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List:       haskell
Subject:    RE: Confused about Random
From:       "Simon Peyton-Jones" <simonpj () microsoft ! com>
Date:       2001-11-30 14:42:31
Message-ID: 1113DDB24D4A2841951BFDF86665EE19066517 () RED-MSG-10 ! redmond ! corp ! microsoft ! com
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Yes, the implementation of
	instance Random Int
in GHC's library pre-dates the existence of the genRange class
operation for RandomGen.  GHC's implementation of the instance
behaves extremely badly when given a random generator with
only two values (as in your example).

I would love someone to send me a better implementation of the
Random library.  Preferably someone who understands the pitfalls.

Simon


| -----Original Message-----
| From: Ian Lynagh [mailto:igloo@earth.li] 
| Sent: 29 November 2001 15:32
| To: haskell@haskell.org
| Subject: Confused about Random
| 
| 
| 
| With the following module:
| 
|     module Main where
| 
|     import Random
| 
|     data Foo = Foo StdGen
| 
|     main :: IO()
|     main = do let rs = randoms (Foo (mkStdGen 39)) :: [Int]
|                   rRs =  randomRs (0,9) (Foo (mkStdGen 39)) :: [Int]
|               putStrLn $ show $ take 100 rs
|               putStrLn $ show $ take 100 rRs
| 
|     instance RandomGen Foo where
|      genRange _ = (0, 1)
|      next (Foo g) = (val `mod` 2, Foo g')
|       where (val, g') = random g
|      split _ = error "Not implemented"
| 
| ghc gives me
| 
| [-2147476078,7482,-2147476078,-2147476078,-2147476079,-2147476
| 078,7483,7482,7482,-2147476079,-2147476078,-2147476079,-214747
| 6079,7482,-2147476078,-2147476078,-2147476078,-2147476079,7482
| ,7483,7482,7483,7482,-2147476078,-2147476078,-2147476078,7483,
| -2147476079,7482,-2147476078,-2147476079,-2147476078,7483,-214
| 7476079,7483,7482,-2147476079,7483,7482,7483,-2147476078,-2147
| 476079,-2147476079,7482,-2147476078,7482,-2147476079,-21474760
| 79,7482,-2147476078,7483,7483,-2147476079,-2147476078,7483,748
| 3,-2147476078,-2147476079,-2147476078,-2147476079,-2147476078,
| 7483,7483,7482,7482,7483,-2147476078,-2147476079,-2147476079,7
| 482,7483,-2147476078,-2147476079,-2147476079,-2147476078,7482,
| 7483,-2147476079,7482,7482,7482,7483,-2147476079,-2147476079,-
| 2147476078,7482,7482,7482,7482,-2147476079,7482,7482,-21474760
| 79,7483,-2147476078,7482,7483,-2147476079,-2147476079,7482]
| [1,2,2,1,1,2,1,2,1,1,1,2,2,2,2,1,2,1,1,1,1,2,1,1,1,1,2,1,1,2,1
| ,2,1,2,1,1,2,1,2,2,2,1,2,2,2,1,1,2,1,2,1,2,2,2,1,1,2,1,1,2,1,1
| ,1,2,2,2,1,1,2,2,2,1,1,1,2,2,2,1,2,2,1,2,1,1,1,1,2,1,1,2,2,1,1
| ,1,1,1,2,1,1,2]
| 
| The first list doesn't seem to cover the whole spectrum and I 
| was expecting the second list to be composed over 0s through 
| 9s. Is my understanding wrong?
| 
| nhc complains
|     Context for Random.Random needed in left hand pattern at 
| 17:11. and hugs doesn't seem to know genRange exists. With it 
| commented out it returns the same as ghc.
| 
| 
| Thanks
| Ian
| 
| 
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