-- The @FamInst@ type: family instance heads {-# LANGUAGE CPP, GADTs, ViewPatterns #-} module FamInst ( FamInstEnvs, tcGetFamInstEnvs, checkFamInstConsistency, tcExtendLocalFamInstEnv, tcLookupDataFamInst, tcLookupDataFamInst_maybe, tcInstNewTyCon_maybe, tcTopNormaliseNewTypeTF_maybe, newFamInst, -- * Injectivity reportInjectivityErrors, reportConflictingInjectivityErrs ) where import GhcPrelude import HscTypes import FamInstEnv import InstEnv( roughMatchTcs ) import Coercion import CoreLint import TcEvidence import LoadIface import TcRnMonad import SrcLoc import TyCon import TcType import CoAxiom import DynFlags import Module import Outputable import Util import RdrName import DataCon ( dataConName ) import Maybes import TyCoRep import TyCoFVs import TyCoPpr ( pprWithExplicitKindsWhen ) import TcMType import Name import Panic import VarSet import FV import Bag( Bag, unionBags, unitBag ) import Control.Monad import Data.List.NonEmpty ( NonEmpty(..) ) import qualified GHC.LanguageExtensions as LangExt #include "HsVersions.h" {- Note [The type family instance consistency story] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ To preserve type safety we must ensure that for any given module, all the type family instances used either in that module or in any module it directly or indirectly imports are consistent. For example, consider module F where type family F a module A where import F( F ) type instance F Int = Bool f :: F Int -> Bool f x = x module B where import F( F ) type instance F Int = Char g :: Char -> F Int g x = x module Bad where import A( f ) import B( g ) bad :: Char -> Int bad c = f (g c) Even though module Bad never mentions the type family F at all, by combining the functions f and g that were type checked in contradictory type family instance environments, the function bad is able to coerce from one type to another. So when we type check Bad we must verify that the type family instances defined in module A are consistent with those defined in module B. How do we ensure that we maintain the necessary consistency? * Call a module which defines at least one type family instance a "family instance module". This flag `mi_finsts` is recorded in the interface file. * For every module we calculate the set of all of its direct and indirect dependencies that are family instance modules. This list `dep_finsts` is also recorded in the interface file so we can compute this list for a module from the lists for its direct dependencies. * When type checking a module M we check consistency of all the type family instances that are either provided by its `dep_finsts` or defined in the module M itself. This is a pairwise check, i.e., for every pair of instances we must check that they are consistent. - For family instances coming from `dep_finsts`, this is checked in checkFamInstConsistency, called from tcRnImports. See Note [Checking family instance consistency] for details on this check (and in particular how we avoid having to do all these checks for every module we compile). - That leaves checking the family instances defined in M itself against instances defined in either M or its `dep_finsts`. This is checked in `tcExtendLocalFamInstEnv'. There are four subtle points in this scheme which have not been addressed yet. * We have checked consistency of the family instances *defined* by M or its imports, but this is not by definition the same thing as the family instances *used* by M or its imports. Specifically, we need to ensure when we use a type family instance while compiling M that this instance was really defined from either M or one of its imports, rather than being an instance that we happened to know about from reading an interface file in the course of compiling an unrelated module. Otherwise, we'll end up with no record of the fact that M depends on this family instance and type safety will be compromised. See #13102. * It can also happen that M uses a function defined in another module which is not transitively imported by M. Examples include the desugaring of various overloaded constructs, and references inserted by Template Haskell splices. If that function's definition makes use of type family instances which are not checked against those visible from M, type safety can again be compromised. See #13251. * When a module C imports a boot module B.hs-boot, we check that C's type family instances are compatible with those visible from B.hs-boot. However, C will eventually be linked against a different module B.hs, which might define additional type family instances which are inconsistent with C's. This can also lead to loss of type safety. See #9562. * The call to checkFamConsistency for imported functions occurs very early (in tcRnImports) and that causes problems if the imported instances use type declared in the module being compiled. See Note [Loading your own hi-boot file] in LoadIface. -} {- ************************************************************************ * * Making a FamInst * * ************************************************************************ -} -- All type variables in a FamInst must be fresh. This function -- creates the fresh variables and applies the necessary substitution -- It is defined here to avoid a dependency from FamInstEnv on the monad -- code. newFamInst :: FamFlavor -> CoAxiom Unbranched -> TcM FamInst -- Freshen the type variables of the FamInst branches newFamInst flavor axiom@(CoAxiom { co_ax_tc = fam_tc }) = ASSERT2( tyCoVarsOfTypes lhs `subVarSet` tcv_set, text "lhs" <+> pp_ax ) ASSERT2( lhs_kind `eqType` rhs_kind, text "kind" <+> pp_ax $$ ppr lhs_kind $$ ppr rhs_kind ) -- We used to have an assertion that the tyvars of the RHS were bound -- by tcv_set, but in error situations like F Int = a that isn't -- true; a later check in checkValidFamInst rejects it do { (subst, tvs') <- freshenTyVarBndrs tvs ; (subst, cvs') <- freshenCoVarBndrsX subst cvs ; dflags <- getDynFlags ; let lhs' = substTys subst lhs rhs' = substTy subst rhs tcvs' = tvs' ++ cvs' ; ifErrsM (return ()) $ -- Don't lint when there are errors, because -- errors might mean TcTyCons. -- See Note [Recover from validity error] in TcTyClsDecls when (gopt Opt_DoCoreLinting dflags) $ -- Check that the types involved in this instance are well formed. -- Do /not/ expand type synonyms, for the reasons discussed in -- Note [Linting type synonym applications]. case lintTypes dflags tcvs' (rhs':lhs') of Nothing -> pure () Just fail_msg -> pprPanic "Core Lint error" (vcat [ fail_msg , ppr fam_tc , ppr subst , ppr tvs' , ppr cvs' , ppr lhs' , ppr rhs' ]) ; return (FamInst { fi_fam = tyConName fam_tc , fi_flavor = flavor , fi_tcs = roughMatchTcs lhs , fi_tvs = tvs' , fi_cvs = cvs' , fi_tys = lhs' , fi_rhs = rhs' , fi_axiom = axiom }) } where lhs_kind = tcTypeKind (mkTyConApp fam_tc lhs) rhs_kind = tcTypeKind rhs tcv_set = mkVarSet (tvs ++ cvs) pp_ax = pprCoAxiom axiom CoAxBranch { cab_tvs = tvs , cab_cvs = cvs , cab_lhs = lhs , cab_rhs = rhs } = coAxiomSingleBranch axiom {- ************************************************************************ * * Optimised overlap checking for family instances * * ************************************************************************ Note [Checking family instance consistency] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ For any two family instance modules that we import directly or indirectly, we check whether the instances in the two modules are consistent, *unless* we can be certain that the instances of the two modules have already been checked for consistency during the compilation of modules that we import. Why do we need to check? Consider module X1 where module X2 where data T1 data T2 type instance F T1 b = Int type instance F a T2 = Char f1 :: F T1 a -> Int f2 :: Char -> F a T2 f1 x = x f2 x = x Now if we import both X1 and X2 we could make (f2 . f1) :: Int -> Char. Notice that neither instance is an orphan. How do we know which pairs of modules have already been checked? For each module M we directly import, we look up the family instance modules that M imports (directly or indirectly), say F1, ..., FN. For any two modules among M, F1, ..., FN, we know that the family instances defined in those two modules are consistent--because we checked that when we compiled M. For every other pair of family instance modules we import (directly or indirectly), we check that they are consistent now. (So that we can be certain that the modules in our `HscTypes.dep_finsts' are consistent.) There is some fancy footwork regarding hs-boot module loops, see Note [Don't check hs-boot type family instances too early] Note [Checking family instance optimization] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ As explained in Note [Checking family instance consistency] we need to ensure that every pair of transitive imports that define type family instances is consistent. Let's define df(A) = transitive imports of A that define type family instances + A, if A defines type family instances Then for every direct import A, df(A) is already consistent. Let's name the current module M. We want to make sure that df(M) is consistent. df(M) = df(D_1) U df(D_2) U ... U df(D_i) where D_1 .. D_i are direct imports. We perform the check iteratively, maintaining a set of consistent modules 'C' and trying to add df(D_i) to it. The key part is how to ensure that the union C U df(D_i) is consistent. Let's consider two modules: A and B from C U df(D_i). There are nine possible ways to choose A and B from C U df(D_i): | A in C only | A in C and B in df(D_i) | A in df(D_i) only -------------------------------------------------------------------------------- B in C only | Already checked | Already checked | Needs to be checked | when checking C | when checking C | -------------------------------------------------------------------------------- B in C and | Already checked | Already checked | Already checked when B in df(D_i) | when checking C | when checking C | checking df(D_i) -------------------------------------------------------------------------------- B in df(D_i) | Needs to be | Already checked | Already checked when only | checked | when checking df(D_i) | checking df(D_i) That means to ensure that C U df(D_i) is consistent we need to check every module from C - df(D_i) against every module from df(D_i) - C and every module from df(D_i) - C against every module from C - df(D_i). But since the checks are symmetric it suffices to pick A from C - df(D_i) and B from df(D_i) - C. In other words these are the modules we need to check: [ (m1, m2) | m1 <- C, m1 not in df(D_i) , m2 <- df(D_i), m2 not in C ] One final thing to note here is that if there's lot of overlap between subsequent df(D_i)'s then we expect those set differences to be small. That situation should be pretty common in practice, there's usually a set of utility modules that every module imports directly or indirectly. This is basically the idea from #13092, comment:14. -} -- This function doesn't check ALL instances for consistency, -- only ones that aren't involved in recursive knot-tying -- loops; see Note [Don't check hs-boot type family instances too early]. -- We don't need to check the current module, this is done in -- tcExtendLocalFamInstEnv. -- See Note [The type family instance consistency story]. checkFamInstConsistency :: [Module] -> TcM () checkFamInstConsistency directlyImpMods = do { (eps, hpt) <- getEpsAndHpt ; traceTc "checkFamInstConsistency" (ppr directlyImpMods) ; let { -- Fetch the iface of a given module. Must succeed as -- all directly imported modules must already have been loaded. modIface mod = case lookupIfaceByModule hpt (eps_PIT eps) mod of Nothing -> panicDoc "FamInst.checkFamInstConsistency" (ppr mod $$ pprHPT hpt) Just iface -> iface -- Which family instance modules were checked for consistency -- when we compiled `mod`? -- Itself (if a family instance module) and its dep_finsts. -- This is df(D_i) from -- Note [Checking family instance optimization] ; modConsistent :: Module -> [Module] ; modConsistent mod = if mi_finsts (mi_final_exts (modIface mod)) then mod:deps else deps where deps = dep_finsts . mi_deps . modIface $ mod ; hmiModule = mi_module . hm_iface ; hmiFamInstEnv = extendFamInstEnvList emptyFamInstEnv . md_fam_insts . hm_details ; hpt_fam_insts = mkModuleEnv [ (hmiModule hmi, hmiFamInstEnv hmi) | hmi <- eltsHpt hpt] } ; checkMany hpt_fam_insts modConsistent directlyImpMods } where -- See Note [Checking family instance optimization] checkMany :: ModuleEnv FamInstEnv -- home package family instances -> (Module -> [Module]) -- given A, modules checked when A was checked -> [Module] -- modules to process -> TcM () checkMany hpt_fam_insts modConsistent mods = go [] emptyModuleSet mods where go :: [Module] -- list of consistent modules -> ModuleSet -- set of consistent modules, same elements as the -- list above -> [Module] -- modules to process -> TcM () go _ _ [] = return () go consistent consistent_set (mod:mods) = do sequence_ [ check hpt_fam_insts m1 m2 | m1 <- to_check_from_mod -- loop over toCheckFromMod first, it's usually smaller, -- it may even be empty , m2 <- to_check_from_consistent ] go consistent' consistent_set' mods where mod_deps_consistent = modConsistent mod mod_deps_consistent_set = mkModuleSet mod_deps_consistent consistent' = to_check_from_mod ++ consistent consistent_set' = extendModuleSetList consistent_set to_check_from_mod to_check_from_consistent = filterOut (`elemModuleSet` mod_deps_consistent_set) consistent to_check_from_mod = filterOut (`elemModuleSet` consistent_set) mod_deps_consistent -- Why don't we just minusModuleSet here? -- We could, but doing so means one of two things: -- -- 1. When looping over the cartesian product we convert -- a set into a non-deterministicly ordered list. Which -- happens to be fine for interface file determinism -- in this case, today, because the order only -- determines the order of deferred checks. But such -- invariants are hard to keep. -- -- 2. When looping over the cartesian product we convert -- a set into a deterministically ordered list - this -- adds some additional cost of sorting for every -- direct import. -- -- That also explains why we need to keep both 'consistent' -- and 'consistentSet'. -- -- See also Note [ModuleEnv performance and determinism]. check hpt_fam_insts m1 m2 = do { env1' <- getFamInsts hpt_fam_insts m1 ; env2' <- getFamInsts hpt_fam_insts m2 -- We're checking each element of env1 against env2. -- The cost of that is dominated by the size of env1, because -- for each instance in env1 we look it up in the type family -- environment env2, and lookup is cheap. -- The code below ensures that env1 is the smaller environment. ; let sizeE1 = famInstEnvSize env1' sizeE2 = famInstEnvSize env2' (env1, env2) = if sizeE1 < sizeE2 then (env1', env2') else (env2', env1') -- Note [Don't check hs-boot type family instances too early] -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -- Family instance consistency checking involves checking that -- the family instances of our imported modules are consistent with -- one another; this might lead you to think that this process -- has nothing to do with the module we are about to typecheck. -- Not so! Consider the following case: -- -- -- A.hs-boot -- type family F a -- -- -- B.hs -- import {-# SOURCE #-} A -- type instance F Int = Bool -- -- -- A.hs -- import B -- type family F a -- -- When typechecking A, we are NOT allowed to poke the TyThing -- for F until we have typechecked the family. Thus, we -- can't do consistency checking for the instance in B -- (checkFamInstConsistency is called during renaming). -- Failing to defer the consistency check lead to #11062. -- -- Additionally, we should also defer consistency checking when -- type from the hs-boot file of the current module occurs on -- the left hand side, as we will poke its TyThing when checking -- for overlap. -- -- -- F.hs -- type family F a -- -- -- A.hs-boot -- import F -- data T -- -- -- B.hs -- import {-# SOURCE #-} A -- import F -- type instance F T = Int -- -- -- A.hs -- import B -- data T = MkT -- -- In fact, it is even necessary to defer for occurrences in -- the RHS, because we may test for *compatibility* in event -- of an overlap. -- -- Why don't we defer ALL of the checks to later? Well, many -- instances aren't involved in the recursive loop at all. So -- we might as well check them immediately; and there isn't -- a good time to check them later in any case: every time -- we finish kind-checking a type declaration and add it to -- a context, we *then* consistency check all of the instances -- which mentioned that type. We DO want to check instances -- as quickly as possible, so that we aren't typechecking -- values with inconsistent axioms in scope. -- -- See also Note [Tying the knot] -- for why we are doing this at all. ; let check_now = famInstEnvElts env1 ; mapM_ (checkForConflicts (emptyFamInstEnv, env2)) check_now ; mapM_ (checkForInjectivityConflicts (emptyFamInstEnv,env2)) check_now } getFamInsts :: ModuleEnv FamInstEnv -> Module -> TcM FamInstEnv getFamInsts hpt_fam_insts mod | Just env <- lookupModuleEnv hpt_fam_insts mod = return env | otherwise = do { _ <- initIfaceTcRn (loadSysInterface doc mod) ; eps <- getEps ; return (expectJust "checkFamInstConsistency" $ lookupModuleEnv (eps_mod_fam_inst_env eps) mod) } where doc = ppr mod <+> text "is a family-instance module" {- ************************************************************************ * * Lookup * * ************************************************************************ -} -- | If @co :: T ts ~ rep_ty@ then: -- -- > instNewTyCon_maybe T ts = Just (rep_ty, co) -- -- Checks for a newtype, and for being saturated -- Just like Coercion.instNewTyCon_maybe, but returns a TcCoercion tcInstNewTyCon_maybe :: TyCon -> [TcType] -> Maybe (TcType, TcCoercion) tcInstNewTyCon_maybe = instNewTyCon_maybe -- | Like 'tcLookupDataFamInst_maybe', but returns the arguments back if -- there is no data family to unwrap. -- Returns a Representational coercion tcLookupDataFamInst :: FamInstEnvs -> TyCon -> [TcType] -> (TyCon, [TcType], Coercion) tcLookupDataFamInst fam_inst_envs tc tc_args | Just (rep_tc, rep_args, co) <- tcLookupDataFamInst_maybe fam_inst_envs tc tc_args = (rep_tc, rep_args, co) | otherwise = (tc, tc_args, mkRepReflCo (mkTyConApp tc tc_args)) tcLookupDataFamInst_maybe :: FamInstEnvs -> TyCon -> [TcType] -> Maybe (TyCon, [TcType], Coercion) -- ^ Converts a data family type (eg F [a]) to its representation type (eg FList a) -- and returns a coercion between the two: co :: F [a] ~R FList a. tcLookupDataFamInst_maybe fam_inst_envs tc tc_args | isDataFamilyTyCon tc , match : _ <- lookupFamInstEnv fam_inst_envs tc tc_args , FamInstMatch { fim_instance = rep_fam@(FamInst { fi_axiom = ax , fi_cvs = cvs }) , fim_tys = rep_args , fim_cos = rep_cos } <- match , let rep_tc = dataFamInstRepTyCon rep_fam co = mkUnbranchedAxInstCo Representational ax rep_args (mkCoVarCos cvs) = ASSERT( null rep_cos ) -- See Note [Constrained family instances] in FamInstEnv Just (rep_tc, rep_args, co) | otherwise = Nothing -- | 'tcTopNormaliseNewTypeTF_maybe' gets rid of top-level newtypes, -- potentially looking through newtype /instances/. -- -- It is only used by the type inference engine (specifically, when -- solving representational equality), and hence it is careful to unwrap -- only if the relevant data constructor is in scope. That's why -- it get a GlobalRdrEnv argument. -- -- It is careful not to unwrap data/newtype instances if it can't -- continue unwrapping. Such care is necessary for proper error -- messages. -- -- It does not look through type families. -- It does not normalise arguments to a tycon. -- -- If the result is Just (rep_ty, (co, gres), rep_ty), then -- co : ty ~R rep_ty -- gres are the GREs for the data constructors that -- had to be in scope tcTopNormaliseNewTypeTF_maybe :: FamInstEnvs -> GlobalRdrEnv -> Type -> Maybe ((Bag GlobalRdrElt, TcCoercion), Type) tcTopNormaliseNewTypeTF_maybe faminsts rdr_env ty -- cf. FamInstEnv.topNormaliseType_maybe and Coercion.topNormaliseNewType_maybe = topNormaliseTypeX stepper plus ty where plus :: (Bag GlobalRdrElt, TcCoercion) -> (Bag GlobalRdrElt, TcCoercion) -> (Bag GlobalRdrElt, TcCoercion) plus (gres1, co1) (gres2, co2) = ( gres1 `unionBags` gres2 , co1 `mkTransCo` co2 ) stepper :: NormaliseStepper (Bag GlobalRdrElt, TcCoercion) stepper = unwrap_newtype `composeSteppers` unwrap_newtype_instance -- For newtype instances we take a double step or nothing, so that -- we don't return the representation type of the newtype instance, -- which would lead to terrible error messages unwrap_newtype_instance rec_nts tc tys | Just (tc', tys', co) <- tcLookupDataFamInst_maybe faminsts tc tys = mapStepResult (\(gres, co1) -> (gres, co `mkTransCo` co1)) $ unwrap_newtype rec_nts tc' tys' | otherwise = NS_Done unwrap_newtype rec_nts tc tys | Just con <- newTyConDataCon_maybe tc , Just gre <- lookupGRE_Name rdr_env (dataConName con) -- This is where we check that the -- data constructor is in scope = mapStepResult (\co -> (unitBag gre, co)) $ unwrapNewTypeStepper rec_nts tc tys | otherwise = NS_Done {- ************************************************************************ * * Extending the family instance environment * * ************************************************************************ -} -- Add new locally-defined family instances, checking consistency with -- previous locally-defined family instances as well as all instances -- available from imported modules. This requires loading all of our -- imports that define family instances (if we haven't loaded them already). tcExtendLocalFamInstEnv :: [FamInst] -> TcM a -> TcM a -- If we weren't actually given any instances to add, then we don't want -- to go to the bother of loading family instance module dependencies. tcExtendLocalFamInstEnv [] thing_inside = thing_inside -- Otherwise proceed... tcExtendLocalFamInstEnv fam_insts thing_inside = do { -- Load family-instance modules "below" this module, so that -- allLocalFamInst can check for consistency with them -- See Note [The type family instance consistency story] loadDependentFamInstModules fam_insts -- Now add the instances one by one ; env <- getGblEnv ; (inst_env', fam_insts') <- foldlM addLocalFamInst (tcg_fam_inst_env env, tcg_fam_insts env) fam_insts ; let env' = env { tcg_fam_insts = fam_insts' , tcg_fam_inst_env = inst_env' } ; setGblEnv env' thing_inside } loadDependentFamInstModules :: [FamInst] -> TcM () -- Load family-instance modules "below" this module, so that -- allLocalFamInst can check for consistency with them -- See Note [The type family instance consistency story] loadDependentFamInstModules fam_insts = do { env <- getGblEnv ; let this_mod = tcg_mod env imports = tcg_imports env want_module mod -- See Note [Home package family instances] | mod == this_mod = False | home_fams_only = moduleUnitId mod == moduleUnitId this_mod | otherwise = True home_fams_only = all (nameIsHomePackage this_mod . fi_fam) fam_insts ; loadModuleInterfaces (text "Loading family-instance modules") $ filter want_module (imp_finsts imports) } {- Note [Home package family instances] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Optimization: If we're only defining type family instances for type families *defined in the home package*, then we only have to load interface files that belong to the home package. The reason is that there's no recursion between packages, so modules in other packages can't possibly define instances for our type families. (Within the home package, we could import a module M that imports us via an hs-boot file, and thereby defines an instance of a type family defined in this module. So we can't apply the same logic to avoid reading any interface files at all, when we define an instances for type family defined in the current module. -} -- Check that the proposed new instance is OK, -- and then add it to the home inst env -- This must be lazy in the fam_inst arguments, see Note [Lazy axiom match] -- in FamInstEnv.hs addLocalFamInst :: (FamInstEnv,[FamInst]) -> FamInst -> TcM (FamInstEnv, [FamInst]) addLocalFamInst (home_fie, my_fis) fam_inst -- home_fie includes home package and this module -- my_fies is just the ones from this module = do { traceTc "addLocalFamInst" (ppr fam_inst) -- Unlike the case of class instances, don't override existing -- instances in GHCi; it's unsound. See #7102. ; mod <- getModule ; traceTc "alfi" (ppr mod) -- Fetch imported instances, so that we report -- overlaps correctly. -- Really we ought to only check consistency with -- those instances which are transitively imported -- by the current module, rather than every instance -- we've ever seen. Fixing this is part of #13102. ; eps <- getEps ; let inst_envs = (eps_fam_inst_env eps, home_fie) home_fie' = extendFamInstEnv home_fie fam_inst -- Check for conflicting instance decls and injectivity violations ; ((), no_errs) <- askNoErrs $ do { checkForConflicts inst_envs fam_inst ; checkForInjectivityConflicts inst_envs fam_inst ; checkInjectiveEquation fam_inst } ; if no_errs then return (home_fie', fam_inst : my_fis) else return (home_fie, my_fis) } {- ************************************************************************ * * Checking an instance against conflicts with an instance env * * ************************************************************************ Check whether a single family instance conflicts with those in two instance environments (one for the EPS and one for the HPT). -} -- | Checks to make sure no two family instances overlap. checkForConflicts :: FamInstEnvs -> FamInst -> TcM () checkForConflicts inst_envs fam_inst = do { let conflicts = lookupFamInstEnvConflicts inst_envs fam_inst ; traceTc "checkForConflicts" $ vcat [ ppr (map fim_instance conflicts) , ppr fam_inst -- , ppr inst_envs ] ; reportConflictInstErr fam_inst conflicts } checkForInjectivityConflicts :: FamInstEnvs -> FamInst -> TcM () -- see Note [Verifying injectivity annotation] in FamInstEnv, check 1B1. checkForInjectivityConflicts instEnvs famInst | isTypeFamilyTyCon tycon -- as opposed to data family tycon , Injective inj <- tyConInjectivityInfo tycon = let conflicts = lookupFamInstEnvInjectivityConflicts inj instEnvs famInst in reportConflictingInjectivityErrs tycon conflicts (coAxiomSingleBranch (fi_axiom famInst)) | otherwise = return () where tycon = famInstTyCon famInst -- | Check whether a new open type family equation can be added without -- violating injectivity annotation supplied by the user. Returns True when -- this is possible and False if adding this equation would violate injectivity -- annotation. This looks only at the one equation; it does not look for -- interaction between equations. Use checkForInjectivityConflicts for that. -- Does checks (2)-(4) of Note [Verifying injectivity annotation] in FamInstEnv. checkInjectiveEquation :: FamInst -> TcM () checkInjectiveEquation famInst | isTypeFamilyTyCon tycon -- type family is injective in at least one argument , Injective inj <- tyConInjectivityInfo tycon = do { dflags <- getDynFlags ; let axiom = coAxiomSingleBranch fi_ax -- see Note [Verifying injectivity annotation] in FamInstEnv ; reportInjectivityErrors dflags fi_ax axiom inj } -- if there was no injectivity annotation or tycon does not represent a -- type family we report no conflicts | otherwise = return () where tycon = famInstTyCon famInst fi_ax = fi_axiom famInst -- | Report a list of injectivity errors together with their source locations. -- Looks only at one equation; does not look for conflicts *among* equations. reportInjectivityErrors :: DynFlags -> CoAxiom br -- ^ Type family for which we generate errors -> CoAxBranch -- ^ Currently checked equation (represented by axiom) -> [Bool] -- ^ Injectivity annotation -> TcM () reportInjectivityErrors dflags fi_ax axiom inj = ASSERT2( any id inj, text "No injective type variables" ) do let lhs = coAxBranchLHS axiom rhs = coAxBranchRHS axiom fam_tc = coAxiomTyCon fi_ax (unused_inj_tvs, unused_vis, undec_inst_flag) = unusedInjTvsInRHS dflags fam_tc lhs rhs inj_tvs_unused = not $ isEmptyVarSet unused_inj_tvs tf_headed = isTFHeaded rhs bare_variables = bareTvInRHSViolated lhs rhs wrong_bare_rhs = not $ null bare_variables when inj_tvs_unused $ reportUnusedInjectiveVarsErr fam_tc unused_inj_tvs unused_vis undec_inst_flag axiom when tf_headed $ reportTfHeadedErr fam_tc axiom when wrong_bare_rhs $ reportBareVariableInRHSErr fam_tc bare_variables axiom -- | Is type headed by a type family application? isTFHeaded :: Type -> Bool -- See Note [Verifying injectivity annotation], case 3. isTFHeaded ty | Just ty' <- coreView ty = isTFHeaded ty' isTFHeaded ty | (TyConApp tc args) <- ty , isTypeFamilyTyCon tc = args `lengthIs` tyConArity tc isTFHeaded _ = False -- | If a RHS is a bare type variable return a set of LHS patterns that are not -- bare type variables. bareTvInRHSViolated :: [Type] -> Type -> [Type] -- See Note [Verifying injectivity annotation], case 2. bareTvInRHSViolated pats rhs | isTyVarTy rhs = filter (not . isTyVarTy) pats bareTvInRHSViolated _ _ = [] ------------------------------------------------------------------ -- Checking for the coverage condition for injective type families ------------------------------------------------------------------ {- Note [Coverage condition for injective type families] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The Injective Type Families paper describes how we can tell whether or not a type family equation upholds the injectivity condition. Briefly, consider the following: type family F a b = r | r -> a -- NB: b is not injective type instance F ty1 ty2 = ty3 We need to make sure that all variables mentioned in ty1 are mentioned in ty3 -- that's how we know that knowing ty3 determines ty1. But they can't be mentioned just anywhere in ty3: they must be in *injective* positions in ty3. For example: type instance F a Int = Maybe (G a) This is no good, if G is not injective. However, if G is indeed injective, then this would appear to meet our needs. There is a trap here, though: while knowing G a does indeed determine a, trying to compute a from G a might not terminate. This is precisely the same problem that we have with functional dependencies and their liberal coverage condition. Here is the test case: type family G a = r | r -> a type instance G [a] = [G a] [W] G alpha ~ [alpha] We see that the equation given applies, because G alpha equals a list. So we learn that alpha must be [beta] for some beta. We then have [W] G [beta] ~ [[beta]] This can reduce to [W] [G beta] ~ [[beta]] which then decomposes to [W] G beta ~ [beta] right where we started. The equation G [a] = [G a] thus is dangerous: while it does not violate the injectivity assumption, it might throw us into a loop, with a particularly dastardly Wanted. We thus do what functional dependencies do: require -XUndecidableInstances to accept this. Checking the coverage condition is not terribly hard, but we also want to produce a nice error message. A nice error message has at least two properties: 1. If any of the variables involved are invisible or are used in an invisible context, we want to print invisible arguments (as -fprint-explicit-kinds does). 2. If we fail to accept the equation because we're worried about non-termination, we want to suggest UndecidableInstances. To gather the right information, we can talk about the *usage* of a variable. Every variable is used either visibly or invisibly, and it is either not used at all, in a context where acceptance requires UndecidableInstances, or in a context that does not require UndecidableInstances. If a variable is used both visibly and invisibly, then we want to remember the fact that it was used invisibly: printing out invisibles will be helpful for the user to understand what is going on. If a variable is used where we need -XUndecidableInstances and where we don't, we can similarly just remember the latter. We thus define Visibility and NeedsUndecInstFlag below. These enumerations are *ordered*, and we used their Ord instances. We then define VarUsage, which is just a pair of a Visibility and a NeedsUndecInstFlag. (The visibility is irrelevant when a variable is NotPresent, but this extra slack in the representation causes no harm.) We finally define VarUsages as a mapping from variables to VarUsage. Its Monoid instance combines two maps, using the Semigroup instance of VarUsage to combine elements that are represented in both maps. In this way, we can compositionally analyze types (and portions thereof). To do the injectivity check: 1. We build VarUsages that represent the LHS (rather, the portion of the LHS that is flagged as injective); each usage on the LHS is NotPresent, because we hvae not yet looked at the RHS. 2. We also build a VarUsage for the RHS, done by injTyVarUsages. 3. We then combine these maps. Now, every variable in the injective components of the LHS will be mapped to its correct usage (either NotPresent or perhaps needing -XUndecidableInstances in order to be seen as injective). 4. We look up each var used in an injective argument on the LHS in the map, making a list of tvs that should be determined by the RHS but aren't. 5. We then return the set of bad variables, whether any of the bad ones were used invisibly, and whether any bad ones need -XUndecidableInstances. If -XUndecidableInstances is enabled, than a var that needs the flag won't be bad, so it won't appear in this list. 6. We use all this information to produce a nice error message, (a) switching on -fprint-explicit-kinds if appropriate and (b) telling the user about -XUndecidableInstances if appropriate. -} -- | Return the set of type variables that a type family equation is -- expected to be injective in but is not. Suppose we have @type family -- F a b = r | r -> a@. Then any variables that appear free in the first -- argument to F in an equation must be fixed by that equation's RHS. -- This function returns all such variables that are not indeed fixed. -- It also returns whether any of these variables appear invisibly -- and whether -XUndecidableInstances would help. -- See Note [Coverage condition for injective type families]. unusedInjTvsInRHS :: DynFlags -> TyCon -- type family -> [Type] -- LHS arguments -> Type -- the RHS -> ( TyVarSet , Bool -- True <=> one or more variable is used invisibly , Bool ) -- True <=> suggest -XUndecidableInstances -- See Note [Verifying injectivity annotation] in FamInstEnv. -- This function implements check (4) described there, further -- described in Note [Coverage condition for injective type families]. -- In theory (and modulo the -XUndecidableInstances wrinkle), -- instead of implementing this whole check in this way, we could -- attempt to unify equation with itself. We would reject exactly the same -- equations but this method gives us more precise error messages by returning -- precise names of variables that are not mentioned in the RHS. unusedInjTvsInRHS dflags tycon@(tyConInjectivityInfo -> Injective inj_list) lhs rhs = -- Note [Coverage condition for injective type families], step 5 (bad_vars, any_invisible, suggest_undec) where undec_inst = xopt LangExt.UndecidableInstances dflags inj_lhs = filterByList inj_list lhs lhs_vars = tyCoVarsOfTypes inj_lhs rhs_inj_vars = fvVarSet $ injectiveVarsOfType undec_inst rhs bad_vars = lhs_vars `minusVarSet` rhs_inj_vars any_bad = not $ isEmptyVarSet bad_vars invis_vars = fvVarSet $ invisibleVarsOfTypes [mkTyConApp tycon lhs, rhs] any_invisible = any_bad && (bad_vars `intersectsVarSet` invis_vars) suggest_undec = any_bad && not undec_inst && (lhs_vars `subVarSet` fvVarSet (injectiveVarsOfType True rhs)) -- When the type family is not injective in any arguments unusedInjTvsInRHS _ _ _ _ = (emptyVarSet, False, False) --------------------------------------- -- Producing injectivity error messages --------------------------------------- -- | Report error message for a pair of equations violating an injectivity -- annotation. No error message if there are no branches. reportConflictingInjectivityErrs :: TyCon -> [CoAxBranch] -> CoAxBranch -> TcM () reportConflictingInjectivityErrs _ [] _ = return () reportConflictingInjectivityErrs fam_tc (confEqn1:_) tyfamEqn = addErrs [buildInjectivityError fam_tc herald (confEqn1 :| [tyfamEqn])] where herald = text "Type family equation right-hand sides overlap; this violates" $$ text "the family's injectivity annotation:" -- | Injectivity error herald common to all injectivity errors. injectivityErrorHerald :: SDoc injectivityErrorHerald = text "Type family equation violates the family's injectivity annotation." -- | Report error message for equation with injective type variables unused in -- the RHS. Note [Coverage condition for injective type families], step 6 reportUnusedInjectiveVarsErr :: TyCon -> TyVarSet -> Bool -- True <=> print invisible arguments -> Bool -- True <=> suggest -XUndecidableInstances -> CoAxBranch -> TcM () reportUnusedInjectiveVarsErr fam_tc tvs has_kinds undec_inst tyfamEqn = let (loc, doc) = buildInjectivityError fam_tc (injectivityErrorHerald $$ herald $$ text "In the type family equation:") (tyfamEqn :| []) in addErrAt loc (pprWithExplicitKindsWhen has_kinds doc) where herald = sep [ what <+> text "variable" <> pluralVarSet tvs <+> pprVarSet tvs (pprQuotedList . scopedSort) , text "cannot be inferred from the right-hand side." ] $$ extra what | has_kinds = text "Type/kind" | otherwise = text "Type" extra | undec_inst = text "Using UndecidableInstances might help" | otherwise = empty -- | Report error message for equation that has a type family call at the top -- level of RHS reportTfHeadedErr :: TyCon -> CoAxBranch -> TcM () reportTfHeadedErr fam_tc branch = addErrs [buildInjectivityError fam_tc (injectivityErrorHerald $$ text "RHS of injective type family equation cannot" <+> text "be a type family:") (branch :| [])] -- | Report error message for equation that has a bare type variable in the RHS -- but LHS pattern is not a bare type variable. reportBareVariableInRHSErr :: TyCon -> [Type] -> CoAxBranch -> TcM () reportBareVariableInRHSErr fam_tc tys branch = addErrs [buildInjectivityError fam_tc (injectivityErrorHerald $$ text "RHS of injective type family equation is a bare" <+> text "type variable" $$ text "but these LHS type and kind patterns are not bare" <+> text "variables:" <+> pprQuotedList tys) (branch :| [])] buildInjectivityError :: TyCon -> SDoc -> NonEmpty CoAxBranch -> (SrcSpan, SDoc) buildInjectivityError fam_tc herald (eqn1 :| rest_eqns) = ( coAxBranchSpan eqn1 , hang herald 2 (vcat (map (pprCoAxBranchUser fam_tc) (eqn1 : rest_eqns))) ) reportConflictInstErr :: FamInst -> [FamInstMatch] -> TcRn () reportConflictInstErr _ [] = return () -- No conflicts reportConflictInstErr fam_inst (match1 : _) | FamInstMatch { fim_instance = conf_inst } <- match1 , let sorted = sortWith getSpan [fam_inst, conf_inst] fi1 = head sorted span = coAxBranchSpan (coAxiomSingleBranch (famInstAxiom fi1)) = setSrcSpan span $ addErr $ hang (text "Conflicting family instance declarations:") 2 (vcat [ pprCoAxBranchUser (coAxiomTyCon ax) (coAxiomSingleBranch ax) | fi <- sorted , let ax = famInstAxiom fi ]) where getSpan = getSrcLoc . famInstAxiom -- The sortWith just arranges that instances are dislayed in order -- of source location, which reduced wobbling in error messages, -- and is better for users tcGetFamInstEnvs :: TcM FamInstEnvs -- Gets both the external-package inst-env -- and the home-pkg inst env (includes module being compiled) tcGetFamInstEnvs = do { eps <- getEps; env <- getGblEnv ; return (eps_fam_inst_env eps, tcg_fam_inst_env env) }