import ListSetOps
import Outputable
import Bag
+
+import Monad (unless)
\end{code}
%************************************************************************
full_tc_args = tc_args ++ mkTyVarTys extra_tvs
full_tvs = tvs ++ extra_tvs
- ; (rep_tc, rep_tc_args) <- tcLookupFamInst tycon full_tc_args
+ ; (rep_tc, rep_tc_args) <- tcLookupFamInstExact tycon full_tc_args
; gla_exts <- doptM Opt_GlasgowExts
; overlap_flag <- getOverlapFlag
baleOut err = addErrTc err >> returnM (Nothing, Nothing)
\end{code}
+Auxiliary lookup wrapper which requires that looked up family instances are
+not type instances.
+
+\begin{code}
+tcLookupFamInstExact :: TyCon -> [Type] -> TcM (TyCon, [Type])
+tcLookupFamInstExact tycon tys
+ = do { result@(rep_tycon, rep_tys) <- tcLookupFamInst tycon tys
+ ; let { tvs = map (Type.getTyVar
+ "TcDeriv.tcLookupFamInstExact")
+ rep_tys
+ ; variable_only_subst = all Type.isTyVarTy rep_tys &&
+ sizeVarSet (mkVarSet tvs) == length tvs
+ -- renaming may have no repetitions
+ }
+ ; unless variable_only_subst $
+ famInstNotFound tycon tys [result]
+ ; return result
+ }
+
+\end{code}
+
%************************************************************************
%* *
; let ordinary_constraints
= [ mkClassPred cls [arg_ty]
| data_con <- tyConDataCons rep_tc,
- arg_ty <- dataConInstOrigArgTys data_con rep_tc_args,
+ arg_ty <- ASSERT( isVanillaDataCon data_con )
+ dataConInstOrigArgTys data_con rep_tc_args,
not (isUnLiftedType arg_ty) ] -- No constraints for unlifted types?
tiresome_subst = zipTopTvSubst (tyConTyVars rep_tc) rep_tc_args
gen_soln :: DerivEqn -> TcM [PredType]
gen_soln (loc, orig, _, tyvars, clas, inst_ty, deriv_rhs)
= setSrcSpan loc $
+ addErrCtxt (derivInstCtxt clas [inst_ty]) $
do { theta <- tcSimplifyDeriv orig tyvars deriv_rhs
- ; addErrCtxt (derivInstCtxt theta clas [inst_ty]) $
- do { checkNoErrs (checkValidInstance tyvars theta clas [inst_ty])
- -- See Note [Deriving context]
- -- If this fails, don't continue
+ -- checkValidInstance tyvars theta clas [inst_ty]
+ -- Not necessary; see Note [Exotic derived instance contexts]
+ -- in TcSimplify
-- Check for a bizarre corner case, when the derived instance decl should
-- have form instance C a b => D (T a) where ...
-- Claim: the result instance declaration is guaranteed valid
-- Hence no need to call:
-- checkValidInstance tyvars theta clas inst_tys
- ; return (sortLe (<=) theta) } } -- Canonicalise before returning the solution
+ ; return (sortLe (<=) theta) } -- Canonicalise before returning the solution
------------------------------------------------------------------
mk_inst_spec :: DerivEqn -> DerivSoln -> Instance
; setGblEnv env' thing_inside }
\end{code}
-Note [Deriving context]
-~~~~~~~~~~~~~~~~~~~~~~~
-With -fglasgow-exts, we allow things like (C Int a) in the simplified
-context for a derived instance declaration, because at a use of this
-instance, we might know that a=Bool, and have an instance for (C Int
-Bool)
-
-We nevertheless insist that each predicate meets the termination
-conditions. If not, the deriving mechanism generates larger and larger
-constraints. Example:
- data Succ a = S a
- data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show
-
-Note the lack of a Show instance for Succ. First we'll generate
- instance (Show (Succ a), Show a) => Show (Seq a)
-and then
- instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)
-and so on. Instead we want to complain of no instance for (Show (Succ a)).
-
%************************************************************************
%* *
-- In case of a family instance, we need to use the representation
-- tycon (after all, it has the data constructors)
- ; (tycon, _) <- tcLookupFamInst visible_tycon tyArgs
+ ; (tycon, _) <- tcLookupFamInstExact visible_tycon tyArgs
; let (meth_binds, aux_binds) = genDerivBinds clas fix_env tycon
-- Bring the right type variables into
standaloneCtxt :: LHsType Name -> SDoc
standaloneCtxt ty = ptext SLIT("In the stand-alone deriving instance for") <+> quotes (ppr ty)
-derivInstCtxt theta clas inst_tys
- = hang (ptext SLIT("In the derived instance:"))
- 2 (pprThetaArrow theta <+> pprClassPred clas inst_tys)
--- Used for the ...Thetas variants; all top level
+derivInstCtxt clas inst_tys
+ = ptext SLIT("When deriving the instance for") <+> parens (pprClassPred clas inst_tys)
badDerivedPred pred
= vcat [ptext SLIT("Can't derive instances where the instance context mentions"),