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System.Random.Stateful

As explained in Learn Haskell by Example

System.Random.Stateful is a Haskell library module providing stateful random-generation interfaces designed to support safer concurrent use.

System.Random.Stateful provides a modern interface for generating random values of many types. The examples focus on StdGen, a typical pseudo-random generator, and contrast its safer interface with deprecated random-generation functions. section 9.1.2 section 9.1.4

The main practical pieces are:

  • Add random, for example random >= 1.2.1.1, to package.yaml dependencies. section 9.1.2
  • Use StdGen explicitly when a function receives and returns generator state alongside its random result. section 9.1.2
  • Use globalStdGen with applyAtomicGen to update the global generator atomically inside IO actions. section 9.1.4
  • Use random for general random values or uniformR for bounded values such as a range from 0 to 100. section 9.1.4

Generating a random value in IO

applyGlobalStdGen :: (StdGen -> (a, StdGen)) -> IO a applyGlobalStdGen f = applyAtomicGen f globalStdGen This convenience function lets an IO action generate values without receiving or initializing a StdGen. For example, applyAtomicGen random globalStdGen :: IO Int produces an Int, while applyAtomicGen (uniformR (0, 100)) globalStdGen :: IO Float produces a bounded Float. The atomic update makes this global-generator pattern safe for concurrent use. section 9.1.4

Listing 9.5 An IO action that applies a function to the global pseudo-random generator
applyGlobalStdGen :: (StdGen -> (a, StdGen)) -> IO a
applyGlobalStdGen f = applyAtomicGen f globalStdGen  #1

Inspect the compact wrapper that applies a StdGen transformation to the atomic global generator. section 9.1.4

How Learn Haskell by Example covers it

How deep it goes
Explains StdGen and atomic global access with code, but uses only one typical generator rather than surveying the full API.
Examples
Demonstrates random lists, explicit generator state, global generation, bounded values, and interactive GHCi results.
What it assumes
Uses package.yaml dependency configuration and IO actions while introducing explicit generator state through StdGen.
What you can do after
Apply it via applyAtomicGen and globalStdGen to generate random values safely in concurrent IO actions.
How it connects
Connects pseudo-random generation with StdGen, Random, uniformR, IO, concurrency, and property testing.
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Related concepts

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