Matt Harden wrote:
>
> ... Darn. Now I want the genRange operation again. The
> "clarification" can really be a significant performance cost. Not to
> mention, you're also shortening the period of the RNG (perhaps by a
> factor of 4!).
>
> Can we have both (see below)?
>
> > class RandomGen g where
> > next :: g -> (Int, g)
> > split :: g -> (g, g)
> > genRange :: g -> (Int, Int)
> > genRange _ = (minBound, maxBound)
>
> Put the "clarification" in, but then allow the implementer to optionally
> provide a different range? If a genRange is provided, it should fit the
> requirements described earlier, of course. And as someone suggested,
> (genRange _|_) should always be defined (not _|_).
>
> Regards,
> Matt Harden
Simon, do you think we could have the *optional* genRange described
above? If that approach is chosen, then the changes to GHC and Hugs
are rather painless. In fact, below I've attached a patch based on
Hugs98-Sept1999 (a context diff). After this patch, my tests that
failed before now pass with flying colors. I do need to put some
comments in though. By the way, I think it's clear that much of the
code in Random.hs could be made more efficient. I went for the
simplest possible change in this case, however.
*** Random.hs.orig Fri Oct 15 21:50:13 1999
--- Random.hs Fri Feb 4 22:57:39 2000
***************
*** 28,33 ****
--- 28,35 ----
class RandomGen g where
next :: g -> (Int, g)
split :: g -> (g, g)
+ genRange :: g -> (Int, Int)
+ genRange = const (minBound, maxBound)
-- An efficient and portable combined random number generator: ---------------
***************
*** 84,94 ****
--- 86,100 ----
StdGen t1 t2 = snd (next std)
+ stdRange :: StdGen -> (Int, Int)
+ stdRange = const (1, 2147483562)
+
-- A standard instance of RandomGen: -----------------------------------------
instance RandomGen StdGen where
next = stdNext
split = stdSplit
+ genRange = stdRange
instance Show StdGen where
showsPrec p (StdGen s1 s2)
***************
*** 167,182 ****
randomIvalInteger :: (RandomGen g, Num a) => (Integer, Integer) -> g -> (a, g)
randomIvalInteger (l,h) rng
| l > h = randomIvalInteger (h,l) rng
! | otherwise = case (f n 1 rng) of
! (v, rng') -> (fromInteger (l + v `mod` k), rng')
where
k = h - l + 1
! b = 2147483561
n = iLogBase b k
f 0 acc g = (acc, g)
f n acc g = let (x,g') = next g
! in f (n-1) (fromInt x + acc * b) g'
randomIvalDouble :: (RandomGen g, Fractional a)
=> (Double, Double) -> (Double -> a) -> g -> (a, g)
--- 173,190 ----
randomIvalInteger :: (RandomGen g, Num a) => (Integer, Integer) -> g -> (a, g)
randomIvalInteger (l,h) rng
| l > h = randomIvalInteger (h,l) rng
! | otherwise = case (f n 0 rng) of
! (v, rng') | v < (b^n `mod` k) -> randomIvalInteger (l,h) rng'
! | otherwise -> (fromInteger (l + v `mod` k), rng')
where
k = h - l + 1
! (c,d) = genRange rng
! b = (toInteger d) - (toInteger c) + 1
n = iLogBase b k
f 0 acc g = (acc, g)
f n acc g = let (x,g') = next g
! in f (n-1) ((fromInt x - fromInt c) + acc * b) g'
randomIvalDouble :: (RandomGen g, Fractional a)
=> (Double, Double) -> (Double -> a) -> g -> (a, g)
***************
*** 197,203 ****
intRange = toInteger (maxBound::Int) - toInteger (minBound::Int)
iLogBase :: Integer -> Integer -> Integer
! iLogBase b i = if i < b then 1 else 1 + iLogBase b (i `div` b)
-- The global standard random number generator: ------------------------------
--- 205,212 ----
intRange = toInteger (maxBound::Int) - toInteger (minBound::Int)
iLogBase :: Integer -> Integer -> Integer
! iLogBase b 1 = 0
! iLogBase b i | i>1 = 1 + iLogBase b ((i-1) `div` b + 1)
-- The global standard random number generator: ------------------------------
