95b4b5508c
* Use cabal-doctest - Haskell build tools run in slightly different environments (meaning different package databases are available). - The nixpkgs build for polysemy-plugin is failing due to a missing package database, which causes the doctest to fail (more information here: https://github.com/NixOS/nixpkgs/issues/71164). - By using cabal-doctest we can expose the Haskell packages required to the doctests no matter the build tool we're using. * Use cabal-doctest in polysemy, build on GHC 8.8.1 - Use @googleson78 's changes to build polysemy on GHC 8.8.1, with slight modifications. The source distribution is now found in "dist-newstyle/sdist", so we've updated the command to point at that folder. Additionally, cabal v2-install doesn't support installing .tar.gz files in the same way v1-install did, so updated the command to use "cabal v1-install". - Modified polysemy to use "cabal-doctest" and so overcome issues with the doctest tests (see issue #258, PR #265). |
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| .. | ||
| src/Polysemy | ||
| test | ||
| .gitignore | ||
| ChangeLog.md | ||
| LICENSE | ||
| package.yaml | ||
| polysemy-plugin.cabal | ||
| README.md | ||
| Setup.hs | ||
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.
Example
Consider the following program:
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:
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.
bar :: Members '[ State Int
, State Double
] r => Sem r ()
bar = put @Int 10
Acknowledgments
This plugin is copied almost verbatim from simple-effects.