+ check_sig gbl_tvs (TcSigInfo {sig_id = id, sig_tvs = tvs,
+ sig_theta = theta, sig_tau = tau})
+ = addErrCtxt (ptext SLIT("In the type signature for") <+> quotes (ppr id)) $
+ addErrCtxtM (sigCtxt id tvs theta tau) $
+ do { tvs' <- checkDistinctTyVars tvs
+ ; ifM (any (`elemVarSet` gbl_tvs) tvs')
+ (bleatEscapedTvs gbl_tvs tvs tvs')
+ ; return tvs' }
+
+checkDistinctTyVars :: [TcTyVar] -> TcM [TcTyVar]
+-- (checkDistinctTyVars tvs) checks that the tvs from one type signature
+-- are still all type variables, and all distinct from each other.
+-- It returns a zonked set of type variables.
+-- For example, if the type sig is
+-- f :: forall a b. a -> b -> b
+-- we want to check that 'a' and 'b' haven't
+-- (a) been unified with a non-tyvar type
+-- (b) been unified with each other (all distinct)
+
+checkDistinctTyVars sig_tvs
+ = do { zonked_tvs <- mapM zonk_one sig_tvs
+ ; foldlM check_dup emptyVarEnv (sig_tvs `zip` zonked_tvs)
+ ; return zonked_tvs }
+ where
+ zonk_one sig_tv = do { ty <- zonkTcTyVar sig_tv
+ ; return (tcGetTyVar "checkDistinctTyVars" ty) }
+ -- 'ty' is bound to be a type variable, because SigSkolTvs
+ -- can only be unified with type variables
+
+ check_dup :: TyVarEnv TcTyVar -> (TcTyVar, TcTyVar) -> TcM (TyVarEnv TcTyVar)
+ -- The TyVarEnv maps each zonked type variable back to its
+ -- corresponding user-written signature type variable
+ check_dup acc (sig_tv, zonked_tv)
+ = case lookupVarEnv acc zonked_tv of
+ Just sig_tv' -> bomb_out sig_tv sig_tv'
+
+ Nothing -> return (extendVarEnv acc zonked_tv sig_tv)
+
+ bomb_out sig_tv1 sig_tv2
+ = failWithTc (ptext SLIT("Quantified type variable") <+> quotes (ppr tidy_tv1)
+ <+> ptext SLIT("is unified with another quantified type variable")
+ <+> ppr tidy_tv2)
+ where
+ (env1, tidy_tv1) = tidyOpenTyVar emptyTidyEnv sig_tv1
+ (_env2, tidy_tv2) = tidyOpenTyVar env1 sig_tv2
+\end{code}
+