HTree

The purpose of this module is to transform hierarchical lists into trees, where trees are as in zwilias/elm-rosetree. A hierarchical list is by definition a list of items endowed with a function

   level : Item -> Int

and a root label, e.g., "*" in the case of lists of strings.

The level function, which associates non-negative integers to items, defines the hierarchy. As an example, consider the course outline below

Introduction
Data Structures
  Lists
  Trees
  Queues
Algorithms
  Brute-force-search
  Binary search
  Probabalistic methods
Compational complexity
  O(n) notation
  Polynomial time 
  Exponential time
...

If we take the root label to be Book and the level of an item to be the number of leading spaces divided by two, then this outline defines a tree. The childreen of Book are are Introduction, Data Structures, etc. Introduction has no children, while Data Structures has three. Etc.

One can turn the above outline into a tree like this:

> import HTree.String

> ourtline = """
Introduction
Data Structures
  Lists
  Trees
  ...
"""

> t = HTree.String.parse ourtline
  Tree "*" 
     [ Tree "Introduction" []
      ,Tree ("Data Structures") [Tree "Lists" [],Tree "Trees" [],Tree "Queues" []]
      ,Tree "Algorithms" [Tree "Brute-force-search" []
      ,Tree ("Binary search") [],Tree ("Probabalistic methods") []]
      ,Tree ("Compational complexity") [Tree ("O(n) notation") []
      ,Tree ("Polynomial time") [],Tree ("Exponential time") []],Tree "" []]

Conversely, a tree can be turned into an outline:

> HTree.String.toOutline t 
  ...