in-other-words-plugin

Overview

A typechecker plugin that can disambiguate "obvious" uses of effects when using in-other-words.

Example

Consider the following program:

foo :: Eff (State Int) m => m ()
foo = put 10

What does this program do? Any human will tell you that it changes the state of the Int to 10, which is clearly what's meant.

Unfortunately, in-other-words can't work this out on its own. Its reasoning is "maybe you wanted to change some other State effect which is also a Num, but you just forgot to add some Eff/s constraints for it."

This is obviously insane, but it's the way the cookie crumbles. in-other-words-plugin is a typechecker plugin which will disambiguate the above program (and others) so the compiler will do what you want.

Usage

Add the following line to your package configuration:

ghc-options: -fplugin=Control.Effect.Plugin

Limitations

in-other-words-plugin will only disambiguate effects if there is exactly one relevant constraint in scope. For example, it will not disambiguate the following program:

bar :: Effs '[ State Int
             , State Double
             ] m
    => m ()
bar = put 10

because it is now unclear whether you're attempting to set the Int or the Double. Instead, you can manually write a type application in this case.

bar :: Effs '[ State Int
             , State Double
             ] m
    => m ()
bar = put @Int 10

Acknowledgments

This plugin and this README is copied almost verbatim from polysemy-plugin, which itself is copied almost verbatim from simple-effects.