Random
Documentation
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 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).
genRange :: g -> (Int, Int)Source
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.
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.
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)Source
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)Source
The same as randomR, but using a default range determined by the type:
randomRs :: RandomGen g => (a, a) -> g -> [a]Source
Plural variant of randomR, producing an infinite list of
random values instead of returning a new generator.
randoms :: RandomGen g => g -> [a]Source
Plural variant of random, producing an infinite list of
random values instead of returning a new generator.
randomRIO :: (a, a) -> IO aSource
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).
getStdRandom :: (StdGen -> (a, StdGen)) -> IO aSource
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))