Hi apfelmus, I wrote:
...a tool for recursing through directories... How about a built-in function that represents a directory tree as a lazy Data.Tree?
-- List all directories in the tree rooted at the given path traverseDirectories :: MonadIO m => TraversalDirection -> FilePath -> ListT m Directory
Or - getting back to the "lazy Data.Tree" idea - we could define TreeT, analgous to ListT: newtype TreeT m a = NodeT (m (a, TreeT (ListT m) a)) and give it a nice Monad instance. Then you can prune and filter trees in a natural way, inside the monad.
apfelmus wrote:
Eh, isn't that what ListT is already for? ...allows you to prune the directory tree in whatever way you like it. Here's an example top-down traversal that lists all non-directories:
allFiles :: MonadIO m => Directory -> m [FilePath] allFiles d = runList $ do f <- liftList $ contents d if isDirectory f then allFiles f else return f
In other words, ListT is like one of those iterators I keep hearing from the Python/Java/.../imperative world
Well, yes, you could also use it that way. And yes, years ago they used to do it that way in Python. Where every user has to build in the logic to discover the tree structure manually, as they iterate along. But it turns out that it is much more natural and convenient to be given the tree structure pre-built, and then just traverse it. You need some laziness to avoid visiting directories that you don't care about and might be costly to visit. Python currently uses generators to get the laziness. Bryan uses unsafe IO, together with his "recursion predicates and filter predicates". My first proposal was just a minor modification of Bryan's ideas. I presented the tree pre-traversed - i.e., flattened, like Bryan, and also used his predicates. But I used ListT to provide the laziness instead of unsafe IO. My second proposal is to present the entire tree inside a lazy monad transformer. While the internal plumbing is slightly more complex, I think it will provide the most natural and convenient interface for the end user (who needs not understand all that monad stuff). -Yitz