[Haskell-cafe] foldl and space problems

[Andrew Pimlott]

On Mon, Jun 13, 2005 at 03:29:21PM +0200, Gracjan Polak wrote:
> Bernard Pope wrote:
> >Perhaps you could post the definition of the state type? Or even better,
> >a small example of code that runs badly.
> 
> I still don't know where old code had problems, but after rewriting 
> everything it seems to run smoothly now :) Thanks for all ideas, btw.
> 
> I "invented" something like this:
> 
> type RestAndState = (MyState -> [MyData]) -> MyState -> [MyData]
> 
> this is the type of funtions that take current state and continuation as 
> parameters. Example:
> 
> putValues v1 v2 v3 rest state =
> 	[v1,v2,v3] ++ rest state
> 
> there is a bit of syntactic ugliness when state chages:
> 
> putValueEx v1 rest state =
> 	let newstate = state { ... } in
>         [v1] ++ rest nstate
> 
> good thing is that function can be composed quite easily with ($):
> 
> putV v1 v2 v3 v4 rest = -- state skipped here :)
> 	putValue v1 v2 v3 $
> 	putValueEx v4 rest
> 
> 
> It works as I want it to. But I have strange feeling that there must be 
> a better way to "compose" foldr and state... Does anybody have any idea 
> how to put monad into this?

I haven't followed the thread from the beginning, but here is one way of
expressing your examples, using State and foldr1:

    putValues :: a -> a -> a -> State s ([a] -> [a])
    putValues v1 v2 v3 = return ([v1,v2,v3] ++)
                                  
    putValueEx :: a -> State s ([a] -> [a])
    putValueEx v1 = do modify (\s -> undefined)
                       return (v1 :)

    putV :: a -> a -> a -> a -> State s ([a] -> [a]) 
    putV v1 v2 v3 v4 = foldr1 (liftM2 (.))
                              [putValues v1 v2 v3, putValueEx v4]

    test = evalState (putV 1 2 3 4) undefined []

I'm not sure exactly what you're trying to accomplish, but maybe this
will give you some ideas.

Andrew