effectful-plugin

A GHC plugin for improving disambiguation of effects.

LTS Haskell 24.35:2.0.0.1
Stackage Nightly 2026-04-03:2.1.0.0
Latest on Hackage:2.1.0.0

See all snapshots effectful-plugin appears in

BSD-3-Clause licensed by Andrzej Rybczak
Maintained by [email protected]
This version can be pinned in stack with:effectful-plugin-2.1.0.0@sha256:2edfd898b23cf3a286bba4933f0e95e012da8f4a28cf77c37103f07548d0c529,2963

Module documentation for 2.1.0.0

Depends on 3 packages(full list with versions):
base, containers, ghc

effectful-plugin

A GHC plugin for improving disambiguation of effects.

Usage

To enable the plugin, add the following GHC option to your project file:

ghc-options: -fplugin=Effectful.Plugin

What it does

The following code:

action :: (State Int :> es, State String :> es) => Eff es ()
action = do
  x <- get
  put (x + 1)

will not compile out of the box because GHC doesn’t know that you meant to get an Int since the function + as well as the literal 1 are polymorphic. You have to write:

action :: (State Int :> es, State String :> es) => Eff es ()
action = do
  x <- get @Int
  put (x + 1)

Which is slightly annoying. This plugin tells GHC extra information so code like this can type-check without having to spell types to the compiler.

Acknowledgements

Thanks to Xy Ren for her work on cleff-plugin effectful-plugin is based on.

Changes

effectful-plugin-2.1.0.0 (2026-04-02)

  • Drop support for GHC < 9.6.
  • Consider built-in instances when filtering candidates.

effectful-plugin-2.0.0.1 (2025-08-30)

  • Small optimization of checking suitable effects for unification.
  • Add timing flag for tracking execution time of the plugin.
  • Expect wanted constraints that are already solved.

effectful-plugin-2.0.0.0 (2025-06-09)

  • Drop support for GHC < 9.4.
  • Fix various bugs and shortcomings.
  • Add verbose flag for tracing execution of the plugin.

effectful-plugin-1.1.0.4 (2024-10-08)

  • Fix inference in presence of implicit parameters.

effectful-plugin-1.1.0.3 (2024-06-07)

  • Drop support for GHC 8.8.

effectful-plugin-1.1.0.2 (2023-09-13)

  • Add support for GHC 9.8.

effectful-plugin-1.1.0.1 (2023-03-13)

  • Show accurate error message in GHC >= 9.4 if Effectful.Internal.Effect is not found.

effectful-plugin-1.1.0.0 (2023-01-23)

  • Add support for GHC 9.4 and 9.6.

effectful-plugin-1.0.0.0 (2022-07-13)

  • Initial release.