Скачать или смотреть Handling starred Nodes in Tree Transformations with Haskell

Learn how to restructure trees in Haskell by lifting starred nodes to direct children of the root, using recursion and pattern matching.
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Introduction

When working with trees in Haskell, you may encounter nodes distinguished by a special property—here, nodes whose names end with a * are called starred nodes. A common transformation is to lift any starred node that is not a child of another starred node directly under the root, preserving the nested starred nodes inside starred parents.

Problem Statement

Starred nodes: Nodes with labels ending in *.

Goal: Move every starred node that is not under another starred node to be a direct child of the root node.

Starred nodes nested inside other starred nodes stay put.

For example, given:

[[See Video to Reveal this Text or Code Snippet]]

Transform to:

[[See Video to Reveal this Text or Code Snippet]]

Solution Approach

Key Insight

Recursively traverse the tree while categorizing nodes:

If the current node is starred, it becomes part of the lifted forest (subtrees to elevate).

Otherwise, collectibles from children are gathered but not lifted.

Implementation Highlights

Using Haskell's Data.Tree:

[[See Video to Reveal this Text or Code Snippet]]

Explanation

If the current node is starred, form a new subtree using this node and its starred children.

Otherwise, no starred subtree starts here, so collect all starred subtrees found in children.

Combining Results

At the top level, combine the two forests:

[[See Video to Reveal this Text or Code Snippet]]

This yields the forest of starred nodes lifted to root's direct children.

Additional Notes

The concept of a "direct child" is fundamental in tree structures and traversals.

This approach cleanly separates starred subtrees and lifts them without complex state management.

Conclusion

This recursive strategy elegantly lifts starred nodes that aren't beneath other starred nodes, a common pattern in tree data transformations. Using Haskell's powerful pattern matching and recursion makes the logic both concise and clear.