Hierarchy

Documentation

type TreeT m = CofreeT Maybe m

A TreeT is a tree of values, where the (possible) branches are ListTs over some Monad m.

select :: MonadPlus m => [a] -> m a

Turn a list into a series of possibilities:

>>> observeAllT $ considering [1,2,3]
[1,2,3]

descend :: MonadPlus m => TreeT m a -> m (a, Maybe (TreeT m a))

Descend one level into a TreeT, yielding a list of values and their possible associated trees.

walk :: MonadPlus m => TreeT m a -> m a

Perform a depth-first traversal of a TreeT, yielding a ListT of its contents. Note that breadth-first traversals cannot offer static memory guarantees, so they are not provided by this module.

winnow :: MonadPlus m => TreeT m a -> CondT a m () -> TreeT m a

Given a TreeT, produce another TreeT which yields only those elements (and sub-trees) matching the given monadic conditional. This conditional (see CondT) can choose both elements and points of recursion, making it capable of expressing any tree traversal in the form of a predicate DSL. This differs from an expression-based traversal, like XPath or Lens, in that effects in m may be used to guide selection.

For example, to print all Haskell files under the current directory:

let files = winnow (directoryFiles ".") $ do
        path <- query
        liftIO $ putStrLn $ "Considering " ++ path
        when (path `elem` ["./.git", "./dist", "./result"])
            prune  -- ignore these, and don't recurse into them
        guard_ (".hs" `isInfixOf`)  -- implicitly references path
runEffect $ for (runListT (walk files)) $ liftIO . print