streamly-fsnotify: Folder watching as a Streamly stream.

What's the deal with this library?

streamly is an undoubtedly awesome library - fast, flexible, and well-documented. File system watching is a natural fit for a streaming library, and this is exactly what streamly-fsnotify provides you.

As an example, here is a program which watches /home/koz/c-project/ and any of its subdirectories for added or modified C source files (which we take to be anything with a .c extension). This program then writes that the event occurred, to what file, and when, forever.

{-# LANGUAGE LambdaCase #-}

import System.FilePath ((</>))
import Streamly.FSNotify (EventPredicate, hasExtension, isDirectory, invert, isDeletion, conj, watchTree)
import qualified Streamly.Prelude as SP

-- conj -> both must be true
-- invert -> true when the argument would be false and vice versa
isCSourceFile :: EventPredicate
isCSourceFile = hasExtension "c" `conj` invert isDirectory

notDeletion :: EventPredicate
notDeletion = invert isDeletion

srcPath :: FilePath
srcPath = "home" </> "koz" </> "c-project"

-- first value given by watchTree stops the watcher
-- we don't use it here, but if you want to, just call it
main :: IO ()
main = do
    (_, stream) <- watchTree srcPath $ isCSourceFile `conj` notDeletion
    SP.drain . SP.mapM go $ stream
  where
    go = \case
        Added p t _ -> putStrLn $ "Created: " ++ show p ++ " at " ++ show t
        Modified p t _ -> putStrLn $ "Modified: " ++ show p ++ " at " ++ show t
        _ -> pure ()

That seems pretty cool! What kind of features can I expect?

• Cross-platform - should work anywhere both streamly and fsnotify do.
• Efficient (event-driven, so won't shred your CPU or load your RAM).
• Able to do one-level and recursive watching.
• Compositional and principled treatment of event filtering predicates.
• Extensive set of filtering predicates, so you don't have to see events you don't care about!

Sounds good? Can I use it?

We've test-built this library for GHCs 8.6.5 through 8.10.1 on GNU/Linux. In theory, streamly-fsnotify should work everywhere both streamly and fsnotify will, which includes other OSes (such as Windows). However, we haven't tried it ourselves - let us know if you do!