unionmount

Haskell library to "union mount" a bunch of folders onto an in-memory data structure, and keeping the latter in sync as the files change over time. Used in Ema and Emanote.

Usage

Both the mount and unionMount functions return a tuple value of type Dynamic, giving direct access to the initial value as well as the updater function that may be run in a separate thread. See how Ema uses it for an illustration.

Here's a simple example of loading Markdown files onto a TVar of Map FilePath Text (file contents keyed by path).

import System.UnionMount qualified as UM
import Data.Map.Strict qualified as Map

main :: IO ()
main = do
  runStdoutLoggingT $ do
    let baseDir = "/Users/srid/Documents/Notebook"
    (model0, modelF) <- UM.mount baseDir (one ((), "*.md")) [] mempty (const $ handlePathUpdate baseDir)
    modelVar <- newTVarIO model0
    modelF $ \newModel -> do
      atomically $ writeTVar modelVar newModel

handlePathUpdate ::
  (MonadIO m) =>
  FilePath -> FilePath -> UM.FileAction () -> m (Map FilePath Text -> Map FilePath Text)
handlePathUpdate baseDir path action = do
  case action of
    UM.Refresh _ _ -> do
      s <- decodeUtf8 <$> readFileBS (baseDir </> path)
      pure $ Map.insert path s
    UM.Delete -> do
      pure $ Map.delete path

Examples

See this example illustrating mounting a directory of Markdown files into (effectively) a Map FilePath String. A more involved example from Emanote demonstrates the "union" aspect of the library.