Low level filesystem operations for polysemy.

Stackage Nightly 2022-01-25:
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BSD-3-Clause licensed by Daniel Firth
This version can be pinned in stack with:polysemy-fs-,958

Module documentation for


This package offers small filesystem effects for polysemy, such as:

data FSExist m a where
  DoesFileExist :: Path b File -> FSExist m Bool
  DoesDirExist  :: Path b Dir  -> FSExist m Bool

It should be noted that these are very weak semantic abstractions, as it does not provide a way to speak to the filesystem as a whole, and therefore is somewhat unsuitable for mocking. These effects exist primarily to give interpreters a way to be precise in the type of filesystem operations they use, and should be treated in the same way as Embed IO in that regard. However, if you need a quick fix - then using this directly in application code is a slight improvement over using Embed IO directly.

Using these as very low level compilation units allows you to inject debugging everywhere you use a filesystem operation. For example, using co-log-polysemy we can do this:

data FileExists where
  FileExists    :: Path b File -> FileExists
  FileNotExists :: Path b File -> FileExists

data DirExists where
  DirExists    :: Path b Dir -> DirExists
  DirNotExists :: Path b Dir -> DirExists

logFileExists :: Members '[FSExist, Log FileExists] r => Sem r a -> Sem r a
logFileExists = intercept \case
  DoesFileExist x -> do
    z <- doesFileExist x
    case z of
      True  -> log $ FileExists x
      False -> log $ FileNotExists x
    return z
  DoesDirExist x -> doesDirExist x

logDirExists :: Members '[FSExist, Log DirExists] r => Sem r a -> Sem r a
logDirExists = intercept \case
  DoesDirExist x  -> do
    z <- doesDirExist x
    case z of
      True  -> log $ DirExists x
      False -> log $ DirNotExists x
    return z
  DoesFileExist x -> doesFileExist x

For a look at a unified version of filesystem operations, check out polysemy-fskvstore


Changelog for polysemy-fs


  • Small filesystem effects for polysemy.