X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcRules.lhs;h=e4898708d8eafe23ebd8e0ed6c029e74861fa46b;hb=2662dbc5b2c30fc11ccb99e7f9b2dba794d680ba;hp=3ee95cf4a28109d8ec3039be942ca80bcf99b4dd;hpb=3220e36e51df973b2634cd800f9d6e761d4f01be;p=ghc-hetmet.git

diff --git a/compiler/typecheck/TcRules.lhs b/compiler/typecheck/TcRules.lhs
index 3ee95cf..e489870 100644
--- a/compiler/typecheck/TcRules.lhs
+++ b/compiler/typecheck/TcRules.lhs
@@ -6,13 +6,6 @@
 TcRules: Typechecking transformation rules
 
 \begin{code}
-{-# OPTIONS -w #-}
--- The above warning supression flag is a temporary kludge.
--- While working on this module you are encouraged to remove it and fix
--- any warnings in the module. See
---     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
--- for details
-
 module TcRules ( tcRules ) where
 
 import HsSyn
@@ -31,6 +24,22 @@ import Outputable
 import FastString
 \end{code}
 
+Note [Typechecking rules]
+~~~~~~~~~~~~~~~~~~~~~~~~~
+We *infer* the typ of the LHS, and use that type to *check* the type of 
+the RHS.  That means that higher-rank rules work reasonably well. Here's
+an example (test simplCore/should_compile/rule2.hs) produced by Roman:
+
+   foo :: (forall m. m a -> m b) -> m a -> m b
+   foo f = ...
+
+   bar :: (forall m. m a -> m a) -> m a -> m a
+   bar f = ...
+
+   {-# RULES "foo/bar" foo = bar #-}
+
+He wanted the rule to typecheck.
+
 \begin{code}
 tcRules :: [LRuleDecl Name] -> TcM [LRuleDecl TcId]
 tcRules decls = mapM (wrapLocM tcRule) decls
@@ -39,15 +48,14 @@ tcRule :: RuleDecl Name -> TcM (RuleDecl TcId)
 tcRule (HsRule name act vars lhs fv_lhs rhs fv_rhs)
   = addErrCtxt (ruleCtxt name)			$ do
     traceTc (ptext (sLit "---- Rule ------") <+> ppr name)
-    rule_ty <- newFlexiTyVarTy openTypeKind
 
 	-- Deal with the tyvars mentioned in signatures
-    (ids, lhs', rhs', lhs_lie, rhs_lie) <-
+    (ids, lhs', rhs', lhs_lie, rhs_lie, rule_ty) <-
       tcRuleBndrs vars $ \ ids -> do
-		-- Now LHS and RHS
-        (lhs', lhs_lie) <- getLIE (tcMonoExpr lhs rule_ty)
+		-- Now LHS and RHS; see Note [Typechecking rules]
+        ((lhs', rule_ty), lhs_lie) <- getLIE (tcInferRho lhs)
         (rhs', rhs_lie) <- getLIE (tcMonoExpr rhs rule_ty)
-        return (ids, lhs', rhs', lhs_lie, rhs_lie)
+        return (ids, lhs', rhs', lhs_lie, rhs_lie, rule_ty)
 
 		-- Check that LHS has no overloading at all
     (lhs_dicts, lhs_binds) <- tcSimplifyRuleLhs lhs_lie
@@ -93,7 +101,7 @@ tcRule (HsRule name act vars lhs fv_lhs rhs fv_rhs)
 		    (mkHsDictLet lhs_binds lhs') fv_lhs
 		    (mkHsDictLet rhs_binds rhs') fv_rhs)
 
-
+tcRuleBndrs :: [RuleBndr Name] -> ([Id] -> TcM a) -> TcM a
 tcRuleBndrs [] thing_inside = thing_inside []
 tcRuleBndrs (RuleBndr var : vars) thing_inside
   = do 	{ ty <- newFlexiTyVarTy openTypeKind
@@ -113,6 +121,7 @@ tcRuleBndrs (RuleBndrSig var rn_ty : vars) thing_inside
 	  tcExtendIdEnv [id] $
 	  tcRuleBndrs vars (\ids -> thing_inside (id:ids)) }
 
+ruleCtxt :: FastString -> SDoc
 ruleCtxt name = ptext (sLit "When checking the transformation rule") <+> 
 		doubleQuotes (ftext name)
 \end{code}