Apache-2.0 licensed by Yuji Yamamoto
Maintained by [email protected]
This version can be pinned in stack with:fakefs-,1164

Module documentation for


Extensible fake file system for testing.

Provides a simple Monad transformer FileSystemT, intended to make a fake file system.
Against its name, FileSystemT imitates very limited feature of a file system:
It’s essentially just a newtype of StateT (Map FilePath contents) m a, where the type variable contents can be any type suitable for your tests.
For example:

  • If your tests have to handle only binary contents of a file, contents should be ByteString.
  • If your tests have to handle only text contents of a file, contents should be Text.
  • If your tests have to handle binary contents of a file with its permisson, contents should be (Permission, ByteString).
  • etc.



import Control.Monad
import Control.Monad.Catch.Pure
import Data.List
import qualified Data.Map.Strict as M
import Data.Maybe
import Test.Hspec
import Test.FileSystem.Fake

import Prelude hiding (readFile, appendFile)

type YourFsM = FileSystemT String Catch

-- | Record of functions to stub IO actions
data FsActions m = FsActions
    { readFile :: FilePath -> m String
    , appendFile :: FilePath -> String -> m ()

fsActionsForTesting :: FsActions YourFsM
fsActionsForTesting = FsActions
    { readFile = readPath
    , appendFile = \path contents ->
        modifyPath path (Just . (++ contents) . fromMaybe "")

-- | The function you test.
collectMatchedPaths :: MonadThrow m => FsActions m -> (String -> Bool) -> FilePath -> [FilePath] -> m ()
collectMatchedPaths acts p outPath = mapM_ $ \path -> do
    contents <- readFile acts path
    when (p contents) $
        appendFile acts outPath $ path ++ "\n"

main :: IO ()
main = hspec $
    describe "collectMatchedPaths" $
        it "collect paths whose content matches the given condition" $ do
            let initialFs = M.fromList
                    [ ("a.txt", "aaaaa\n")
                    , ("b.hs", "main = undefined\n")
                    , ("c.c", "#include <stdio.h>\n")
                outPath = "result.txt"
                args = M.keys initialFs
                Right resultFs = runCatch . (`execFileSystemT` initialFs)
                    $ collectMatchedPaths fsActionsForTesting (isInfixOf "in") outPath args

            resultFs M.! outPath `shouldBe` "b.hs\nc.c\n"

-- END


Changelog for fakefs (2020-09-05)

  • :bug: Support GHC 8.6 or later.
  • :memo: Update the example in README to follow the changes in (2020-08-11)

  • :memo: Update README to follow the changes in (2020-08-11)

  • Redesign: Add ability to throw IOExceptions in a more composable manner.
    • By deleting ExceptT from FileSystemT, it’s more GeneralizedNewtypeDeriving-friendly.
    • Generalize the monad of readPath and writePath etc. by the new constraint synonym MonadFileSystem. (2019-07-16)

  • :memo: Update READE with an example.
  • :memo: Correct wrong repository URL in the cabal file. (2019-03-30)

  • Simplify the monad stack and the file object.
  • Rename main APIs to express its generality.
    • readFileT -> readPath
    • writeFileT -> writePath
  • Add utility functions: evalFileSystemT, execFileSystemT, etc. (2019-03-29)

Initial Release