[project @ 2002-02-01 11:38:32 by simonpj]

More wibbles on deriving with -fallow-undecidable-instances

diff --git a/ghc/compiler/typecheck/TcDeriv.lhs b/ghc/compiler/typecheck/TcDeriv.lhs
index de7b2b3..6655ad0 100644 (file)
--- a/ghc/compiler/typecheck/TcDeriv.lhs
+++ b/ghc/compiler/typecheck/TcDeriv.lhs
@@ -148,6 +148,9 @@ type DerivEqn = (Name, Class, TyCon, [TyVar], DerivRhs)
                -- The Name is the name for the DFun we'll build
                -- The tyvars bind all the variables in the RHS
 
+pprDerivEqn (n,c,tc,tvs,rhs)
+  = parens (hsep [ppr n, ppr c, ppr tc, ppr tvs] <+> equals <+> ppr rhs)
+
 type DerivRhs  = ThetaType
 type DerivSoln = DerivRhs
 \end{code}
@@ -486,7 +489,7 @@ solveDerivEqns :: InstEnv
                                -- This bunch is Absolutely minimal...
 
 solveDerivEqns inst_env_in orig_eqns
-  = iterateDeriv initial_solutions
+  = iterateDeriv 1 initial_solutions
   where
        -- The initial solutions for the equations claim that each
        -- instance has an empty context; this solution is certainly
@@ -499,8 +502,14 @@ solveDerivEqns inst_env_in orig_eqns
        -- compares it with the current one; finishes if they are the
        -- same, otherwise recurses with the new solutions.
        -- It fails if any iteration fails
-    iterateDeriv :: [DerivSoln] ->TcM [DFunId]
-    iterateDeriv current_solns
+    iterateDeriv :: Int -> [DerivSoln] ->TcM [DFunId]
+    iterateDeriv n current_solns
+      | n > 20         -- Looks as if we are in an infinite loop
+               -- This can happen if we have -fallow-undecidable-instances
+               -- (See TcSimplify.tcSimplifyDeriv.)
+      = pprPanic "solveDerivEqns: probable loop" 
+                (vcat (map pprDerivEqn orig_eqns) $$ ppr current_solns)
+      | otherwise
       =        getDOptsTc                              `thenNF_Tc` \ dflags ->
         let 
            dfuns    = zipWithEqual "add_solns" mk_deriv_dfun orig_eqns current_solns
@@ -515,7 +524,7 @@ solveDerivEqns inst_env_in orig_eqns
        if (current_solns == new_solns) then
            returnTc dfuns
        else
-           iterateDeriv new_solns
+           iterateDeriv (n+1) new_solns
 
     ------------------------------------------------------------------
 

diff --git a/ghc/compiler/typecheck/TcSimplify.lhs b/ghc/compiler/typecheck/TcSimplify.lhs
index 775cf75..13c847f 100644 (file)
--- a/ghc/compiler/typecheck/TcSimplify.lhs
+++ b/ghc/compiler/typecheck/TcSimplify.lhs
@@ -1738,30 +1738,38 @@ tcSimplifyDeriv tyvars theta
     simpleReduceLoop doc reduceMe wanteds              `thenTc` \ (frees, _, irreds) ->
     ASSERT( null frees )                       -- reduceMe never returns Free
 
+    doptsTc Opt_AllowUndecidableInstances              `thenNF_Tc` \ undecidable_ok ->
     let
        tv_set      = mkVarSet tvs
        simpl_theta = map dictPred irreds       -- reduceMe squashes all non-dicts
 
        check_pred pred
+         | isEmptyVarSet pred_tyvars   -- Things like (Eq T) should be rejected
+         = addErrTc (noInstErr pred)
+
+         | not undecidable_ok && not (isTyVarClassPred pred)
          -- Check that the returned dictionaries are all of form (C a b)
          --    (where a, b are type variables).  
-         -- At one time we allowed this if we had -fallow-undecidable-instances,
-         -- but that risks non-termination in the 'deriving' context-inference
-         -- fixpoint loop. If you want fancy stuff you just have to write the
-         -- instance decl yourself.
-         | not (isTyVarClassPred pred)
+         -- We allow this if we had -fallow-undecidable-instances,
+         -- but note that risks non-termination in the 'deriving' context-inference
+         -- fixpoint loop.   It is useful for situations like
+         --    data Min h a = E | M a (h a)
+         -- which gives the instance decl
+         --    instance (Eq a, Eq (h a)) => Eq (Min h a)
           = addErrTc (noInstErr pred)
   
+         | not (pred_tyvars `subVarSet` tv_set) 
          -- Check for a bizarre corner case, when the derived instance decl should
          -- have form  instance C a b => D (T a) where ...
          -- Note that 'b' isn't a parameter of T.  This gives rise to all sorts
          -- of problems; in particular, it's hard to compare solutions for
          -- equality when finding the fixpoint.  So I just rule it out for now.
-         | not (tyVarsOfPred pred `subVarSet` tv_set) 
          = addErrTc (badDerivedPred pred)
   
          | otherwise
          = returnNF_Tc ()
+         where
+           pred_tyvars = tyVarsOfPred pred
 
        rev_env = mkTopTyVarSubst tvs (mkTyVarTys tyvars)
                -- This reverse-mapping is a Royal Pain,