Re: [Haskell-cafe] Question about ST arrays

(re-sending with "reply to all") If I understand you correctly, you want to create updates to your hash function throughout execution of your program. If this is the case, based on my own experience, I believe the ST monad was not designed for this purpose and so I think it is probably impossible. You must use the IO monad for a hash table that receives regular updates during program execution. ST is designed for doing rapid construction of a large immutable data by evaluating many stateful updates to structure during the time before it is made immutable. When monad evaluation terminates, the data structure becomes immutable. Furthermore, you may not use the immutable data structure until the ST evaluation terminates, producing the pure value. For something like a game, you need to take input then update a hash in an event loop. The IO monad is a good model of this sort of behavior, but the ST monad is not. Another possibility is that you can use a pure binary tree structure such as Data.Map, if you would prefer to maintain purity, and pass this Map structure around in a State monad transformer. If your State monad transformer is of type (StateT (Map k v) IO a), you can use "liftIO" to take inputs, and the "modify" function to update the Map state after each input. On Sat, Dec 30, 2017 at 1:05 AM, Jean-Marc Alliot wrote:

Well I am going to try to explain why I want to use STArrays and the runST function (and I am absolutely ready to be proven wrong, and directed to a better way to do it).

I have, as a simple way to learn the language, implemented an Awele program in Haskell (I have been programming games for years). It was easy, elegant (around 70 lines, 150 with the graphical interface) and completely functional, but I soon stumbled upon one problem. While implementing a vanilla alpha-beta is easy in functional style, implementing hash-tables (I mean hash-tables of the kind we use in game programming, not exactly regular hash-tables) is not that easy, and hash tables are mandatory to have a fast alpha-beta.

Currently, my idea is to create a module which will hide the implementation, and have only two functions in its interface: 1) A test_and_retrieve which take as only parameter a position and return a Maybe object which contains Nothing if the position has never been evaluated or (Just v) if there is already an evaluation for it (I know that I need more information than just a value, but for the sake of simplicity let's stick with just a value) 2) A store function which will take a position and its evaluation and store it in the table.

Now if I use IO Arrays, I will have to live in the IO Monad if I understand correctly how the IO Monad works, while using ST Arrays seems to give me the possibility to hide all the non functional code in my hash module, and keep my main code almost purely functional.

Le 29/12/2017 à 16:36, Ramin Honary a écrit :

You should use "runSTArray" or "runSTUArray" instead of "runST" to convert your STArray to an immutable array: https://hackage. haskell.org/package/array-0.5.2.0/docs/Data-Array-ST.html#v:runSTUArray

Or another option is to use "stToIO" to convert the "STArray" to an "IOArray." https://hackage.haskell.org/package/base-4.10. 1.0/docs/Control-Monad-ST-Lazy.html#v:stToIO but if you want to build an IOArray, it is better to just start with an IOArray rather than converting an STArray to an IOArray.

The design of ST arrays is to allow you to construct them quickly and then make them immutable once you are done constructing it.

An immutable array must have the whole array copied after every single update, but ST arrays allow you to make many updates without copying, then when you have completed constructing the ST array, you must freeze it to an immutable array using the "runSTUArray" function. Freezing happens without copying the array, after that it is immutable and may not be made into an STArray again unless you unfreeze it using "thaw", which creates a copy of it: https://hackage.haskell.org/package/array-0.5.2.0/ docs/Data-Array-MArray.html#v:thaw

Once you have constructed your immutable array, you can access it arbitrarily using the immutable operator (!).

If you want to make multiple updates at multiple times, you must use an IOArray or IOUArray. The ST monad is designed for you to construct pure, referentially transparent, immutable values in an isolated and strictly evaluated "environment" that lets you perform strict updates during construction. Once evaluation of the "ST" monad is complete, the returned value becomes pure, immutable, and referentially transparent. The for-all'd "s" parameter of the "runST" function ensures you do not mix separate "environments," and this is the reason you got your type error. Using "runSTArray" or "runSTUArray" does not have this restriction.

I am not sure of the reason for this design decision, but I know it has something to do with the compiler guaranteeing that pure immutable referentially transparent types are constructed without effecting each other, preventing race conditions. There is discussion of this on the Haskell wiki: https://wiki.haskell.org/Monad/ST#An_explanation_ in_Haskell-Cafe

On Fri, Dec 29, 2017 at 11:36 PM, Jean-Marc Alliot wrote:

...

Hi,

This is my first post to this list so I apologize in advance if I don't use it properly, or if my question is too simple or inapropriate.

I come from the Caml world and I am quite new to Haskell (but not to functional programming). I am currently trying to get the hang of Haskell arrays. I have gone through regular arrays, IO Arrays and I am now working with ST Arrays.

This is the problem I am currently stuck with. I write the following code:

arr = newArray (-1, 1) 0 :: ST s (STArray s Int Int) get :: Int -> Int get i = runST (arr >>= (\b -> readArray b i))

Here everything is perfectly OK.

Now I want a more general version that could deal with any array like arr. So I write:

get2 :: ST s (STArray s Int Int) -> Int -> Int get2 tab i = runST (tab >>= (\b -> readArray b i))

And the compiler is clearly very upset by my code:

Couldn't match type ‘s’ with ‘s1’ ‘s’ is a rigid type variable bound by the type signature for: get2 :: forall s. ST s (STArray s Int Int) -> Int -> Int at testst.hs:17:9 ‘s1’ is a rigid type variable bound by a type expected by the context: forall s1. ST s1 Int at testst.hs:18:14 Expected type: ST s1 Int Actual type: ST s Int I am pretty sure that the compiler is right and I am wrong, but I don't get why... Anyone could help?

Thanks

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.