Disable form-checking for rule LHSs

Previously we checked the form of the arguments of a RULE lhs, to
ensure that they were simple applications. There was no good reason
for that, save to prevent you writing LHSs that were unlikely to match.

And Don Stewart found he wanted to do something we didn't allow (a section,
I think).  So I have just disabled the check.

diff --git a/compiler/rename/RnSource.lhs b/compiler/rename/RnSource.lhs
index bb26a3e..38211b9 100644 (file)
--- a/compiler/rename/RnSource.lhs
+++ b/compiler/rename/RnSource.lhs
@@ -357,16 +357,27 @@ rnHsRuleDecl (HsRule rule_name act vars lhs fv_lhs rhs fv_rhs)
     rn_var (RuleBndrSig (L loc v) t, id)
        = rnHsTypeFVs doc t     `thenM` \ (t', fvs) ->
          returnM (RuleBndrSig (L loc id) t', fvs)
+
+badRuleVar name var
+  = sep [ptext SLIT("Rule") <+> doubleQuotes (ftext name) <> colon,
+        ptext SLIT("Forall'd variable") <+> quotes (ppr var) <+> 
+               ptext SLIT("does not appear on left hand side")]
 \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.)
+Note [Rule LHS validity checking]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+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.  
 
-NB: if you add new cases here, make sure you add new ones to TcRule.ruleLhsTvs
+We used restrict the form of the 'ei' to prevent you writing rules
+with LHSs with a complicated desugaring (and hence unlikely to match);
+(e.g. a case expression is not allowed: too elaborate.)
 
+But there are legitimate non-trivial args ei, like sections and
+lambdas.  So it seems simmpler not to check at all, and that is why
+check_e is commented out.
+       
 \begin{code}
 validRuleLhs :: [Name] -> LHsExpr Name -> Maybe (HsExpr Name)
 -- Nothing => OK
@@ -381,8 +392,10 @@ validRuleLhs foralls lhs
     check (HsVar v) | v `notElem` foralls = Nothing
     check other                                  = Just other  -- Failure
 
-    checkl_e (L loc e) = check_e e
+       -- Check an argument
+    checkl_e (L loc e) = Nothing       -- Was (check_e e); see Note [Rule LHS validity checking]
 
+{-     Commented out; see Note [Rule LHS validity checking] above 
     check_e (HsVar v)     = Nothing
     check_e (HsPar e)    = checkl_e e
     check_e (HsLit e)    = Nothing
@@ -396,6 +409,7 @@ validRuleLhs foralls lhs
     check_e other               = Just other   -- Fails
 
     checkl_es es = foldr (seqMaybe . checkl_e) Nothing es
+-}
 
 badRuleLhsErr name lhs (Just bad_e)
   = sep [ptext SLIT("Rule") <+> ftext name <> colon,
@@ -403,11 +417,6 @@ badRuleLhsErr name lhs (Just bad_e)
                       ptext SLIT("in left-hand side:") <+> ppr lhs])]
     $$
     ptext SLIT("LHS must be of form (f e1 .. en) where f is not forall'd")
-
-badRuleVar name var
-  = sep [ptext SLIT("Rule") <+> doubleQuotes (ftext name) <> colon,
-        ptext SLIT("Forall'd variable") <+> quotes (ppr var) <+> 
-               ptext SLIT("does not appear on left hand side")]
 \end{code}