tardis: Bidirectional state monad transformer

[ bsd3, control, library ] [ Propose Tags ] [ Report a vulnerability ]

A Tardis is a combination of both a forwards and a backwards state transformer, providing two state values that "travel" in opposite directions.

A detailed description of what a Tardis is and how to use it can be found in the documentation for Control.Monad.Tardis.

[Skip to Readme]

Modules

[Index] [Quick Jump]

Control
- Monad
  - Control.Monad.Tardis
    - Control.Monad.Tardis.Class
  - Trans
    - Control.Monad.Trans.Tardis

Downloads

tardis-0.5.0.1.tar.gz [browse] (Cabal source package)
Package description (as included in the package)

Maintainer's Corner

Package maintainers

DanBurton

For package maintainers and hackage trustees

edit package information

Candidates

No Candidates

Versions [RSS]	0.2.0.0, 0.3.0.0, 0.4.1.0, 0.4.2.0, 0.4.3.0, 0.4.4.0, 0.5.0, 0.5.0.1
Change log	ChangeLog.md
Dependencies	base (>=4.8 && <5), mmorph (>=1 && <2), mtl (>=2 && <3) [details]
Tested with	ghc ==9.14.1, ghc ==9.12.2, ghc ==9.10.3, ghc ==9.8.4, ghc ==9.6.7, ghc ==9.4.8
License	BSD-3-Clause
Author	Dan Burton
Maintainer	danburton.email@gmail.com
Uploaded	by DanBurton at 2026-01-12T23:43:48Z
Category	Control
Home page	https://github.com/DanBurton/tardis
Bug tracker	https://github.com/DanBurton/tardis/issues
Source repo	head: git clone https://github.com/DanBurton/tardis.git
Distributions	LTSHaskell:0.5.0, NixOS:0.5.0, Stackage:0.5.0
Reverse Dependencies	5 direct, 3 indirect [details]
Downloads	5177 total (20 in the last 30 days)
Rating	2.0 (votes: 1) [estimated by Bayesian average]
Your Rating	λ λ λ
Status	Docs available [build log] Last success reported on 2026-01-12 [all 1 reports]

Readme for tardis-0.5.0.1

[back to package description]

TardisT

The State monad allows you to send information forwards to the future, or to receive such information from the past. The Reverse State monad allows you to do the reverse: send information backwards to the past, or receive information from the future.

TardisT is a monad transformer that provides state operations that allow you to send information to both the future and the past, as well as receive information from both directions. It is isomorphic to a StateT on top of a ReverseStateT, or vice-versa.

See test/Example.hs for an example.

This was inspired by Luke Palmer's blog post on the "reverse state monad".

http://lukepalmer.wordpress.com/2008/08/10/mindfuck-the-reverse-state-monad/