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 <bugfact@gmail.com> wrote:
From: Peter Verswyvelen <bugfact@gmail.com>
Subject: Re: [Haskell-cafe] Simple program. Simple problem?
To: "michael rice" <nowgate@yahoo.com>
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 <bugfact@gmail.com> 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 :-)
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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