diff options
Diffstat (limited to 'compiler')
| -rw-r--r-- | compiler/GHC/Tc/TyCl/Instance.hs | 64 |
1 files changed, 57 insertions, 7 deletions
diff --git a/compiler/GHC/Tc/TyCl/Instance.hs b/compiler/GHC/Tc/TyCl/Instance.hs index 0b9e313ce5..a716c9671f 100644 --- a/compiler/GHC/Tc/TyCl/Instance.hs +++ b/compiler/GHC/Tc/TyCl/Instance.hs @@ -77,6 +77,7 @@ import BooleanFormula ( isUnsatisfied, pprBooleanFormulaNice ) import qualified GHC.LanguageExtensions as LangExt import Control.Monad +import Data.Tuple import Maybes import Data.List( mapAccumL ) @@ -460,7 +461,7 @@ tcLocalInstDecl (L loc (TyFamInstD { tfid_inst = decl })) ; return ([], [fam_inst], []) } tcLocalInstDecl (L loc (DataFamInstD { dfid_inst = decl })) - = do { (fam_inst, m_deriv_info) <- tcDataFamInstDecl NotAssociated (L loc decl) + = do { (fam_inst, m_deriv_info) <- tcDataFamInstDecl NotAssociated emptyVarEnv (L loc decl) ; return ([], [fam_inst], maybeToList m_deriv_info) } tcLocalInstDecl (L loc (ClsInstD { cid_inst = decl })) @@ -483,6 +484,9 @@ tcClsInstDecl (L loc (ClsInstDecl { cid_poly_ty = hs_ty, cid_binds = binds ; (subst, skol_tvs) <- tcInstSkolTyVars tyvars ; let tv_skol_prs = [ (tyVarName tv, skol_tv) | (tv, skol_tv) <- tyvars `zip` skol_tvs ] + -- Map from the skolemized Names to the original Names. + -- See Note [Associated data family instances and di_scoped_tvs]. + tv_skol_env = mkVarEnv $ map swap tv_skol_prs n_inferred = countWhile ((== Inferred) . binderArgFlag) $ fst $ splitForAllVarBndrs dfun_ty visible_skol_tvs = drop n_inferred skol_tvs @@ -497,7 +501,7 @@ tcClsInstDecl (L loc (ClsInstDecl { cid_poly_ty = hs_ty, cid_binds = binds mb_info = InClsInst { ai_class = clas , ai_tyvars = visible_skol_tvs , ai_inst_env = mini_env } - ; df_stuff <- mapAndRecoverM (tcDataFamInstDecl mb_info) adts + ; df_stuff <- mapAndRecoverM (tcDataFamInstDecl mb_info tv_skol_env) adts ; tf_insts1 <- mapAndRecoverM (tcTyFamInstDecl mb_info) ats -- Check for missing associated types and build them @@ -634,10 +638,16 @@ For some reason data family instances are a lot more complicated than type family instances -} -tcDataFamInstDecl :: AssocInstInfo - -> LDataFamInstDecl GhcRn -> TcM (FamInst, Maybe DerivInfo) +tcDataFamInstDecl :: + AssocInstInfo + -> TyVarEnv Name -- If this is an associated data family instance, maps the + -- parent class's skolemized type variables to their + -- original Names. If this is a non-associated instance, + -- this will be empty. + -- See Note [Associated data family instances and di_scoped_tvs]. + -> LDataFamInstDecl GhcRn -> TcM (FamInst, Maybe DerivInfo) -- "newtype instance" and "data instance" -tcDataFamInstDecl mb_clsinfo +tcDataFamInstDecl mb_clsinfo tv_skol_env (L loc decl@(DataFamInstDecl { dfid_eqn = HsIB { hsib_ext = imp_vars , hsib_body = FamEqn { feqn_bndrs = mb_bndrs @@ -749,11 +759,12 @@ tcDataFamInstDecl mb_clsinfo ; checkValidCoAxBranch fam_tc ax_branch ; checkValidTyCon rep_tc - ; let m_deriv_info = case derivs of + ; let scoped_tvs = map mk_deriv_info_scoped_tv_pr (tyConTyVars rep_tc) + m_deriv_info = case derivs of L _ [] -> Nothing L _ preds -> Just $ DerivInfo { di_rep_tc = rep_tc - , di_scoped_tvs = mkTyVarNamePairs (tyConTyVars rep_tc) + , di_scoped_tvs = scoped_tvs , di_clauses = preds , di_ctxt = tcMkDataFamInstCtxt decl } @@ -784,6 +795,45 @@ tcDataFamInstDecl mb_clsinfo = go pats (Bndr tv tcb_vis : etad_tvs) go pats etad_tvs = (reverse (map fstOf3 pats), etad_tvs) + -- Create a Name-TyVar mapping to bring into scope when typechecking any + -- deriving clauses this data family instance may have. + -- See Note [Associated data family instances and di_scoped_tvs]. + mk_deriv_info_scoped_tv_pr :: TyVar -> (Name, TyVar) + mk_deriv_info_scoped_tv_pr tv = + let n = lookupWithDefaultVarEnv tv_skol_env (tyVarName tv) tv + in (n, tv) + +{- +Note [Associated data family instances and di_scoped_tvs] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Some care is required to implement `deriving` correctly for associated data +family instances. Consider this example from #18055: + + class C a where + data D a + + class X a b + + instance C (Maybe a) where + data D (Maybe a) deriving (X a) + +When typechecking the `X a` in `deriving (X a)`, we must ensure that the `a` +from the instance header is brought into scope. This is the role of +di_scoped_tvs, which maps from the original, renamed `a` to the skolemized, +typechecked `a`. When typechecking the `deriving` clause, this mapping will be +consulted when looking up the `a` in `X a`. + +A naïve attempt at creating the di_scoped_tvs is to simply reuse the +tyConTyVars of the representation TyCon for `data D (Maybe a)`. This is only +half correct, however. We do want the typechecked `a`'s Name in the /range/ +of the mapping, but we do not want it in the /domain/ of the mapping. +To ensure that the original `a`'s Name ends up in the domain, we consult a +TyVarEnv (passed as an argument to tcDataFamInstDecl) that maps from the +typechecked `a`'s Name to the original `a`'s Name. In the even that +tcDataFamInstDecl is processing a non-associated data family instance, this +TyVarEnv will simply be empty, and there is nothing to worry about. +-} + ----------------------- tcDataFamInstHeader :: AssocInstInfo -> TyCon -> [Name] -> Maybe [LHsTyVarBndr GhcRn] |