[project @ 2002-09-16 07:31:11 by simonpj]

--------------------------------
   Quantify over unbound type vars in RULE lhs's
	--------------------------------

-- We need to gather the type variables mentioned on the LHS so we can
-- quantify over them.  Example:
--   data T a = C
--
--   foo :: T a -> Int
--   foo C = 1
--
--   {-# RULES "myrule"  foo C = 1 #-}
--
-- After type checking the LHS becomes (foo a (C a))
-- and we do not want to zap the unbound tyvar 'a' to (), because
-- that limits the applicability of the rule.  Instead, we
-- want to quantify over it!

This commit fixes the problem, discovered by Manuel.  It uses a
free-variable finder for RULE lhs's (TcRule.ruleLhsTvs) which relies
on the fact that the LHS of a rule can only take ver forms
(c.f RnSource.validRuleLhs).

diff --git a/ghc/compiler/rename/RnSource.lhs b/ghc/compiler/rename/RnSource.lhs
index d8c9a5b..e67b4ad 100644 (file)
--- a/ghc/compiler/rename/RnSource.lhs
+++ b/ghc/compiler/rename/RnSource.lhs
@@ -417,6 +417,8 @@ 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
+
 \begin{code}
 validRuleLhs foralls lhs
   = check lhs

diff --git a/ghc/compiler/typecheck/TcRules.lhs b/ghc/compiler/typecheck/TcRules.lhs
index 533971f..6f9890b 100644 (file)
--- a/ghc/compiler/typecheck/TcRules.lhs
+++ b/ghc/compiler/typecheck/TcRules.lhs
@@ -8,20 +8,21 @@ module TcRules ( tcRules ) where
 
 #include "HsVersions.h"
 
-import HsSyn           ( RuleDecl(..), RuleBndr(..), collectRuleBndrSigTys )
+import HsSyn           ( RuleDecl(..), RuleBndr(..), HsExpr(..), collectRuleBndrSigTys )
 import CoreSyn         ( CoreRule(..) )
 import RnHsSyn         ( RenamedRuleDecl )
-import TcHsSyn         ( TypecheckedRuleDecl, mkHsLet )
+import TcHsSyn         ( TypecheckedRuleDecl, TcExpr, mkHsLet )
 import TcRnMonad
 import TcSimplify      ( tcSimplifyToDicts, tcSimplifyInferCheck )
 import TcMType         ( newTyVarTy )
-import TcType          ( tyVarsOfTypes, openTypeKind )
+import TcType          ( TcTyVarSet, tyVarsOfTypes, openTypeKind )
 import TcIfaceSig      ( tcCoreExpr, tcCoreLamBndrs, tcVar )
 import TcMonoType      ( tcHsSigType, UserTypeCtxt(..), tcAddScopedTyVars )
 import TcExpr          ( tcMonoExpr )
 import TcEnv           ( tcExtendLocalValEnv )
 import Inst            ( instToId )
 import Id              ( idType, mkLocalId )
+import VarSet
 import Outputable
 \end{code}
 
@@ -86,17 +87,18 @@ tcRule (HsRule name act vars lhs rhs src_loc)
        --      RULE:  forall v. fst (ss v) = fst v
        -- The type of the rhs of the rule is just a, but v::(a,(b,c))
        --
-       -- It's still conceivable that there may be type variables mentioned
-       -- in the LHS, but not in the type of the lhs, nor in the binders.
-       -- They'll get zapped to (), but that's over-constraining really.
-       -- Let's see if we get a problem.
+       -- We also need to get the free tyvars of the LHS; see notes 
+       -- below with ruleLhsTvs.
+       --
        forall_tvs = tyVarsOfTypes (rule_ty : map idType tpl_ids)
+                       `unionVarSet`
+                    ruleLhsTvs lhs'
     in
-
        -- RHS can be a bit more lenient.  In particular,
        -- we let constant dictionaries etc float outwards
        --
-       -- 
+       -- NB: tcSimplifyInferCheck zonks the forall_tvs, and 
+       --     knocks out any that are constrained by the environment
     tcSimplifyInferCheck (text "tcRule")
                         forall_tvs
                         lhs_dicts rhs_lie      `thenM` \ (forall_tvs1, rhs_binds) ->
@@ -112,6 +114,34 @@ tcRule (HsRule name act vars lhs rhs src_loc)
     new_id (RuleBndrSig var rn_ty) = tcHsSigType (RuleSigCtxt var) rn_ty       `thenM` \ ty ->
                                     returnM (mkLocalId var ty)
 
+ruleLhsTvs :: TcExpr -> TcTyVarSet
+-- We need to gather the type variables mentioned on the LHS so we can 
+-- quantify over them.  Example:
+--   data T a = C
+-- 
+--   foo :: T a -> Int
+--   foo C = 1
+--
+--   {-# RULES "myrule"  foo C = 1 #-}
+-- 
+-- After type checking the LHS becomes (foo a (C a))
+-- and we do not want to zap the unbound tyvar 'a' to (), because
+-- that limits the applicability of the rule.  Instead, we
+-- want to quantify over it!  
+--
+-- Fortunately the form of the LHS is pretty limited (see RnSource.validRuleLhs)
+-- so we don't need to deal with the whole of HsSyn.
+--
+ruleLhsTvs (OpApp e1 op _ e2)
+  = ruleLhsTvs e1 `unionVarSet` ruleLhsTvs op `unionVarSet` ruleLhsTvs e2
+ruleLhsTvs (HsApp e1 e2)
+  = ruleLhsTvs e1 `unionVarSet` ruleLhsTvs e2
+ruleLhsTvs (HsVar v) = emptyVarSet     -- I don't think we need the tyvars of the Id
+ruleLhsTvs (TyApp e1 tys)
+  = ruleLhsTvs e1 `unionVarSet` tyVarsOfTypes tys
+ruleLhsTvs e = pprPanic "ruleLhsTvs" (ppr e)
+
+
 ruleCtxt name = ptext SLIT("When checking the transformation rule") <+> 
                doubleQuotes (ftext name)
 \end{code}