17  Random Numbers

module Random ( RandomGen(next, split, genRange), StdGen, mkStdGen, Random( random,   randomR,  randoms,  randomRs, randomIO, randomRIO ), getStdRandom, getStdGen, setStdGen, newStdGen   ) where ---------------- The RandomGen class ------------------------ class RandomGen g where   genRange :: g -> (Int, Int)   next     :: g -> (Int, g)   split    :: g -> (g, g) ---------------- A standard instance of RandomGen ----------- data StdGen = ... -- Abstract instance RandomGen StdGen where ... instance Read     StdGen where ... instance Show     StdGen where ... mkStdGen :: Int -> StdGen ---------------- The Random class --------------------------- class Random a where    randomR :: RandomGen g => (a, a) -> g -> (a, g)    random  :: RandomGen g => g -> (a, g)    randomRs :: RandomGen g => (a, a) -> g -> [a]    randoms  :: RandomGen g => g -> [a]    randomRIO :: (a,a) -> IO a    randomIO  :: IO a instance Random Int     where ... instance Random Integer where ... instance Random Float   where ... instance Random Double  where ... instance Random Bool    where ... instance Random Char    where ... ---------------- The global random generator ---------------- newStdGen    :: IO StdGen setStdGen    :: StdGen -> IO () getStdGen    :: IO StdGen getStdRandom :: (StdGen -> (a, StdGen)) -> IO a

The Random library deals with the common task of pseudo-random number generation. The library makes it possible to generate repeatable results, by starting with a specified initial random number generator; or to get different results on each run by using the system-initialised generator, or by supplying a seed from some other source.

The library is split into two layers:

• A core random number generator provides a supply of bits. The class RandomGen provides a common interface to such generators.

• The class Random provides a way to extract particular values from a random number generator. For example, the Float instance of Random allows one to generate random values of type Float.

17.1  The RandomGen class, and the StdGen generator

The class RandomGen provides a common interface to random number generators.

  class RandomGen g where
    genRange :: g -> (Int,Int)
    next     :: g  -> (Int, g)
    split    :: g -> (g, g)
  
    -- Default method
    genRange g = (minBound,maxBound)

• The genRange operation yields the range of values returned by the generator.

  It is required that:

  • If (a,b) = genRange g, then a < b.
  • genRange _|_ /= _|_.
  The second condition ensures that genRange cannot examine its argument, and hence the value it returns can be determined only by the instance of RandomGen. That in turn allows an implementation to make a single call to genRange to establish a generator's range, without being concerned that the generator returned by (say) next might have a different range to the generator passed to next.

• The next operation returns an Int that is uniformly distributed in the range returned by genRange (including both end points), and a new generator.

• The split operation allows one to obtain two independent random number generators. This is very useful in functional programs (for example, when passing a random number generator down to recursive calls), but very little work has been done on statistically robust implementations of split ([1,4] are the only examples we know of).

The Random library provides one instance of RandomGen, the abstract data type StdGen:

  data StdGen = ... -- Abstract
  
  instance RandomGen StdGen where ...
  instance Read      StdGen where ...
  instance Show      StdGen where ...
  
  mkStdGen :: Int -> StdGen

The StgGen instance of RandomGen has a genRange of at least 30 bits.

The result of repeatedly using next should be at least as statistically robust as the "Minimal Standard Random Number Generator" described by [2,3]. Until more is known about implementations of split, all we require is that split deliver generators that are (a) not identical and (b) independently robust in the sense just given.

The Show/Read instances of StdGen provide a primitive way to save the state of a random number generator. It is required that read (show g) == g.

In addition, read may be used to map an arbitrary string (not necessarily one produced by show) onto a value of type StdGen. In general, the read instance of StdGen has the following properties:

• It guarantees to succeed on any string.
• It guarantees to consume only a finite portion of the string.
• Different argument strings are likely to result in different results.

The function mkStdGen provides an alternative way of producing an initial generator, by mapping an Int into a generator. Again, distinct arguments should be likely to produce distinct generators.

Programmers may, of course, supply their own instances of RandomGen.

Implementation warning. A superficially attractive implementation of split is

  instance RandomGen MyGen where
    ...
    split g = (g, variantOf g)

Here, split returns g itself and a new generator derived from g. But now consider these two apparently-independent generators:

  g1 = snd (split g)
  g2 = snd (split (fst (split g)))

If split genuinely delivers independent generators (as specified), then g1 and g2 should be independent, but in fact they are both equal to variantOf g. Implementations of the above form do not meet the specification.

17.2  The Random class

With a source of random number supply in hand, the Random class allows the programmer to extract random values of a variety of types:

class Random a where
   randomR :: RandomGen g => (a, a) -> g -> (a, g)
   random  :: RandomGen g => g -> (a, g)

   randomRs :: RandomGen g => (a, a) -> g -> [a]
   randoms  :: RandomGen g => g -> [a]

   randomRIO :: (a,a) -> IO a
   randomIO :: IO a

     -- Default methods
   randoms g = x : randoms g' 
   where 
     (x,g') = random g
   randomRs = ...similar...

   randomIO        = getStdRandom random
   randomRIO range = getStdRandom (randomR range)


instance Random Int     where ...
instance Random Integer where ...
instance Random Float   where ...
instance Random Double  where ...
instance Random Bool    where ...
instance Random Char    where ...

• randomR takes a range (lo,hi) and a random number generator g, and returns a random value uniformly distributed in the closed interval [lo,hi], together with a new generator. It is unspecified what happens if lo>hi. For continuous types there is no requirement that the values lo and hi are ever produced, but they may be, depending on the implementation and the interval.

• random does the same as randomR, but does not take a range.
  • For bounded types (instances of Bounded, such as Char), the range is normally the whole type.
  • For fractional types, the range is normally the semi-closed interval [0,1).
  • For Integer, the range is (arbitrarily) the range of Int.

• The plural versions, randomRs and randoms, produce an infinite list of random values, and do not return a new generator.

• The IO versions, randomRIO and randomIO, use the global random number generator (see Section 17.3).

17.3  The global random number generator

There is a single, implicit, global random number generator of type StdGen, held in some global variable maintained by the IO monad. It is initialised automatically in some system-dependent fashion, for example, by using the time of day, or Linux's kernel random number generator. To get deterministic behaviour, use setStdGen.

  setStdGen    :: StdGen -> IO ()
  getStdGen    :: IO StdGen
  newStdGen    :: IO StdGen
  getStdRandom :: (StdGen -> (a, StdGen)) -> IO a

• getStdGen and setStdGen get and set the global random number generator, respectively.

• newStdGen applies split to the current global random generator, updates it with one of the results, and returns the other.

• getStdRandom uses the supplied function to get a value from the current global random generator, and updates the global generator with the new generator returned by the function. For example, rollDice gets a random integer between 1 and 6:
  
    rollDice :: IO Int
    rollDice = getStdRandom (randomR (1,6))
  
  

References

[1]

FW Burton and RL Page, "Distributed random number generation", Journal of Functional Programming, 2(2):203-212, April 1992.

[2]

SK Park, and KW Miller, "Random number generators - good ones are hard to find", Comm ACM 31(10), Oct 1988, pp1192-1201.

[3]

DG Carta, "Two fast implementations of the minimal standard random number generator", Comm ACM, 33(1), Jan 1990, pp87-88.

[4]

P Hellekalek, "Don't trust parallel Monte Carlo", Department of Mathematics, University of Salzburg, http://random.mat.sbg.ac.at/~peter/pads98.ps, 1998.

The Web site http://random.mat.sbg.ac.at/ is a great source of information.