There's something deeply satisfying about recursive tools who's inputs can include the definition of the tool itself. Brilliant.
I'd expect most parser generators in this category to be able to parse their own grammars.
Imagine a very weak parser that can only handle LR0. But if it uses a Lisp-like grammar language, it too is self-hosting.
E.g.:
bitstring : bistring '1'
| bitstring '0'
| /* empty */
;
That part of the README goes something like this:
1. It can parse Python and Go efficiently.
2. In fact it's so expressive that it can even parse itself, which is a "language of languages".
If you had first shown some hard-to-parse langcc syntax, then sure, _that_ would be evidence of expressiveness. But there's nothing impressive about being able to parse a "language of languages", since a language of languages can be LR(0).
I wrote an improved version called Meta5ix http://canonical.org/~kragen/sw/dev3/meta5ix.m5 (an interpreter for the virtual machine, with a complete description, is at http://canonical.org/~kragen/sw/dev3/meta5ixrun.py, with the precompiled grammar at http://canonical.org/~kragen/sw/dev3/meta5ix.generated.m5asm) and adapted it to produce C: http://canonical.org/~kragen/sw/dev3/meta5ix2c.m5 (precompiled version at http://canonical.org/~kragen/sw/dev3/meta5ix2c.c).
Meta5ix written in itself is only 18 lines of code. Briefly, "" encloses expected input, {} encloses a line of output, [] encloses repeating constructs, commas separate alternatives (implemented without backtracking), fnord skips leading whitespace, $it copies the last <<>>-enclosed input token to the output, and other $variables interpolate locally-defined assembly labels.
- program: ["-" name @it ":" terms {return}, "#" [:notin ""]]
- terms: term ["," {continue $choice} term] @choice
- term: (factor {else $seq}, output) [factor {assert}, output] @seq
- factor: string {literal $it}
, "(" terms ")"
, "[" @many terms {continue $many} "]"
, name {call $it}
, "<<" {begin} terms ">>" {end}
, ":fnord" {fnord} factor
, ":notin" string {notin $it}
, ":between" string {between $it}
- output: (quasiquote, "@" {dedent} (var, quasiquote)) {writeline}
- quasiquote: "{" [(<<ch [ch]>>, "\" <<:notin "">>) {say "$it"}, "$" var] "}"
- ch: :notin "$}\"
- var: "it" {copyinput}, name {gen $it}
- string: :fnord <<'"' [:notin '"'] '"', "'" [:notin "'"] "'">>
- name: :fnord <<letter [letter, :between "09"]>>
- letter: :between "az", :between "AZ", :between "__"
> OG used to mean Original Gangster allthough some poeple these days use OG as a quicker way of saying Original