[Haskell-beginners] adding state handing to existing code

[Scott Thoman]

Thank you guys very much for the quick responses.  This is very useful
info and I've definitely got more reading and learning to do - in
particular, the whole lifting/transforming/stacking monads area.  Part
of this exercise of learning Haskell is learning how to do things with
a pure functional approach and so I'm thinking about things that might
happen while building and maintaining software.  In this scenario I
was imagining that the "doit" function was something over which I
might not have control.  I like the idea that you can't just jam state
into the code without the type system, and everyone involved, knowing
about it.  So my question is now evolving into looking at it the other
way around - from the designer of "doit".

If I were designing something, for a very simple example, like the map
function but maybe over some custom sequence-like thing, how could I
make it so that the user-supplied function could be monadic/stateful
or not?  Is there a way to make the map function flexible enough so
that the author of the function argument could make it stateful or
pure without any control over the definition of map itself?

That question doesn't really need an answer that just my reasoning at
this point - one of those "what if I had written software with Haskell
and a high priority requirement came along, how would I handle it"
kind of questions.  It's an interesting exercise to think about
applying a purely functional approach to something might happen in day
to day development.

-stt

On Sun, Jan 24, 2010 at 4:11 PM, Stephen Blackheath [to
Haskell-Beginners] <mutilating.cauliflowers.stephen at blacksapphire.com>
wrote:
> Scott,
>
> Here's the most straightforward way to do it:
>
> --
> process ::  Integer -> Integer -> StateT Int IO Integer
> process x y = do
>    s <- get
>    put $ s + 1
>    return $ 2 * x * y
>
> doit :: StateT Int IO ()
> doit = do
>    p <- process 42 43
>    liftIO $ printf "f x y = %d\n" p
>
> main :: IO ()
> main = do
>    n <- execStateT doit 0
>    putStrLn $ "done "++show n++" times"
> --
>
> One thing you'll note is that the type of 'doit' has changed.  There's
> no way to pass state "through" a function without it being reflected in
> the type, and in many ways, that's the point of Haskell - to make
> potentially dangerous things explicit.  An alternative is to use an
> IORef, but that makes your code completely imperative style, which is
> not very Haskellish.
>
> One thing you'll notice is that process is now in IO, which is not
> desirable, since it's pure.  On occasions I've written this helper function:
>
> -- | Adapt a StateT to a pure state monad.
> purely :: Monad m => State s a -> StateT s m a
> purely code = do
>    s <- get
>    let (ret, s') = runState code s
>    put s'
>    return ret
>
> With this you could re-write it as...
>
> --
> process ::  Integer -> Integer -> State Int Integer
> process x y = do
>    s <- get
>    put $ s + 1
>    return $ 2 * x * y
>
> doit :: StateT Int IO ()
> doit = do
>    p <- purely $ process 42 43
>    liftIO $ printf "f x y = %d\n" p
> --
>
> Monad transformer stacks aren't perfect, but they're good if used
> appropriately.  If you use them a lot, then it can lead to a necessity
> to unstack and re-stack them like I did here.  I think monads work best
> if you initially think of your code in plain Haskell terms, and
> introduce them later as a convenience.
>
> As I'm sure you know, the "Haskell way" is to make code as pure as
> possible, using IO types only where necessary.
>
>
> Steve
>
> Scott Thoman wrote:
>> Since I'm very new to Haskell I have what is probably a simple
>> question yet I'm having trouble finding a clear example of how it
>> works.  The basic question is: how do I pass state through existing
>> code without the intermediate code knowing about it.  If I have, for
>> example, several layers of function calls and the innermost function
>> needs to access some state that is only "seeded" by the outermost
>> function, how do I do that without the functions in between knowing
>> about the additional state being threaded through them?
>>
>> I have a simple example (that may *not* be good idiomatic Haskell):
>>
>> --
>> process ::  Integer -> Integer -> Integer
>> process x y =
>>     2 * x * y
>>
>> doit :: IO ()
>> doit = do
>>     printf "f x y = %d\n" $ process 42 43
>>
>> main :: IO ()
>> main = do
>>     doit
>>     putStrLn "done"
>> --
>>
>> (I'm not totally sure about the type of "doit" but the code compiles
>> and runs as expected)
>>
>> What I want to do is add some state handing to "process" to have it,
>> say, count the number of times it's been called (putting
>> threading/thread-local concerns aside for the moment).  I'm trying to
>> understand how to add state to "process" along the lines of:
>>
>> --
>> process ::  Integer -> Integer -> State Integer Integer
>> process x y = do
>>     s <- get
>>     put $ s + 1
>>     return $ 2 * x * y
>> --
>>
>> but I want to only seed the state from "main" without "doit" having to
>> change -- I can call "process" from "doit" like "(execState (process
>> 42 43) 0)" but I want the initial state to be determined at the top
>> level, from main.
>>
>> I have a feeling there's some kind of "ah ha" moment that I'm just not
>> seeing yet.  Any help or pointers to where I can look for myself would
>> be greatly appreciated.
>>
>> Thanks in advance,
>>
>> -thor
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