<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
</head>
<body text="#000000" bgcolor="#FFFFFF">
<div class="moz-cite-prefix">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).<br>
<br>
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.<br>
<br>
Currently, my idea is to create a module which will hide the
implementation, and have only two functions in its interface: <br>
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)<br>
2) A store function which will take a position and its evaluation
and store it in the table.<br>
<br>
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.<br>
<br>
Le 29/12/2017 à 16:36, Ramin Honary a écrit :<br>
</div>
<blockquote type="cite"
cite="mid:CAFhnQTQwu8TdUEEQM1n7R0SU2VtNE=0QUcM2xiqbiHgJ-sAMtw@mail.gmail.com">
<div dir="ltr">
<div>You should use "runSTArray" or "runSTUArray" instead of
"runST" to convert your STArray to an immutable array: <a
href="https://hackage.haskell.org/package/array-0.5.2.0/docs/Data-Array-ST.html#v:runSTUArray"
target="_blank" moz-do-not-send="true">https://hackage.haskell.<wbr>org/package/array-0.5.2.0/<wbr>docs/Data-Array-ST.html#v:<wbr>runSTUArray</a></div>
<div><br>
</div>
<div>Or another option is to use "stToIO" to convert the
"STArray" to an "IOArray." <a
href="https://hackage.haskell.org/package/base-4.10.1.0/docs/Control-Monad-ST-Lazy.html#v:stToIO"
moz-do-not-send="true">https://hackage.haskell.org/package/base-4.10.1.0/docs/Control-Monad-ST-Lazy.html#v:stToIO</a>
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.</div>
<div><br>
</div>
The design of ST arrays is to allow you to construct them
quickly and then make them immutable once you are done
constructing it.
<div><br>
</div>
<div>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: <a
href="https://hackage.haskell.org/package/array-0.5.2.0/docs/Data-Array-MArray.html#v:thaw"
moz-do-not-send="true">https://hackage.haskell.org/package/array-0.5.2.0/docs/Data-Array-MArray.html#v:thaw</a>
<div><br>
</div>
<div>Once you have constructed your immutable array, you can
access it arbitrarily using the immutable operator (!).</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>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: <a
href="https://wiki.haskell.org/Monad/ST#An_explanation_in_Haskell-Cafe"
moz-do-not-send="true">https://wiki.haskell.org/Monad/ST#An_explanation_in_Haskell-Cafe</a><br>
</div>
<div><br>
</div>
</div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Fri, Dec 29, 2017 at 11:36 PM,
Jean-Marc Alliot <span dir="ltr"><<a
href="mailto:jm@alliot.org" target="_blank"
moz-do-not-send="true">jm@alliot.org</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">Hi,<br>
<br>
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.<br>
<br>
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.<br>
<br>
This is the problem I am currently stuck with. I write the
following code:<br>
<br>
arr = newArray (-1, 1) 0 :: ST s (STArray s Int Int)<br>
get :: Int -> Int<br>
get i = runST (arr >>= (\b -> readArray b i))<br>
<br>
Here everything is perfectly OK.<br>
<br>
Now I want a more general version that could deal with any
array like arr. So I write:<br>
<br>
get2 :: ST s (STArray s Int Int) -> Int -> Int<br>
get2 tab i = runST (tab >>= (\b -> readArray b i))<br>
<br>
And the compiler is clearly very upset by my code:<br>
<br>
Couldn't match type ‘s’ with ‘s1’<br>
‘s’ is a rigid type variable bound by<br>
the type signature for:<br>
get2 :: forall s. ST s (STArray s Int Int) ->
Int -> Int<br>
at testst.hs:17:9<br>
‘s1’ is a rigid type variable bound by<br>
a type expected by the context:<br>
forall s1. ST s1 Int<br>
at testst.hs:18:14<br>
Expected type: ST s1 Int<br>
Actual type: ST s Int<br>
I am pretty sure that the compiler is right and I am wrong,
but I don't get why... Anyone could help?<br>
<br>
Thanks<br>
<br>
<br>
______________________________<wbr>_________________<br>
Haskell-Cafe mailing list<br>
To (un)subscribe, modify options or view archives go to:<br>
<a
href="http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe"
rel="noreferrer" target="_blank" moz-do-not-send="true">http://mail.haskell.org/cgi-bi<wbr>n/mailman/listinfo/haskell-caf<wbr>e</a><br>
Only members subscribed via the mailman list are allowed to
post.</blockquote>
</div>
<br>
</div>
</blockquote>
<p><br>
</p>
</body>
</html>