mtl

Monad classes, using functional dependencies

http://github.com/ekmett/mtl

Version on this page:2.2.1@rev:1
LTS Haskell 24.25:2.3.1
Stackage Nightly 2025-12-23:2.3.1
Latest on Hackage:2.3.2

See all snapshots mtl appears in

BSD-3-Clause licensed by Andy Gill
Maintained by Edward Kmett
This version can be pinned in stack with:mtl-2.2.1@sha256:4b5a800fe9edf168fc7ae48c7a3fc2aab6b418ac15be2f1dad43c0f48a494a3b,1749

Module documentation for 2.2.1

Depends on 2 packages(full list with versions):
base, transformers

Monad classes using functional dependencies, with instances for various monad transformers, inspired by the paper Functional Programming with Overloading and Higher-Order Polymorphism, by Mark P Jones, in Advanced School of Functional Programming, 1995 (http://web.cecs.pdx.edu/~mpj/pubs/springschool.html).

Changes

2.2.1

  • Provide MINIMAL pragmas for MonadState, MonadWriter, MonadReader
  • Added a cyclic definition of ask in terms of reader for consistency with get/put vs. state and tell vs. writer
  • Fix deprecation warnings caused by transformers 0.4 deprecating ErrorT.
  • Added Control.Monad.Except in the style of the other mtl re-export modules

2.2.0.1

  • Fixed a bug caused by the change in how transformers 0.4 exports its data types. We will now export runFooT for each transformer again!

2.2

  • transformers 0.4 support
  • Added instances for ExceptT
  • Added modify' to Control.Monad.State.*

2.1.3.1

  • Avoid importing Control.Monad.Instances on GHC 7.8 to build without deprecation warnings.

2.1.3

  • Removed the now-irrelevant Error constraint from the MonadError instance for Either e.