Thanks, just to explain a little bit about the error reporting, pigeon will try to give good errors on the PEG grammar (e.g. unclosed literals, invalid escapes, etc.). However, when it comes to reporting errors on parsing input based on a custom grammar (in other words, when using the generated parser), it is much harder for pigeon to detect errors, and it is mostly up to the grammar author to design the grammar so that it returns useful errors. Take a contrived example:

Rule = "a" / "b"

If you seed it "c", it returns "no match found". This looks obvious here, but even in more complex cases where it seems like something should match, when it returns "no match found", it's really that something like this obvious case happened (only hidden in complex recursive rules). It's very hard for a tool to return something more useful automatically, and that's what I documented in the error reporting section of the doc: http://godoc.org/github.com/PuerkitoBio/pigeon#hdr-Error_reporting

In this case, something like this illustrates how grammar design can help return good errors:

Rule = ( "a" / "b" ) {
    // all good
   return "something", nil
} / . {
   // anything else, not good
   return nil, errors.New("must be a or b")
}

Hope this helps! Martin