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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
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