Refactoring for valid rule checking

[ghc-hetmet.git] / compiler / rename / RnSource.lhs

diff --git a/compiler/rename/RnSource.lhs b/compiler/rename/RnSource.lhs
index bb26a3e..ae994d0 100644 (file)
--- a/compiler/rename/RnSource.lhs
+++ b/compiler/rename/RnSource.lhs
@@ -335,15 +335,9 @@ rnHsRuleDecl (HsRule rule_name act vars lhs fv_lhs rhs fv_rhs)
 
     rnLExpr lhs                                        `thenM` \ (lhs', fv_lhs') ->
     rnLExpr rhs                                        `thenM` \ (rhs', fv_rhs') ->
-    let
-       mb_bad = validRuleLhs ids lhs'
-    in
-    checkErr (isNothing mb_bad)
-            (badRuleLhsErr rule_name lhs' mb_bad)      `thenM_`
-    let
-       bad_vars = [var | var <- ids, not (var `elemNameSet` fv_lhs')]
-    in
-    mappM (addErr . badRuleVar rule_name) bad_vars     `thenM_`
+
+    checkValidRule rule_name ids lhs' fv_lhs'  `thenM_`
+
     returnM (HsRule rule_name act vars' lhs' fv_lhs' rhs' fv_rhs',
             fv_vars `plusFV` fv_lhs' `plusFV` fv_rhs')
   where
@@ -357,17 +351,38 @@ 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}
+checkValidRule rule_name ids lhs' fv_lhs'
+  = do         {       -- Check for the form of the LHS
+         case (validRuleLhs ids lhs') of
+               Nothing  -> return ()
+               Just bad -> failWithTc (badRuleLhsErr rule_name lhs' bad)
+
+               -- Check that LHS vars are all bound
+       ; let bad_vars = [var | var <- ids, not (var `elemNameSet` fv_lhs')]
+       ; mappM (addErr . badRuleVar rule_name) bad_vars }
+
 validRuleLhs :: [Name] -> LHsExpr Name -> Maybe (HsExpr Name)
 -- Nothing => OK
 -- Just e  => Not ok, and e is the offending expression
@@ -381,8 +396,10 @@ 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
@@ -396,18 +413,14 @@ validRuleLhs foralls lhs
     check_e other               = Just other   -- Fails
 
     checkl_es es = foldr (seqMaybe . checkl_e) Nothing es
+-}
 
-badRuleLhsErr name lhs (Just bad_e)
+badRuleLhsErr name lhs bad_e
   = sep [ptext SLIT("Rule") <+> ftext name <> colon,
         nest 4 (vcat [ptext SLIT("Illegal expression:") <+> ppr 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}