On Wed, Nov 25, 2009 at 3:48 PM, Ben Franksen
Derek Elkins wrote:
The following code works fine for me, so it seems you are missing some details that may help. [...snip code...]
Thank you! Indeed I did simplify the code when writing the message -- because I thought that those other bits could not possibly be at fault... ;-)
*trying out many changes to my own code and yours*
Ok, I finally found it. What actually made the difference was the case for variables:
Your version is
eval (Var x) = gets (fromJust . M.lookup x)
which is suitably lazy, whereas mine was (more or less)
eval e@(Var name) = do env <- ask case M.lookup name env of Nothing -> do -- undefined variable reference warning ("reference to undefined variable " ++ show name) let val = Data "" modify (M.insert name val) return val Just val -> return val
Note that whatever I do in the 'Nothing' case is irrelevant, your code with the Var case replaced by
eval e@(Var name) = do env <- ask case M.lookup name env of Just val -> return val
loops as well.
My problem is that I still don't understand why this is so! I know of course that pattern matching is strict, but I thought this should be ok here, since I evaluate the declarations _before_ the body, so when evaluation of the body demands the variable, it will be defined.
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