Simple program. Simple problem?

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michael rice

11 Oct 2009 11 Oct '09

6:10 p.m.

What is going wrong here? Michael ======= import System.Random coinToss :: StdGen -> IO () coinToss gen = putStrLn "What's your guess, heads or tails ('h' or 't')?" >> getChar >>= \c -> let (randInt, _) = randomR(0,1) gen :: (Int, StdGen) in if c == ((!!) "ht" randInt) then putStrLn "You win!" else putStrLn "You lose!" main = do gen <- getStdGen coinToss gen gen <- newStdGen main ======= [michael@localhost ~]$ runhaskell cointoss.hs What's your guess, heads or tails ('h' or 't')? h You win! What's your guess, heads or tails ('h' or 't')? You lose! What's your guess, heads or tails ('h' or 't')? h You lose! What's your guess, heads or tails ('h' or 't')? You lose! What's your guess, heads or tails ('h' or 't')? ^Ccointoss.hs: cointoss.hs: interrupted [michael@localhost ~]$

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Peter Verswyvelen

11 Oct 11 Oct

6:24 p.m.

It always helps to put a Debug.Trace.trace: in if trace (show (fromEnum c)) $ c == ((!!) "ht" randInt) then p What's your guess, heads or tails ('h' or 't')? h 104 You win! What's your guess, heads or tails ('h' or 't')? *10* You lose! What's your guess, heads or tails ('h' or 't')? So getChar also receives the linefeed character. An easy way to get around this, is to use getLine instead and just use the first character, as in >> fmap head getLine But of course we're hacking away here :-) On Mon, Oct 12, 2009 at 12:10 AM, michael rice wrote:

...

What is going wrong here?

Michael

=======

import System.Random

coinToss :: StdGen -> IO () coinToss gen = putStrLn "What's your guess, heads or tails ('h' or 't')?" >> getChar >>= \c -> let (randInt, _) = randomR(0,1) gen :: (Int, StdGen) in if c == ((!!) "ht" randInt) then putStrLn "You win!" else putStrLn "You lose!"

main = do gen <- getStdGen coinToss gen gen <- newStdGen main

=======

[michael@localhost ~]$ runhaskell cointoss.hs What's your guess, heads or tails ('h' or 't')? h You win! What's your guess, heads or tails ('h' or 't')? You lose! What's your guess, heads or tails ('h' or 't')? h You lose! What's your guess, heads or tails ('h' or 't')? You lose! What's your guess, heads or tails ('h' or 't')? ^Ccointoss.hs: cointoss.hs: interrupted [michael@localhost ~]$

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Peter Verswyvelen

6:42 p.m.

btw I always find it amusing to play with interact and lazy IO: guess :: [Char] -> [String] -> [String] guess (c:cs) ~(i:is) = "What's your guess, heads or tails ('h' or 't')?" : (if [c]==i then "You win!" else "You lose!") : guess cs is main = do gen <- getStdGen let rs = randomRs (0,1::Int) gen cs = map ("ht"!!) rs interact $ unlines . guess cs . lines On Mon, Oct 12, 2009 at 12:24 AM, Peter Verswyvelen wrote:

...

It always helps to put a Debug.Trace.trace: in if trace (show (fromEnum c)) $ c == ((!!) "ht" randInt) then p

What's your guess, heads or tails ('h' or 't')? h 104 You win! What's your guess, heads or tails ('h' or 't')? *10* You lose! What's your guess, heads or tails ('h' or 't')?

So getChar also receives the linefeed character.

An easy way to get around this, is to use getLine instead and just use the first character, as in

>> fmap head getLine

But of course we're hacking away here :-)

On Mon, Oct 12, 2009 at 12:10 AM, michael rice wrote:

...
What is going wrong here?

Michael

=======

import System.Random

coinToss :: StdGen -> IO () coinToss gen = putStrLn "What's your guess, heads or tails ('h' or 't')?" >> getChar >>= \c -> let (randInt, _) = randomR(0,1) gen :: (Int, StdGen) in if c == ((!!) "ht" randInt) then putStrLn "You win!" else putStrLn "You lose!"

main = do gen <- getStdGen coinToss gen gen <- newStdGen main

=======

[michael@localhost ~]$ runhaskell cointoss.hs What's your guess, heads or tails ('h' or 't')? h You win! What's your guess, heads or tails ('h' or 't')? You lose! What's your guess, heads or tails ('h' or 't')? h You lose! What's your guess, heads or tails ('h' or 't')? You lose! What's your guess, heads or tails ('h' or 't')? ^Ccointoss.hs: cointoss.hs: interrupted [michael@localhost ~]$

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michael rice

6:56 p.m.

Thanks for the tip, and the lazy example. I think I'm finally beginning to "get" monads, so I decided to test my understanding with this small example. So far so good, except for that little bump in the road. Onward and upward. Michael --- On Sun, 10/11/09, Peter Verswyvelen wrote: From: Peter Verswyvelen Subject: Re: [Haskell-cafe] Simple program. Simple problem? To: "michael rice" Cc: haskell-cafe@haskell.org Date: Sunday, October 11, 2009, 6:42 PM btw I always find it amusing to play with interact and lazy IO: guess :: [Char] -> [String] -> [String]guess (c:cs) ~(i:is) = "What's your guess, heads or tails ('h' or 't')?" : (if [c]==i then "You win!" else "You lose!") : guess cs is main = do gen <- getStdGen let rs = randomRs (0,1::Int) gen cs = map ("ht"!!) rs interact $ unlines . guess cs . lines On Mon, Oct 12, 2009 at 12:24 AM, Peter Verswyvelen wrote: It always helps to put a Debug.Trace.trace: in if trace (show (fromEnum c)) $ c == ((!!) "ht" randInt) then p What's your guess, heads or tails ('h' or 't')? h104You win!What's your guess, heads or tails ('h' or 't')?10You lose!What's your guess, heads or tails ('h' or 't')? So getChar also receives the linefeed character. An easy way to get around this, is to use getLine instead and just use the first character, as in >> fmap head getLine But of course we're hacking away here :-) On Mon, Oct 12, 2009 at 12:10 AM, michael rice wrote: What is going wrong here? Michael ======= import System.Random coinToss :: StdGen -> IO () coinToss gen = putStrLn "What's your guess, heads or tails ('h' or 't')?" >> getChar >>= \c -> let (randInt, _) = randomR(0,1) gen :: (Int, StdGen) in if c == ((!!) "ht" randInt) then putStrLn "You win!" else putStrLn "You lose!" main = do gen <- getStdGen coinToss gen gen <- newStdGen main ======= [michael@localhost ~]$ runhaskell cointoss.hs What's your guess, heads or tails ('h' or 't')? h You win! What's your guess, heads or tails ('h' or 't')? You lose! What's your guess, heads or tails ('h' or 't')? h You lose! What's your guess, heads or tails ('h' or 't')? You lose! What's your guess, heads or tails ('h' or 't')? ^Ccointoss.hs: cointoss.hs: interrupted [michael@localhost ~]$ _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

Felipe Lessa

7:08 p.m.

On Mon, Oct 12, 2009 at 12:42:16AM +0200, Peter Verswyvelen wrote:

...

btw I always find it amusing to play with interact and lazy IO:

I always find it frightening to play with lazy IO :). -- Felipe.

michael rice

7:24 p.m.

There's still something screwy going on here with the random generation or passing the gen to cointoss. Shouldn't newStdGen be updating getStdGen? I've been running it and entering 'h' four (4) times, then aborting with CNTL-C. Each time I either get You lose! You lose! You win! You lose! or You win! You win! You lose! You win! That's all, should be getting some variation. Michael ====================== import System.Random coinToss :: StdGen -> IO () coinToss gen = putStrLn "What's your guess, heads or tails ('h' or 't')?" >> fmap head getLine >>= \c -> let (randInt, _) = randomR(0,1) gen :: (Int, StdGen) in if c == ((!!) "ht" randInt) then putStrLn "You win!" else putStrLn "You lose!" main = do gen <- getStdGen coinToss gen gen <- newStdGen main --- On Sun, 10/11/09, Felipe Lessa wrote: From: Felipe Lessa Subject: Re: [Haskell-cafe] Simple program. Simple problem? To: haskell-cafe@haskell.org Date: Sunday, October 11, 2009, 7:08 PM On Mon, Oct 12, 2009 at 12:42:16AM +0200, Peter Verswyvelen wrote:

...

btw I always find it amusing to play with interact and lazy IO:

I always find it frightening to play with lazy IO :). -- Felipe. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

Reid Barton

8:17 p.m.

New subject: split (was Re: Simple program. Simple problem?)

It seems that the definition of split in System.Random is not really satisfactory. Imagine a tree-like computation of the form f gen = {- some expression using b, g1, g2 -} where b = fst (random gen) :: Bool (gen1, gen2) = split gen g1 = f gen1 g2 = f gen2 Let's look at the first 30 values of b produced along the right side of the tree: GHCi> length $ nub [ take 30 . map (fst . random) . iterate (snd . split) $ mkStdGen i :: [Bool] | i <- take 10000 . randoms $ mkStdGen 0 ] 10000 Great, we tried 10000 different initial gens and got 10000 different sequences. Now let's look at the left side: GHCi> length $ nub [ take 30 . map (fst . random) . iterate (fst . split) $ mkStdGen i :: [Bool] | i <- take 10000 . randoms $ mkStdGen 0 ] 8 This doesn't seem good. Michael's code (below) is effectively doing iterate (fst . split). Regards, Reid Barton On Sun, Oct 11, 2009 at 04:24:55PM -0700, michael rice wrote:

...

main = do gen <- getStdGen coinToss gen gen <- newStdGen main

Reid Barton

12 Oct 12 Oct

12:53 a.m.

New subject: split (was Re: Simple program. Simple problem?)

On Sun, Oct 11, 2009 at 08:17:48PM -0400, Reid Barton wrote:

...

It seems that the definition of split in System.Random is not really satisfactory.

For the curious, the reason for the asymmetry between fst . split and snd . split is that the RNG states produced by mkStdGen have varying first component but second component (virtually) always equal to 1. For full details see http://hackage.haskell.org/trac/ghc/ticket/3575 Regards, Reid Barton

michael rice

11 Oct 11 Oct

7:47 p.m.

For the code below, if I CNTL-C after the list is printed, and run it again, I get a different list of colors, each time. This doesn't seem to be happening with my earlier example. Michael ============= import System.Random import Data.Ord data Color = Red | Blue | Green | Yellow | Orange | Brown deriving (Show, Read, Eq, Enum, Ord, Bounded) main = do gen <- getStdGen let code = map toEnum $ take 4 $ randomRs (0,5) gen :: [Color] putStrLn $ "List is " ++ show code guessString <- getLine newGen <- newStdGen main --- On Sun, 10/11/09, Felipe Lessa wrote: From: Felipe Lessa Subject: Re: [Haskell-cafe] Simple program. Simple problem? To: haskell-cafe@haskell.org Date: Sunday, October 11, 2009, 7:08 PM On Mon, Oct 12, 2009 at 12:42:16AM +0200, Peter Verswyvelen wrote:

...

btw I always find it amusing to play with interact and lazy IO:

Peter Verswyvelen

12 Oct 12 Oct

4:30 p.m.

On Mon, Oct 12, 2009 at 1:08 AM, Felipe Lessa wrote:

...

On Mon, Oct 12, 2009 at 12:42:16AM +0200, Peter Verswyvelen wrote:

...
btw I always find it amusing to play with interact and lazy IO:

I always find it frightening to play with lazy IO :).

yes, I guess that's why I like it, because I'm still an imperative / OO programmer ;-)

...

-- Felipe. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Felipe Lessa
michael rice
Peter Verswyvelen
Reid Barton