BSD-3-Clause licensed by Xy Ren
Maintained by
This version can be pinned in stack with:cleff-plugin-,2895

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This GHC typechecking plugin disambiguates obvious usages of effects for the extensible effect framework cleff.


This plugin works with GHC 8.6 through 9.2 and cleff >= 0.1 && < 0.4. To use the plugin:

  1. Add this plugin as your package’s dependency.
    • If you use stack, then you also need to add these lines your stack.yaml:
      - cleff-plugin-
  2. Enable the plugin by adding the following GHC option to your project file:
    ghc-options: -fplugin=Cleff.Plugin

What it does

When using cleff, the following code would not compile:

action :: '[State Int, State String] :>> es => Eff es ()
action = do
  x <- get
  put (x + 1)
-- • Could not deduce (Cleff.Internal.Rec.Elem (State s0) es)
--     arising from a use of ‘get’

This is because GHC is unable to determine which State effect we’re trying to operate on, although the only viable choice is State Int. We had to write:

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

This is quite unwieldy. This plugin tells GHC extra information so code like this can typecheck without requiring manually annotating which effect to use.


This plugin’s design is largely inspired by polysemy-plugin, which is in turn based on simple-effects.

Refer to docs/ for details on the disambiguation algorithm this typecheck plugin uses.


Changelog for cleff-plugin (2022-03-13)

  • Initial release