    {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-}

    -- ------------------------------------------------------------

    {- |
       Module     : Data.Tree.NavigatableTree.Class
       Copyright  : Copyright (C) 2010 Uwe Schmidt
       License    : MIT

       Maintainer : Uwe Schmidt (uwe\@fh-wedel.de)
       Stability  : experimental
       Portability: portable

       Interface definition for navigatable trees.
       Navigatable trees need to have operations to move up, down, left and right.
       With these elementary operations, most of the XPath axises can be defined.
    -}

    -- ------------------------------------------------------------

    module Data.Tree.NavigatableTree.Class
    where

    -- ------------------------------------------------------------

    -- | The interface for navigatable trees

    class NavigatableTree t where
        -- | move one step towards the root
        mvUp                :: t a -> Maybe (t a)

        -- | descend one step to the leftmost child
        mvDown              :: t a -> Maybe (t a)

        -- | move to the left neighbour
        mvLeft              :: t a -> Maybe (t a)

        -- | move to the right neighbour
        mvRight             :: t a -> Maybe (t a)

    -- ------------------------------------------------------------

    -- | Conversion between trees and navigatable trees,
    --
    -- There is only a single navigatable tree implementation for a given tree allowed
    -- (see the functional dependencies)

    class NavigatableTreeToTree nt t | t -> nt, nt -> t where
        -- | construct a navigatable tree
        fromTree            :: t a -> nt a

        -- | remove navigation
        toTree              :: nt a -> t a

    -- ------------------------------------------------------------

    -- | Edit operation on navigatable trees
    --
    -- There is only a single navigatable tree implementation for a given tree allowed
    -- (see the functional dependencies)

    class NavigatableTreeModify nt t | t -> nt, nt -> t where

        -- | add an ordinary tree in front of the given navigatable tree
        addTreeLeft         :: t a -> nt a -> Maybe (nt a)

        -- | add an ordinary tree behind of the given navigatable tree
        addTreeRight        :: t a -> nt a -> Maybe (nt a)

        -- | drop the direct left sibling tree of the given navigatable tree
        dropTreeLeft        :: nt a -> Maybe (nt a)

        -- | drop the direct right sibling tree of the given navigatable tree
        dropTreeRight       :: nt a -> Maybe (nt a)

        -- | change the tree but remain the navigation
        substThisTree       :: t a -> nt a -> nt a

    -- ------------------------------------------------------------