| Portability | portable |
|---|---|
| Stability | stable |
| Maintainer | libraries@haskell.org |
| Safe Haskell | Trustworthy |
System.Random
Contents
Description
This 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
RandomGenprovides a common interface to such generators. The library provides one instance ofRandomGen, the abstract data typeStdGen. Programmers may, of course, supply their own instances ofRandomGen. - The class
Randomprovides a way to extract values of a particular type from a random number generator. For example, theFloatinstance ofRandomallows one to generate random values of typeFloat.
This implementation uses the Portable Combined Generator of L'Ecuyer [System.Random] for 32-bit computers, transliterated by Lennart Augustsson. It has a period of roughly 2.30584e18.
Random number generators
class RandomGen g where
The class RandomGen provides a common interface to random number
generators.
Methods
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 genRange operation yields the range of values returned by
the generator.
It is required that:
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 default definition spans the full range of Int.
split :: g -> (g, g)
The split operation allows one to obtain two distinct 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 ([System.Random, System.Random]
are the only examples we know of).
Standard random number generators
data StdGen
The StdGen 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
[System.Random, System.Random].
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 and 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, reads 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 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.
getStdRandom :: (StdGen -> (a, StdGen)) -> IO a
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))
Applies split to the current global random generator,
updates it with one of the results, and returns the other.
Random values of various types
class Random a where
With a source of random number supply in hand, the Random class allows the
programmer to extract random values of a variety of types.
Methods
randomR :: RandomGen g => (a, a) -> g -> (a, g)
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 :: RandomGen g => g -> (a, g)
The same as randomR, but using a default range determined by the type:
randomRs :: RandomGen g => (a, a) -> g -> [a]
Plural variant of randomR, producing an infinite list of
random values instead of returning a new generator.
randoms :: RandomGen g => g -> [a]
Plural variant of random, producing an infinite list of
random values instead of returning a new generator.
A variant of randomR that uses the global random number generator
(see System.Random).
A variant of random that uses the global random number generator
(see System.Random).
Instances
References
- FW Burton and RL Page, Distributed random number generation, Journal of Functional Programming, 2(2):203-212, April 1992.
- SK Park, and KW Miller, /Random number generators - good ones are hard to find/, Comm ACM 31(10), Oct 1988, pp1192-1201.
- DG Carta, /Two fast implementations of the minimal standard random number generator/, Comm ACM, 33(1), Jan 1990, pp87-88.
- P Hellekalek, Don\'t trust parallel Monte Carlo, Department of Mathematics, University of Salzburg, http://random.mat.sbg.ac.at/~peter/pads98.ps, 1998.
- Pierre L'Ecuyer, /Efficient and portable combined random number generators/, Comm ACM, 31(6), Jun 1988, pp742-749.
The Web site http://random.mat.sbg.ac.at/ is a great source of information.