# polysemy-plugin
[](https://travis-ci.org/isovector/polysemy)
[](https://hackage.haskell.org/package/polysemy-plugin)
## Dedication
> It doesn't matter how beautiful your theory is, it doesn't matter how smart
> you are. If it doesn't agree with experiment, it's wrong.
>
> Richard Feynman
## Overview
A typechecker plugin that can disambiguate "obvious" uses of effects in
[`polysemy`](https://hackage.haskell.org/package/polysemy).
## Example
Consider the following program:
```haskell
foo :: Member (State Int) r => Sem r ()
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, `polysemy` 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 a `Member` constraint for it."
This is obviously insane, but it's the way the cookie crumbles.
`polysemy-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=Polysemy.Plugin
```
## Limitations
The `polysemy-plugin` will only disambiguate effects if there is exactly one
relevant constraint in scope. For example, it will *not* disambiguate the
following program:
```haskell
bar :: Members '[ State Int
, State Double
] r => Sem r ()
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.
```haskell
bar :: Members '[ State Int
, State Double
] r => Sem r ()
bar = put @Int 10
```
## Acknowledgments
This plugin is copied almost verbatim from [`simple-effects`](https://hackage.haskell.org/package/simple-effects).