyVar subst) tpl_tvs) : ms) us rest -- Does not match, so next check whether the things unify -- See Note [Overlapping instances] -- Ignore ones that are incoherent: Note [Incoherent instances] | isIncoherent item = find ms us rest | otherwise = ASSERT2( tys_tv_set `disjointVarSet` tpl_tv_set, (ppr cls <+> ppr tys) $$ (ppr tpl_tvs <+> ppr tpl_tys) ) -- Unification will break badly if the variables overlap -- They shouldn't because we allocate separate uniques for them -- See Note [Template tyvars are fresh] case tcUnifyTysFG instanceBindFun tpl_tys tys of -- We consider MaybeApart to be a case where the instance might -- apply in the future. This covers an instance like C Int and -- a target like [W] C (F a), where F is a type family. SurelyApart -> find ms us rest -- Note [Infinitary substitution in lookup] MaybeApart MARInfinite _ -> find ms us rest _ -> find ms (item:us) rest where tpl_tv_set = mkVarSet tpl_tvs tys_tv_set = tyCoVarsOfTypes tys --------------- -- This is the common way to call this function. lookupInstEnv :: Bool -- Check Safe Haskell overlap restrictions -> InstEnvs -- External and home package inst-env -> Class -> [Type] -- What we are looking for -> ClsInstLookupResult -- ^ See Note [Rules for instance lookup] -- ^ See Note [Safe Haskell Overlapping Instances] in "GHC.Tc.Solver" -- ^ See Note [Safe Haskell Overlapping Instances Implementation] in "GHC.Tc.Solver" lookupInstEnv check_overlap_safe (InstEnvs { ie_global = pkg_ie , ie_local = home_ie , ie_visible = vis_mods }) cls tys = -- pprTrace "lookupInstEnv" (ppr cls <+> ppr tys $$ ppr home_ie) $ (final_matches, final_unifs, unsafe_overlapped) where (home_matches, home_unifs) = lookupInstEnv' home_ie vis_mods cls tys (pkg_matches, pkg_unifs) = lookupInstEnv' pkg_ie vis_mods cls tys all_matches = home_matches ++ pkg_matches all_unifs = home_unifs ++ pkg_unifs final_matches = foldr insert_overlapping [] all_matches -- Even if the unifs is non-empty (an error situation) -- we still prune the matches, so that the error message isn't -- misleading (complaining of multiple matches when some should be -- overlapped away) unsafe_overlapped = case final_matches of [match] -> check_safe match _ -> [] -- If the selected match is incoherent, discard all unifiers final_unifs = case final_matches of (m:_) | isIncoherent (fst m) -> [] _ -> all_unifs -- NOTE [Safe Haskell isSafeOverlap] -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -- We restrict code compiled in 'Safe' mode from overriding code -- compiled in any other mode. The rationale is that code compiled -- in 'Safe' mode is code that is untrusted by the ghc user. So -- we shouldn't let that code change the behaviour of code the -- user didn't compile in 'Safe' mode since that's the code they -- trust. So 'Safe' instances can only overlap instances from the -- same module. A same instance origin policy for safe compiled -- instances. check_safe (inst,_) = case check_overlap_safe && unsafeTopInstance inst of -- make sure it only overlaps instances from the same module True -> go [] all_matches -- most specific is from a trusted location. False -> [] where go bad [] = bad go bad (i@(x,_):unchecked) = if inSameMod x || isOverlappable x then go bad unchecked else go (i:bad) unchecked inSameMod b = let na = getName $ getName inst la = isInternalName na nb = getName $ getName b lb = isInternalName nb in (la && lb) || (nameModule na == nameModule nb) -- We consider the most specific instance unsafe when it both: -- (1) Comes from a module compiled as `Safe` -- (2) Is an orphan instance, OR, an instance for a MPTC unsafeTopInstance inst = isSafeOverlap (is_flag inst) && (isOrphan (is_orphan inst) || classArity (is_cls inst) > 1) --------------- insert_overlapping :: InstMatch -> [InstMatch] -> [InstMatch] -- ^ Add a new solution, knocking out strictly less specific ones -- See Note [Rules for instance lookup] insert_overlapping new_item [] = [new_item] insert_overlapping new_item@(new_inst,_) (old_item@(old_inst,_) : old_items) | new_beats_old -- New strictly overrides old , not old_beats_new , new_inst `can_override` old_inst = insert_overlapping new_item old_items | old_beats_new -- Old strictly overrides new , not new_beats_old , old_inst `can_override` new_inst = old_item : old_items -- Discard incoherent instances; see Note [Incoherent instances] | isIncoherent old_inst -- Old is incoherent; discard it = insert_overlapping new_item old_items | isIncoherent new_inst -- New is incoherent; discard it = old_item : old_items -- Equal or incomparable, and neither is incoherent; keep both | otherwise = old_item : insert_overlapping new_item old_items where new_beats_old = new_inst `more_specific_than` old_inst old_beats_new = old_inst `more_specific_than` new_inst -- `instB` can be instantiated to match `instA` -- or the two are equal instA `more_specific_than` instB = isJust (tcMatchTys (is_tys instB) (is_tys instA)) instA `can_override` instB = isOverlapping instA || isOverlappable instB -- Overlap permitted if either the more specific instance -- is marked as overlapping, or the more general one is -- marked as overlappable. -- Latest change described in: #9242. -- Previous change: #3877, Dec 10. {- Note [Incoherent instances] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ For some classes, the choice of a particular instance does not matter, any one is good. E.g. consider class D a b where { opD :: a -> b -> String } instance D Int b where ... instance D a Int where ... g (x::Int) = opD x x -- Wanted: D Int Int For such classes this should work (without having to add an "instance D Int Int", and using -XOverlappingInstances, which would then work). This is what -XIncoherentInstances is for: Telling GHC "I don't care which instance you use; if you can use one, use it." Should this logic only work when *all* candidates have the incoherent flag, or even when all but one have it? The right choice is the latter, which can be justified by comparing the behaviour with how -XIncoherentInstances worked when it was only about the unify-check (note [Overlapping instances]): Example: class C a b c where foo :: (a,b,c) instance C [a] b Int instance [incoherent] [Int] b c instance [incoherent] C a Int c Thanks to the incoherent flags, [Wanted] C [a] b Int works: Only instance one matches, the others just unify, but are marked incoherent. So I can write (foo :: ([a],b,Int)) :: ([Int], Int, Int). but if that works then I really want to be able to write foo :: ([Int], Int, Int) as well. Now all three instances from above match. None is more specific than another, so none is ruled out by the normal overlapping rules. One of them is not incoherent, but we still want this to compile. Hence the "all-but-one-logic". The implementation is in insert_overlapping, where we remove matching incoherent instances as long as there are others. ************************************************************************ * * Binding decisions * * ************************************************************************ -} instanceBindFun :: BindFun instanceBindFun tv _rhs_ty | isOverlappableTyVar tv = Apart | otherwise = BindMe -- Note [Binding when looking up instances] {- Note [Binding when looking up instances] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When looking up in the instance environment, or family-instance environment, we are careful about multiple matches, as described above in Note [Overlapping instances] The target tys can contain skolem constants. For existentials and instance variables, we can guarantee that those are never going to be instantiated to anything, so we should not involve them in the unification test. These are called "super skolems". Example: class Foo a where { op :: a -> Int } instance Foo a => Foo [a] -- NB overlap instance Foo [Int] -- NB overlap data T = forall a. Foo a => MkT a f :: T -> Int f (MkT x) = op [x,x] The op [x,x] means we need (Foo [a]). This `a` will never be instantiated, and so it is a super skolem. (See the use of tcInstSuperSkolTyVarsX in GHC.Tc.Gen.Pat.tcDataConPat.) Super skolems respond True to isOverlappableTyVar, and the use of Apart in instanceBindFun, above, means that these will be treated as fresh constants in the unification algorithm during instance lookup. Without this treatment, GHC would complain, saying that the choice of instance depended on the instantiation of 'a'; but of course it isn't *going* to be instantiated. Note that it is necessary that the unification algorithm returns SurelyApart for these super-skolems for GHC to be able to commit to another instance. We do this only for super skolems. For example we reject g :: forall a => [a] -> Int g x = op x on the grounds that the correct instance depends on the instantiation of 'a' -}