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) nottation
  Polynomial time 
  Exponential time
...

If we take the root label to be Book, 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) nottation") []
      ,Tree ("Polynomial time") [],Tree ("Exponential time") []],Tree "" []]

Conversely, a tree can be turned into an outline:

> HTree.String.toOutline t 
  ...