Derek Elkins wrote:
On Wed, Nov 25, 2009 at 3:48 PM, Ben Franksen
What am I missing?
The problem is the liftM2 in the Let branch of eval. You are executing the body while making the bindings, so you are trying to look up x while you are still trying to bind it. One solution is to move the execution of the body after the binding as in:
eval (Let decls body) = mdo let (names,exprs) = unzip decls updateEnv env = foldr (uncurry M.insert) env $ zip names values values <- local updateEnv $ mapM eval exprs local updateEnv $ eval body
I already tried that :( It works for non-recursive expressions like 'example', but not for recursive ones; not even non-recursive ones that merely use a variable before it is defined like this one
example2 = Let [("y", Var "x"),("x", Const 1)] (Var "y")
which again makes eval loop. However, if I use your lazy version
eval (Var x) = gets (fromJust . M.lookup x)
_or_ remove the ErrorT from the monad stack (see my other message) eval does not loop, even with mutually recursive definitions. *some time later* Ok, it seems I have a version that does what I want. It is not very elegant, I have to manually wrap/unwrap the ErrorT stuff just for the Val case, but at least it seems to work. Here it goes:
eval (Var x) = Eval $ ErrorT $ do env <- get v <- case M.lookup x env of Just v -> return v Nothing -> do warning ("reference to undefined variable " ++ show x) let v = Data "" modify (M.insert x v) return v return (Right v)
warning s = tell $ ["Warning: " ++ s]
I suspect what is needed to avoid this is a combinator that convinces ErrorT that a computation is really going to succeed, no matter what. Hmm, now that I think about it this should be possible using catchError. I will investigate. Cheers Ben