Re: [Haskell-cafe] Monad Imparative Usage Example
Kaveh Shahbazian wrote: Thanks All This is about my tries to understand monads and handling state - as you perfectly know - is one of them. I have understood a little about monads but that knowledge does not satidfy me. Again Thankyou There are many tutorials available from the wiki at http://www.haskell.org/haskellwiki/Books_and_tutorials#Using_Monads and http://www.haskell.org/haskellwiki/Monad Another way is to look at the source code for the State monad and StateT monad transformer, then you can see that the mysterious monad is nothing other than a normal data or newtype declaration together with an instance declaration ie: -- from State.hs newtype State s a = S (s - (a,s)) instance Monad (State s) where return a = S (\s - (a, s)) S m = k = S (\s - let (a, s1) = m s S n= k a in n s1) So if you want to understand what's going on when you write: do x - q p a first step is to remove the syntactic sugar to get: q = (\x - p) and then replace the = with it's definition for the monad you're using. For example with the State monad, (q) must be some expression which evaluates to something of the form S fq where fq is a function with type s - (a,s), and similarly, (\x - p) must have type a -S ( s - (a,s)). If we choose names for these values which describe the types we have: q = S s_as p = a_S_s_as soq = (\x - p) ===S s_as = a_S_s_as ===S (\s0 - let (a1, s1) = s_as s0 S s_a2s2 = a_S_s_as a1 in s_a2s2 s1) If we use State.runState s0 (q = (\x - p)) to execute this composite action, from the source we see that: runState :: s - State s a - (a,s) runState s (S m) = m s so runState s0 (q = (\x - p)) ===runState s0 (S (\s0 - let ... in s_a2s2 s1)) ===(\s0 - let ... in s_a2s2 s1) s0 ===s_a2s2 s1 ===a2s2 -- ie (a2, s2) Anyway I hope I haven't made things more complicated! ;-) The best thing is to just try and work through some examples yourself with pencil and paper and read lots of tutorials until things start clicking into place. Regards, Brian. -- Logic empowers us and Love gives us purpose. Yet still phantoms restless for eras long past, congealed in the present in unthought forms, strive mightily unseen to destroy us. http://www.metamilk.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Monad Imparative Usage Example
Brian Hulley wrote:
q = (\x - p)
For example with the State monad, (q) must be some expression which
evaluates to something of the form S fq where fq is a function with
type s - (a,s), and similarly, (\x - p) must have type a -S ( s -
(a,s)). If we choose names for these values which describe the types
we have:
q = S s_as
p = a_S_s_as
Sorry I meant:
(\x - p) = a_S_s_as
('p' and 'q' stand for arbitrary expressions that evaluate to monadic
values)
Regards, Brian.
--
Logic empowers us and Love gives us purpose.
Yet still phantoms restless for eras long past,
congealed in the present in unthought forms,
strive mightily unseen to destroy us.
http://www.metamilk.com
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Re: [Haskell-cafe] Monad Imparative Usage Example
Very Thankyou I am starting to feel it. I think about it as a 'context' that wraps some computations, which are handled by compiler environment (please make me correct if I am wrong). Now I think I need to find out how this 'monads' fit in solving problems. And for that I must go through bigger programs to write. Thanks again ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Monad Imparative Usage Example
Ooops - more bugs in my explanation... Brian Hulley wrote: -- from State.hs newtype State s a = S (s - (a,s)) I used the source given in ghc-6.4.2\libraries\monads\Monad\State.hs but the version of state monad that comes with the hierarchical libs is in ghc-6.4.2\libraries\mtl\Control\Monad\State.hs - the bits related to the explanation behave in the same way but you might find it interesting to decide which implementation is more readable since mtl uses record syntax and the other version doesn't. q = (\x - p) means that both q and p are expressions that evaluate to monadic values ie values whose type is of the form S (s - (a, s)) So we have: q :: S (s - (a, s)) (\x - p) :: a - S (s - (b, s)) Ooops! I meant: q :: State s a (\x - p) :: a - State s b therefore the *value* of q is of the form S (s - (a,s)) and the value of (\x - p) is of the form (a - S(s - (b, s))) To make the explanation simpler, we can rename the variables in the definition of = to reflect their types: to reflect the structure of their values Apologies for the millions of corrections and re-posts for this explanation - no matter how hard I try to proof read my posts something always slips through... ;-) Regards, Brian. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Monad Imparative Usage Example
Thanks All
This is about my tries to understand monads and handling state - as
you perfectly know - is one of them. I have understood a little about
monads but that knowledge does not satidfy me. Again Thankyou
On 8/2/06, Duncan Coutts [EMAIL PROTECTED] wrote:
On Wed, 2006-08-02 at 13:26 +0330, Kaveh Shahbazian wrote:
Haskell is the most powerfull and interesting thing I'v ever
encountered in IT world. But with an imparative background and lack of
understanding (because of any thing include that maybe I am not that
smart) has brought me problems. I know this is an old issue. But
please help it.
Question : Could anyone show me a sample of using a monad as a
statefull variable?
For example see this code in C# :
//
public class Test
{
int var;
static void Fun1() { var = 0; Console.Write(var); }
static void Fun2() { var = var + 4; Console.Write(var); }
static void Main() { Fun1(); Fun2(); var = 10; Console.Write(var
= + var.ToString()); }
}
//
I want to see this code in haskell.
As other people have noted, you probably don't want to to see this code
in Haskell. It's possible to translate stateful code in a 1-1 style but
that's not really the point. You'll not get much of the advantages of
the language if you do that.
You can certainly use console IO etc but for your object containing
mutable state, well in a functional style you'd simply not do that and
solve the problem in a different way.
That's why you see the code people have suggested as translations are
bigger than the code you started with, because the language is not
naturally imperative.
So the trick is to solve your problem in Haskell, not translate your
imperative solution to Haskell.
Duncan
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[Haskell-cafe] Monad Imparative Usage Example
Haskell is the most powerfull and interesting thing I'v ever
encountered in IT world. But with an imparative background and lack of
understanding (because of any thing include that maybe I am not that
smart) has brought me problems. I know this is an old issue. But
please help it.
Question : Could anyone show me a sample of using a monad as a
statefull variable?
For example see this code in C# :
//
public class Test
{
int var;
static void Fun1() { var = 0; Console.Write(var); }
static void Fun2() { var = var + 4; Console.Write(var); }
static void Main() { Fun1(); Fun2(); var = 10; Console.Write(var
= + var.ToString()); }
}
//
I want to see this code in haskell.
Thankyou
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Re: [Haskell-cafe] Monad Imparative Usage Example
kaveh.shahbazian:
Haskell is the most powerfull and interesting thing I'v ever
encountered in IT world. But with an imparative background and lack of
understanding (because of any thing include that maybe I am not that
smart) has brought me problems. I know this is an old issue. But
please help it.
Question : Could anyone show me a sample of using a monad as a
statefull variable?
For example see this code in C# :
//
public class Test
{
int var;
static void Fun1() { var = 0; Console.Write(var); }
static void Fun2() { var = var + 4; Console.Write(var); }
static void Main() { Fun1(); Fun2(); var = 10; Console.Write(var
= + var.ToString()); }
}
//
I want to see this code in haskell.
Ok, here you go. A state monad on top of IO, storing just your variable. Its
even 'initialised' to undefined at the start :)
import Control.Monad.State
main = execStateT (do f1; f2; put 10) undefined
f1 = do
let v = 0
put v
liftIO $ print v
f2 = do
v - get
let v' = v + 4
put v'
liftIO $ print v'
Running:
$ runhaskell A.hs
0
4
10
Of course, there are many other ways to do this, too.
-- Don
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Re: [Haskell-cafe] Monad Imparative Usage Example
On 8/2/06, Kaveh Shahbazian [EMAIL PROTECTED] wrote:
Haskell is the most powerfull and interesting thing I'v ever
encountered in IT world. But with an imparative background and lack of
understanding (because of any thing include that maybe I am not that
smart) has brought me problems. I know this is an old issue. But
please help it.
Question : Could anyone show me a sample of using a monad as a
statefull variable?
For example see this code in C# :
//
public class Test
{
int var;
static void Fun1() { var = 0; Console.Write(var); }
static void Fun2() { var = var + 4; Console.Write(var); }
static void Main() { Fun1(); Fun2(); var = 10; Console.Write(var
= + var.ToString()); }
}
//
I want to see this code in haskell.
Thankyou
You're doing IO so I guess the IO monad would be the way to go here.
So something like this:
import Data.IORef
main = do var - newIORef 0
fun1 var
fun2 var
writeIORef var 10
val - readIORef var
putStrLn ( var ++ show val)
fun1 var = do writeIORef var 0
val - readIORef var
print val
fun2 var = do modifyIORef var (+4)
val - readIORef var
print val
Notice that you have to pass the mutable reference around (no globals)
and extract its value explicitly whenever you want to use it.
You can also use mutable values using the ST monad rather than the IO
monad. This allows you to run the resulting actions from within
purely functional code, whereas there is no way to run an IO action
(i.e. you can't convert an IO Int to an Int - once you're in IO you
don't get out).
/S
--
Sebastian Sylvan
+46(0)736-818655
UIN: 44640862
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[Haskell-cafe] Monad Imparative Usage Example
Monad Imparative Usage Example
Thanks for your replies. I have not haskell on this computer and I
will try this solutions tonight.
I must notice that IO computations is not the point here. My target is
to have this code for mutable variable 'var'.
#
Haskell is the most powerfull and interesting thing I'v ever
encountered in IT world. But with an imparative background and lack of
understanding (because of any thing include that maybe I am not that
smart) has brought me problems. I know this is an old issue. But
please help it.
Question : Could anyone show me a sample of using a monad as a
statefull variable?
For example see this code in C# :
//
public class Test
{
int var;
static void Fun1() { var = 0; Console.Write(var); }
static void Fun2() { var = var + 4; Console.Write(var); }
static void Main() { Fun1(); Fun2(); var = 10; Console.Write(var
= + var.ToString()); }
}
//
I want to see this code in haskell.
Thankyou
#
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Re: [Haskell-cafe] Monad Imparative Usage Example
Kaveh Shahbazian wrote:
Haskell is the most powerfull and interesting thing I'v ever
encountered in IT world. But with an imparative background and lack of
understanding (because of any thing include that maybe I am not that
smart) has brought me problems. I know this is an old issue. But
please help it.
Question : Could anyone show me a sample of using a monad as a
statefull variable?
That question is a bit ill-posed. A monad is a type of interface. A stateful
variable would probably be an IORef or a STRef which can be created and used in
the IO and ST monads, respectively.
For example see this code in C# :
//
public class Test
{
int var;
static void Fun1() { var = 0; Console.Write(var); }
static void Fun2() { var = var + 4; Console.Write(var); }
static void Main() { Fun1(); Fun2(); var = 10; Console.Write(var
= + var.ToString()); }
}
//
I want to see this code in haskell.
Thankyou
___
Here is one translation:
module Imp where
import Data.IORef
data Test = Test {var :: IORef Int
,fun1 :: IO ()
,fun2 :: IO ()
,testMain :: IO ()
}
newTest :: IO Test
newTest = do var - newIORef 0
let fun1 = do writeIORef var 0
print = readIORef var
fun2 = do modifyIORef var (+4)
print = readIORef var
main = do fun1
fun2
writeIORef var 10
value - readIORef var
print (var = ++show value)
return Test {var = var
,fun1 = fun1
,fun2 = fun2
,testMain = main}
main :: IO ()
main = do
test - newTest
fun1 test
fun2 test
testMain test
print = readIORef (var test)
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Re: [Haskell-cafe] Monad Imparative Usage Example
kaveh.shahbazian: Monad Imparative Usage Example Thanks for your replies. I have not haskell on this computer and I will try this solutions tonight. I must notice that IO computations is not the point here. My target is to have this code for mutable variable 'var'. Still not entirely clear what your goal in the translation is. The most Haskell way would be to emulate a mutable variable with a state monad, but you seem to want an actual named mutable variable? So here's an example, like Seb's, where we create a mutable variable (in the IO monad) and mutate it all over the place. This uses similar scoping to your original code. For fun we use MVars instead of IORefs. import Control.Concurrent.MVar main = do var - newEmptyMVar let write = withMVar var print f1 = do putMVar var 0 write f2 = do modifyMVar_ var (return.(+4)) write f1 f2 swapMVar var 10 write Produces: $ runhaskell A.hs 0 4 10 Of course, if you're learning Haskell, you should probably try to /avoid/ mutable variables for a while. -- Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Monad Imparative Usage Example
Am Mittwoch, 2. August 2006 11:56 schrieb Kaveh Shahbazian:
Haskell is the most powerfull and interesting thing I'v ever
encountered in IT world. But with an imparative background and lack of
understanding (because of any thing include that maybe I am not that
smart) has brought me problems. I know this is an old issue. But
please help it.
Question : Could anyone show me a sample of using a monad as a
statefull variable?
For example see this code in C# :
//
public class Test
{
int var;
static void Fun1() { var = 0; Console.Write(var); }
static void Fun2() { var = var + 4; Console.Write(var); }
static void Main() { Fun1(); Fun2(); var = 10; Console.Write(var
= + var.ToString()); }
}
//
I want to see this code in haskell.
Thankyou
Well, I don't know C#, so maybe I misinterpreted Console.Write, but probably
not, so:
import Control.Monad.State
fun1 :: StateT Int IO ()
fun1 = do put 0
var - get
lift $ print var
fun2 :: StateT Int IO ()
fun2 = do modify (+4)
var - get
lift $ print var
mfun :: StateT Int IO ()
mfun = do fun1
fun2
put 10
var - get
lift $ putStrLn $ var = ++ show var
main :: IO ()
main = evalStateT mfun 0
-- since the initial state isn't used, even undefined would do
Another possibility would be (using State Int (IO ()) instead of
StateT Int IO ()):
import Control.Monad.State
fun1 = put (0::Int) get = return . print
fun2 = modify (+4) get = return . print
fin = put 10 get = return . putStrLn . (++) var = . show
mfun = sequence [fun1, fun2, fin]
main = sequence_ $ evalState mfun 0
but I deem the first preferable.
Also take a look at monad tutorials, e.g. Jeff Newbern's All About Monads
(sorry, I forgot the URL).
Cheers,
Daniel
--
In My Egotistical Opinion, most people's C programs should be
indented six feet downward and covered with dirt.
-- Blair P. Houghton
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Re: [Haskell-cafe] Monad Imparative Usage Example
Hello Kaveh, Wednesday, August 2, 2006, 1:56:10 PM, you wrote: Question : Could anyone show me a sample of using a monad as a statefull variable? monad is not an stateful variable, it's the way to organize computations, rule to join them (as the Ring of Supreme Power ;) ). i recommend you to read http://haskell.org/haskellwiki/IO_inside and all about monads tutorials -- Best regards, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Monad Imparative Usage Example
On Wed, 2 Aug 2006, Donald Bruce Stewart wrote: ... Of course, if you're learning Haskell, you should probably try to /avoid/ mutable variables for a while. Along the same line, I note that proposed solutions seem to use features relatively recently added to the language, is that true? StateT requires multi-parameter type class, for example, so it can't have been there all along. MVar is pretty new, isn't it? IORef must be the oldest of them, but hardly there from the start, I suspect. To learn core concepts, maybe it's a good idea to stay away from GHC in the beginning, and use Hugs or something that tends not to be so much of a magnet for new features. That forces you to look for a solution on the terms of the basic language concepts. Donn ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
