Thaks a lot for your contribution, this helps me a lot, I see what I've got to do. However, I understand the first version (Stack.hs), but I can't get what StateM.hs is. Is it the same version but using state transformers, so as to be able to do IO (which I would need)? In fact, could you give me a simple example of how to use StackM.hs, a simple example that pushes some ints and add the toppest two. Thanks a lot anyway, Sam. PS: In fact I'm trying to implement a simple RPN "calculator". So at first I need +, push, pop and view which shows the whole list. Attached is what I started to do before I get your mail. Mark Carroll wrote:
On Thu, 3 Mar 2005, Sam G. wrote:
I need a Monad to represent an internal stack. I mean I've got a lot of functions which operates on lists and I would not like to pass the list as an argument everytime.
Could you help me writing this monad? To start, I just need a + function which will return the sum of the 2 toppest elements of the stack.
I wrote one which is at,
http://www.aetion.com/src/Stack.hs http://www.aetion.com/src/StackM.hs
Then,
add :: Num a => Stack a ()
add = do x <- pop y <- pop push (x + y)
or whatever.
Of course, if you use Control.Monad.State where you store the stack as a list then you can probably just do something like,
add = (x:y:z) <- get put ((x+y):z)
I hope that helps.
-- Mark
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module Main where main = do putStrLn "Forth Environment\nCopyright (C) Sam G. 2005.\n" doLoop [] doLoop list = do l <- getLine let w = execute (pushWords (words l)) list in do write w doLoop w write [] = return () write (s:ss) = do putStrLn $ show (s::Int) write ss pushWords [] = return [] pushWords (s:ss) = do push $ read (s) pushWords ss newtype State state value = State (state -> (state, value)) instance Monad (State state) where return v = State $ \s -> (s, v) State f >>= k = State $ \s -> let (s0, v0) = f s State g = k v0 in g s0 push a = State $ \s -> (a:s, a) execute (State program) = fst . program