checkInstTermination, checkValidTypeInst, checkTyFamFreeness,
checkUpdateMeta, updateMeta, checkTauTvUpdate, fillBoxWithTau, unifyKindCtxt,
unifyKindMisMatch, validDerivPred, arityErr, notMonoType, notMonoArgs,
+ growPredTyVars, growTyVars, growThetaTyVars,
--------------------------------
-- Zonking
= mapM_ complain (filter is_ambig theta)
where
complain pred = addErrTc (ambigErr pred)
- extended_tau_vars = grow theta tau_tyvars
+ extended_tau_vars = growThetaTyVars theta tau_tyvars
-- See Note [Implicit parameters and ambiguity] in TcSimplify
is_ambig pred = isClassPred pred &&
= sep [ptext (sLit "Ambiguous constraint") <+> quotes (pprPred pred),
nest 4 (ptext (sLit "At least one of the forall'd type variables mentioned by the constraint") $$
ptext (sLit "must be reachable from the type after the '=>'"))]
+
+--------------------------
+-- For this 'grow' stuff see Note [Growing the tau-tvs using constraints] in Inst
+
+growThetaTyVars :: TcThetaType -> TyVarSet -> TyVarSet
+-- Finds a fixpoint
+growThetaTyVars theta tvs
+ | null theta = tvs
+ | otherwise = fixVarSet mk_next tvs
+ where
+ mk_next tvs = foldr growPredTyVars tvs theta
+
+
+growPredTyVars :: TcPredType -> TyVarSet -> TyVarSet
+-- Here is where the special case for inplicit parameters happens
+growPredTyVars (IParam _ ty) tvs = tvs `unionVarSet` tyVarsOfType ty
+growPredTyVars pred tvs = growTyVars (tyVarsOfPred pred) tvs
+
+growTyVars :: TyVarSet -> TyVarSet -> TyVarSet
+growTyVars new_tvs tvs
+ | new_tvs `intersectsVarSet` tvs = tvs `unionVarSet` new_tvs
+ | otherwise = tvs
\end{code}
In addition, GHC insists that at least one type variable