- rn_var (RuleBndr v, id) = returnRn (RuleBndr id, emptyFVs)
- rn_var (RuleBndrSig v t, id) = rnHsTypeFVs doc t `thenRn` \ (t', fvs) ->
- returnRn (RuleBndrSig id t', fvs)
+ rn_var (RuleBndr v, id) = returnM (RuleBndr id, emptyFVs)
+ rn_var (RuleBndrSig v t, id) = rnHsTypeFVs doc t `thenM` \ (t', fvs) ->
+ returnM (RuleBndrSig id t', fvs)
+\end{code}
+
+Check the shape of a transformation rule LHS. Currently
+we only allow LHSs of the form @(f e1 .. en)@, where @f@ is
+not one of the @forall@'d variables. We also restrict the form of the LHS so
+that it may be plausibly matched. Basically you only get to write ordinary
+applications. (E.g. a case expression is not allowed: too elaborate.)
+
+NB: if you add new cases here, make sure you add new ones to TcRule.ruleLhsTvs
+
+\begin{code}
+validRuleLhs :: [Name] -> RenamedHsExpr -> Maybe RenamedHsExpr
+-- Nothing => OK
+-- Just e => Not ok, and e is the offending expression
+validRuleLhs foralls lhs
+ = check lhs
+ where
+ check (OpApp e1 op _ e2) = check op `seqMaybe` check_e e1 `seqMaybe` check_e e2
+ check (HsApp e1 e2) = check e1 `seqMaybe` check_e e2
+ check (HsVar v) | v `notElem` foralls = Nothing
+ check other = Just other -- Failure
+
+ check_e (HsVar v) = Nothing
+ check_e (HsPar e) = check_e e
+ check_e (HsLit e) = Nothing
+ check_e (HsOverLit e) = Nothing
+
+ check_e (OpApp e1 op _ e2) = check_e e1 `seqMaybe` check_e op `seqMaybe` check_e e2
+ check_e (HsApp e1 e2) = check_e e1 `seqMaybe` check_e e2
+ check_e (NegApp e _) = check_e e
+ check_e (ExplicitList _ es) = check_es es
+ check_e (ExplicitTuple es _) = check_es es
+ check_e other = Just other -- Fails
+
+ check_es es = foldr (seqMaybe . check_e) Nothing es