Twitch is a monadic DSL and library for file watching. It conveniently utilizes 'do' notation in the style of Shake and clay to expose the functionality of the fsnotify cross-platform file system watcher.

Here is an example that converts Markdown files to HTML and reloads Safari whenever the input files change.

{-# LANGUAGE OverloadedStrings #-}
import Twitch 
import Filesystem.Path.CurrentOS

main = defaultMain $ do
  "*.md"   |> \filePath -> system $ "pandoc -t html " ++ encodeString filePath 
  "*.html" |> \_ -> system $ "osascript refreshSafari.AppleScript"

Rules are specified in the Dep (for Dependency) monad. The library takes advantage of the OverloadedStrings extension to create a Dep value from a glob pattern.

After creating a Dep value using a glob, event callbacks are added using prefix or infix API.

There are three types of events: 'add', 'modify' and 'delete'. In many cases, the 'add' and 'modify' responses are the same, so an 'add and modify' API is provided

In the example above, an 'add and modify' callback was added to both the "*.md" and "*.html" globs using the |> operator.

Although this is the common case, differing callbacks can be added with |+ (or add) and |% (or modify) functions. Finally, delete callbacks are added with |- (of delete).

Here is a more complex usage example, handling all three events separately.

handleHaskellFiles :: Dep 
handleHaskellFiles = "src/**/*.hs" |+ addToCabalFile |% reloadFile |- removeFromCabalFile

The glob above is also more complicated and incorporates a recursive wildcard. For complete documentation on the glob syntax, consult the Glob library's documentation.

Since a command pattern is calling system commands with a file path, a useful addition to twitch is the file-command-qq quasiquoter.

Here is a slightly more complicated version the example from earlier, using the FileCommand quasiquoter.

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE QuasiQuotes #-}
import Twitch 
import FileCommand

main = defaultMain $ do
  "*.md"    |> [s|pandoc -t html -o$directory$basename-test.html $path|]
  "*.html"  |> [s|osascript refreshSafari.AppleScript|]