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<TITLE>State Monad - using the updated state in an adhoc manner</TITLE>
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<FONT SIZE="4"><FONT FACE="Calibri, Verdana, Helvetica, Arial"><SPAN STYLE='font-size:11pt'>Hi,<BR>
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I’m a newbie looking to get my head around using the State Monad for random number generation. I’ve written non-monad code that achieves this no problem. When attempting to use the state monad I can get what I know to be the correct initial value and state, but can’t figure out for the life of me how to then increment it without binding more calls there and then. Doing several contiguous calls is not what I want to do here – and the examples I’ve read all show this (using something like liftM2 (,) myRandom myRandom). I want to be able to do:<BR>
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Get_a_random_number<BR>
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< a whole load of other stuff ><BR>
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Get the next number as defined by the updated state in the first call<BR>
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<some more stuff><BR>
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Get another number, and so on.<BR>
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I get the first number fine, but am lost at how to get the second, third, forth etc without binding there and then. I just want each number one at a time where and when I want it, rather than saying give 1,2,10 or even ‘n’ numbers now. I’m sure it’s blindly obvious!<BR>
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Note: I’m not using Haskell’s built in Random functionality (nor is that an option), I’ll spare the details of the method I’m using (NRC’s ranq1) as I know it works for the non-Monad case, and it’s irrelevent to the question. So the code is:<BR>
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ranq1 :: Word64 -> ( Double, Word64 )<BR>
ranq1 state = ( output, newState )<BR>
where<BR>
newState = ranq1Increment state<BR>
output = convert_to_double newState<BR>
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ranq1Init :: Word64 -> Word64<BR>
ranq1Init = convert_to_word64 . ranq1Increment . xor_v_init <BR>
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-- I’ll leave the detail of how ranq1Increment works out for brevity. I know this bit works fine. Same goes for the init function it’s just providing an initial state.<BR>
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-- The Monad State Attempt<BR>
getRanq1 :: State Word64 Double<BR>
getRanq1 = do<BR>
state <- get<BR>
let ( randDouble, newState ) = ranq1 state<BR>
put newState<BR>
return randDouble<BR>
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_________ And then in my main _________<BR>
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-- 124353542542 is just an arbitrary seed<BR>
main :: IO()<BR>
main = do<BR>
let x = evalState getRanq1 (ranq1Init 124353542542)<BR>
print (x)<BR>
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As I said this works fine; x gives me the correct first value for this sequence, but how do I then get the second and third without writing the giveMeTenRandoms style function? I guess what I want is a next() type function, imperatively speaking.<BR>
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Many thanks for any help,<BR>
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Phil.<BR>
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