System.Random.randomIvalInteger makes invalid assumptions about RandomGen
The existing API for System.Random.RandomGen
allows a random number generator (RNG) to produce Ints within an arbitrary range specified by genRange
.
For example, the following RandomGen
produces only zeros and ones, but should be legitimate:
import System.Random
data BinRNG = BinRNG StdGen
instance RandomGen BinRNG where
next (BinRNG g) = (x `mod` 2, BinRNG g')
where (x,g') = next g
split (BinRNG g) = (BinRNG g1, BinRNG g2)
where (g1,g2) = split g
genRange _ = (0,1)
ls :: [Int]
ls = randoms (BinRNG$ mkStdGen 38388)
main = print $ take 20 ls
But System.Random.randomIvalInteger
makes invalid assumptions about the amount of randomness produced by next
. (Specifically, assuming that it creates the same amount as StdGen
.) Thus, the above program will create an output with only a couple of unique ints (rather than 2^64).
For example:
[4611734781337924537,4611734781337924537,-9223323645458902796,
-9223323645458902797,4611734783485408099,4611734783485408098,
-9223323645458902796,-9223323647606386357,4611734781337924538,
-9223323645458902796,-9223323645458902797,
-9223323647606386357,4611734783485408098,4611734783485408098,
-9223323647606386357,4611734781337924538,4611734781337924537,
-9223323645458902796,4611734783485408099,4611734781337924538]
Trac metadata
Trac field | Value |
---|---|
Version | 7.0.3 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | libraries/random |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |