[Haskell-cafe] Re: Haskell code for this example of flow control
Here are some even older discussions on the subject. I don't know if anyone ever put them into a library or on the wiki. Dominic. http://haskell.org/pipermail/haskell-cafe/2005-May/009784.html http://www.haskell.org//pipermail/libraries/2005-February/003143.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Haskell code for this example of flow control
Hello Maurício, Friday, February 03, 2006, 7:28:16 PM, you wrote: M>I wonder if I could write a generic while based on your example: while :: (a ->> IO a) -> (a -> Bool) -> IO () M>I'll probably learn something trying that. i have about 5-10 imperative control structures defined in my common lib, including while, until, forever, doInChunks -- Best regards, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Haskell code for this example of flow control
On Feb 3, 2006, at 11:28 AM, Maurício wrote:
Kurt Hutchinson wrote:
On 2/2/06, Maurício <[EMAIL PROTECTED]> wrote:
I understand those examples, but I really would like to know
how to
do that with monads. I would like to ask the same question, but
now with
this code:
double a = 1000;
double b = 0;
while (a != b) {
a /= 2;
cout << a; // Prints a
cin << b; // User gives a number, stored in b
};
An idiomatic approach:
example :: Double -> Double -> IO ()
example a b
| a == b= return ()
| otherwise = do
let a' = a / 2
print a'
b' <- readLn
example a' b'
main = example 1000 0
Thanks! Robert's, Chris' and yours examples solved many of my
questions. I understand I can insert modifications in IORefs (as
used by Robert and Chris) inside the loop above:
| otherwise = do
let a' = a / 2
...
modifyIORef some_ioref some_function
...
example a' b'
I wonder if I could write a generic while based on your example:
while :: (a -> IO a) -> (a -> Bool) -> IO ()
I'll probably learn something trying that.
FYI, here's a thread from a few months back about monad control
structures; it may also provide some enlightenment.
http://www.haskell.org/pipermail/haskell-cafe/2005-October/011890.html
Rob Dockins
Speak softly and drive a Sherman tank.
Laugh hard; it's a long way to the bank.
-- TMBG
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[Haskell-cafe] Re: Haskell code for this example of flow control
Kurt Hutchinson wrote:
On 2/2/06, Maurício <[EMAIL PROTECTED]> wrote:
I understand those examples, but I really would like to know how to
do that with monads. I would like to ask the same question, but now with
this code:
double a = 1000;
double b = 0;
while (a != b) {
a /= 2;
cout << a; // Prints a
cin << b; // User gives a number, stored in b
};
An idiomatic approach:
example :: Double -> Double -> IO ()
example a b
| a == b= return ()
| otherwise = do
let a' = a / 2
print a'
b' <- readLn
example a' b'
main = example 1000 0
Thanks! Robert's, Chris' and yours examples solved many of my
questions. I understand I can insert modifications in IORefs (as used by
Robert and Chris) inside the loop above:
| otherwise = do
let a' = a / 2
...
modifyIORef some_ioref some_function
...
example a' b'
I wonder if I could write a generic while based on your example:
while :: (a -> IO a) -> (a -> Bool) -> IO ()
I'll probably learn something trying that.
Best,
Maurício
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Re: [Haskell-cafe] Re: Haskell code for this example of flow control
On 2/2/06, Maurício <[EMAIL PROTECTED]> wrote:
>I understand those examples, but I really would like to know how to
> do that with monads. I would like to ask the same question, but now with
> this code:
>
> double a = 1000;
> double b = 0;
> while (a != b) {
> a /= 2;
> cout << a; // Prints a
> cin << b; // User gives a number, stored in b
> };
An idiomatic approach:
example :: Double -> Double -> IO ()
example a b
| a == b= return ()
| otherwise = do
let a' = a / 2
print a'
b' <- readLn
example a' b'
main = example 1000 0
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Re: [Haskell-cafe] Re: Haskell code for this example of flow control
Maurício wrote:
> I understand those examples, but I really would like to know how to do
> that with monads. I would like to ask the same question, but now with
> this code:
>
> double a = 1000;
> double b = 0;
> while (a != b) {
> a /= 2;
> cout << a; // Prints a
> cin << b; // User gives a number, stored in b
> };
>
A close to line-for-line translation:
import Data.IORef
import Control.Monad(liftM2,when)
main = example 1000
example :: Double -> IO ()
example originalA = do
refA <- newIORef originalA -- allocate local variable a
refB <- newIORef 0 -- allocate local variable b
let loop = do -- loop in scope of refA, refB
flag <- liftM2 (/=) (readIORef refA) (readIORef refB)
when flag $ do
modifyIORef refA (/2)
print =<< readIORef refA
-- This will give an error if not a number:
writeIORef refB =<< readIO =<< getLine
loop
loop -- start executing loop
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Re: [Haskell-cafe] Re: Haskell code for this example of flow control
I think you're looking for IORef http://www.haskell.org/ghc/docs/
latest/html/libraries/base/Data-IORef.html
Something like this (untested) should do what you want:
example :: IO ()
example = do { ref <- newIORef 1000; loop ref }
where loop ref = do
x <- readIORef ref
print x
when (x>1) (writeIORef ref (x/2) >> loop ref)
On Feb 2, 2006, at 10:57 AM, Maurício wrote:
Donald Bruce Stewart wrote:
briqueabraque:
Hi,
I would like to know what options I have in Haskell to do
something similar to this C++ code:
double a = 1000;
while (a>1) a/=2;
I'm able to do that with lists, but I would like to know how to
do that with monads and variables with state.
You'll get good code using a normal recusive loop:
main = print (loop 1000)
where
loop a | a <= 1= a| otherwise
= loop (a/2)
All such control structures may be implemented using recursion.
-- Don
I understand those examples, but I really would like to know how
to do that with monads. I would like to ask the same question, but
now with this code:
double a = 1000;
double b = 0;
while (a != b) {
a /= 2;
cout << a; // Prints a
cin << b; // User gives a number, stored in b
};
Best,
Maurício
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Laugh hard; it's a long way to the bank.
-- TMBG
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[Haskell-cafe] Re: Haskell code for this example of flow control
Donald Bruce Stewart wrote:
briqueabraque:
Hi,
I would like to know what options I have in Haskell to do something
similar to this C++ code:
double a = 1000;
while (a>1) a/=2;
I'm able to do that with lists, but I would like to know how to do
that with monads and variables with state.
You'll get good code using a normal recusive loop:
main = print (loop 1000)
where
loop a | a <= 1= a
| otherwise = loop (a/2)
All such control structures may be implemented using recursion.
-- Don
I understand those examples, but I really would like to know how to
do that with monads. I would like to ask the same question, but now with
this code:
double a = 1000;
double b = 0;
while (a != b) {
a /= 2;
cout << a; // Prints a
cin << b; // User gives a number, stored in b
};
Best,
Maurício
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