haskell-effectful/effectful
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effectful/effectful-plugin/README.md
Andrzej Rybczak 0c66324baf
Deprecate (:>>) because GHC can't efficiently handle recursive type families (#101) 
It's really saddening since (:>>) is a great QoL feature, but if people use it
in a lot of type signatures in a non-trivial codebase, it'll start taking
forever to compile their code and they'll think it's the fault of the library.

This was already encountered in the context of effect libraries:
https://github.com/joshvera/effects/issues/49

Relevant GHC issue: https://gitlab.haskell.org/ghc/ghc/-/issues/19238
2022-10-06 16:12:29 +02:00

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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.