Breadth First Traversal

JoostVisser and I have looked at a breadth first for Tools.JJTraveler. The breadth first in the Stratego library is incorrect:

• It doesn't apply s to the root (and hence not to any node);
• It's not overall breadht-first, only breadth-first per node.

-- ArieVanDeursen; 25 Nov 2001.

It is indeed not a correct implementation of breadth first, but it does actually apply transformations. Consider the definition:

  breadth-first(s) = 
    rec x(all(s); all(x))

This first applies s to all direct subterms of the root, and then recursively applies x to the direct sub-terms. The difference with topdown is that all subterms are visited, before visiting subterms recursively. What is a better name for this strategy? kids-first maybe?

An implementation of proper breadth-first should employ a global queue data structure to which terms to be visited should be added. It is also interesting to consider using TermAnnotations to let a term point to the next term in a breadth-first traversal.

-- EelcoVisser - 09 Dec 2001

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