unparser | Turn Ruby AST into semantically equivalent Ruby source | Parser library

The UnparseResult actual doesn't contain the result of the unparse (yeah, maybe we could name that better ;). The UnparseResult actually just contains whether or not the unparse succeeded (via the isError method) and any diagnostics when something fails. The unparse data is written to the WritableByteChannel that you pass into unparse() as a parameter.

The problem is that in your case, you have the following to define that channel:

Since I didn't receive an answer from here I went ahead and asked in the Nearley GitHub repo.

According to the maintainers, nearley-unparse can't currently generate strings to match a regular expression. There also aren't any plans to add that functionality as it would be a project in and of itself.

Here is their full response:

Hello there! Thanks for trying to post a StackOverflow question first, I’m sorry there wasn’t anyone able to help there :-)

This is a limitation of the unparser: it doesn’t know how to generate random strings satisfying a regexp, nor are we planning to do so (that would be a project in itself!).

Your grammar looks fine to me, at a brief glance; if you test it with nearley-test, hopefully you’ll find you get the parse trees you expect.

This runs in constant heap space. The idea is to first try p, and explicitly perform case analysis on the result of its success to decide whether to run go or not, so that go ends up in tail call position.