MonadRandom

Random-number generation monad.

Version on this page:0.6@rev:4
LTS Haskell 24.16:0.6.2
Stackage Nightly 2025-10-24:0.6.2
Latest on Hackage:0.6.2

See all snapshots MonadRandom appears in

BSD-3-Clause licensed by Cale Gibbard and others
Maintained by Brent Yorgey
This version can be pinned in stack with:MonadRandom-0.6@sha256:039a76b6bcd714188d84f0f63c380440eb3fec9956777a74e4b8dcd4bda0e843,1511

Module documentation for 0.6

Support for computations which consume random values.

Changes

0.6 (5 Nov 2022)

  • Remove instances for deprecated ErrorT and ListT transformers
  • Allow building with transformers-0.6 and mtl-2.3.1
  • Drop support for GHC 7.6 and 7.8

0.5.3 (8 April 2021)

  • StatefulGen instances for RandT

  • Addition of RandGen

  • Additioon of withRandGen and withRandGen_

  • r1 (28 April 2021): require base >= 4.6

  • r2 (9 Aug 2021): allow transformers-compat-0.7.

  • r3 (9 Aug 2022): Test with GHC 9.0-9.4

0.5.2 (24 June 2020)

  • Support for random-1.2.
  • Drop support for GHC 7.4, test with GHC 8.10.

0.5.1.2 (18 Jan 2020)

  • Better error message when total probability sum is negative in fromListMay, weightedMay, weighted

0.5.1.1 (21 May 2018)

  • Only depend on fail package when GHC < 8.0.

0.5.1 (9 February 2017)

Re-export System.Random from Control.Monad.Random.{Lazy,Strict} and hence also from Control.Monad.Random.

  • Hackage r1: allow transformers-compat-0.6.x.

0.5 (3 January 2017)

This release has quite a few small additions as well as a big module reorganization. However, thanks to module re-exports, most existing code using the library should continue to work with no changes; the major version bump reflects the large reorganization and my inability to 100% guarantee that existing user code will not break.

The biggest changes that may be of interest to users of the library include new lazy vs strict variants of the Rand monad; a new MonadInterleave class which is a big improvement over MonadSplit; new PrimMonad instances; and new random selection functions like weighted, weightedMay, uniformMay, etc.. See the list below for full details.

Although there was some discussion of generalizing MonadRandom to work for a wider range of underlying generators (see #26, #31, and comments on this blog post), I decided to punt on that for now. It seems rather complicated and there are already good alternatives so I decided to keep things simple for this release. I’m still open to proposals for generalizing future releases.

Changes in 0.5 include:

  • Refactor to reflect structure of mtl and transformers libraries.
  • Add lazy and strict variants of RandT.
  • Add MonadRandom and MonadSplit instances for ListT.
  • Add (but do not export) unRandT field to RandT.
  • Add MonadCont, MonadError, MonadRWS, PrimMonad, and MonadFail instances for RandT.
  • Add evalRandTIO operation.
  • Move fromList and uniform operations to Control.Monad.Random.Class.
  • fromList now raises an error when the total weight of elements is zero.
  • Generalize the type of uniform to work over any Foldable.
  • Add new operations weighted, weightedMay, fromListMay, and uniformMay. weighted is like fromList but generalized to work over any Foldable. The May variants return a Maybe result instead of raising an error.
  • New MonadInterleave class for random monads which can interleave random generation using split. In some ways this is similar to MonadSplit but much more useful.
  • Improved documentation.

0.4.2.3 (21 April 2016)

  • Mark Control.Monad.Random as Trustworthy.

0.4.2.2 (18 January 2016)

  • Allow transformers-0.5.

0.4.2.1 (16 January 2016)

  • Allow transformers-compat-0.5.

0.4.2 (16 January 2016)

  • Add MonadPlus and Alternative instances for RandT.

0.4.1 (20 November 2015)

  • Remove unnecessary Monad m constraint from liftRandT and runRandT.

This should again technically require a major version bump, but I’m not doing it this time in the interest of not being super annoying. If this breaks something for you, just yell, and I will deprecate this version and do a proper 0.5 release.

0.4 (12 May 2015)

  • Remove unnecessary RandomGen g constraints from liftRandT, liftRand, evalRandT, evalRand, runRandT, runRand.

A major version bump is required by the PVP since the types of all the above methods have changed, but this release is again very unlikely to break any client code.

0.3.0.2 (30 March 2015)

  • Add transformers-compat to allow building with newer mtl

0.3.0.1 (24 November 2014)

  • Improve documentation: ranges are exclusive at the upper bound

0.3 (4 September 2014)

  • Eta-reduce definition of Rand
  • Remove unnecessary Random a constraint from types of liftRand and liftRandT.

Note that a major version bump is required by the PVP since the types of liftRand and liftRandT have changed, but this release is highly unlikely to break any client code.

0.2.0.1 (24 August 2014)

  • Allow building with both transformers-0.3 and 0.4.

0.2 (20 August 2014)

  • change Rand from a newtype to a type synonym
  • MonadRandom and MonadSplit instances for
    • ExceptT
    • strict variants of StateT and WriterT
    • both lazy and strict variants of RWST
  • remove unneeded RandomGen constraint from MonadState RandT instance

0.1.13 (9 February 2014)

  • add simple uniform function for creating a uniform distribution over a list of values

0.1.12 (30 September 2013)

  • add liftRandT and liftRand functions, for lifting explicit generator-passing functions into RandT and Rand, respectively.

0.1.11 (1 August 2013)

  • add MonadRandom and MonadSplit instances for IdentityT
  • derive MonadReader and MonadWriter instances instead of declaring them explicitly (thanks again to James Koppel)

0.1.10 (16 July 2013)

  • add MonadRandom and MonadSplit instances for ContT (thanks to James Koppel for the patch)

0.1.9 (26 April 2013)

  • add MonadRandom and MonadSplit instances for MaybeT