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78 lines
2.1 KiB
Markdown
78 lines
2.1 KiB
Markdown
# polysemy-plugin
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[](https://travis-ci.org/isovector/polysemy-research)
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[](https://hackage.haskell.org/package/polysemy-plugin)
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## Dedication
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> It doesn't matter how beautiful your theory is, it doesn't matter how smart
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> you are. If it doesn't agree with experiment, it's wrong.
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>
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> Richard Feynman
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## Overview
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A typechecker plugin that can disambiguate "obvious" uses of effects in
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[`polysemy`](https://hackage.haskell.org/package/polysemy).
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## Example
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Consider the following program:
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```haskell
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foo :: Member (State Int) r => Sem r ()
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foo = put 10
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```
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What does this program do? Any human will tell you that it changes the state of
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the `Int` to 10, which is clearly what's meant.
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Unfortunately, `polysemy` can't work this out on its own. Its reasoning is
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"maybe you wanted to change some other `State` effect which is *also* a `Num`,
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but you just forgot to add a `Member` constraint for it."
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This is obviously insane, but it's the way the cookie crumbles.
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`polysemy-plugin` is a typechecker plugin which will disambiguate the above
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program (and others) so the compiler will do what you want.
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## Usage
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Add the following line to your package configuration:
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```
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ghc-options: -fplugin=Polysemy.Plugin
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```
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## Limitations
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The `polysemy-plugin` will only disambiguate effects if there is exactly one
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relevant constraint in scope. For example, it will *not* disambiguate the
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following program:
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```haskell
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bar :: Members '[ State Int
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, State Double
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] r => Sem r ()
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bar = put 10
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```
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because it is now unclear whether you're attempting to set the `Int` or the
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`Double`. Instead, you can manually write a type application in this case.
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```haskell
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bar :: Members '[ State Int
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, State Double
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] r => Sem r ()
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bar = put @Int 10
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```
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## Acknowledgments
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This plugin is copied almost verbatim from [`simple-effects`](https://hackage.haskell.org/package/simple-effects).
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