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effectful-plugin: A GHC plugin for improving disambiguation of effects.

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Instruct GHC to do a better job with disambiguation of effects. . See the README for more information.

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timing

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verbose

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Use -f <flag> to enable a flag, or -f -<flag> to disable that flag. More info

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Versions [RSS] 1.0.0.0, 1.1.0.0, 1.1.0.1, 1.1.0.2, 1.1.0.3, 1.1.0.4, 2.0.0.0, 2.0.0.1
Change log CHANGELOG.md
Dependencies base (>=4.16 && <5), containers (>=0.5), ghc (>=9.4 && <9.15) [details]
Tested with ghc ==9.4.8 || ==9.6.7 || ==9.8.4 || ==9.10.2 || ==9.12.2 || ==9.14.1
License BSD-3-Clause
Author Andrzej Rybczak
Maintainer andrzej@rybczak.net
Category Control
Bug tracker https://github.com/haskell-effectful/effectful/issues
Source repo head: git clone https://github.com/haskell-effectful/effectful.git
Uploaded by arybczak at 2025-08-30T18:21:01Z
Distributions LTSHaskell:2.0.0.1, NixOS:1.1.0.4, Stackage:2.0.0.1
Reverse Dependencies 4 direct, 0 indirect [details]
Downloads 1001 total (16 in the last 30 days)
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Status Docs available [build log]
Last success reported on 2025-08-30 [all 1 reports]

Readme for effectful-plugin-2.0.0.1

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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.