Improvement to typecheck higher-rank rules better

[ghc-hetmet.git] / compiler / typecheck / TcRules.lhs

diff --git a/compiler/typecheck/TcRules.lhs b/compiler/typecheck/TcRules.lhs
index 50ad098..e489870 100644 (file)
--- a/compiler/typecheck/TcRules.lhs
+++ b/compiler/typecheck/TcRules.lhs
@@ -6,17 +6,8 @@
 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
 
-#include "HsVersions.h"
-
 import HsSyn
 import TcRnMonad
 import TcSimplify
@@ -33,6 +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
@@ -40,16 +47,15 @@ tcRules decls = mapM (wrapLocM tcRule) decls
 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
+    traceTc (ptext (sLit "---- Rule ------") <+> ppr name)
 
        -- 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
@@ -95,7 +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
@@ -115,7 +121,8 @@ tcRuleBndrs (RuleBndrSig var rn_ty : vars) thing_inside
          tcExtendIdEnv [id] $
          tcRuleBndrs vars (\ids -> thing_inside (id:ids)) }
 
-ruleCtxt name = ptext SLIT("When checking the transformation rule") <+> 
+ruleCtxt :: FastString -> SDoc
+ruleCtxt name = ptext (sLit "When checking the transformation rule") <+> 
                doubleQuotes (ftext name)
 \end{code}