11–20 of 34 posts
Earlier quoted context omitted.
Try PEGs https://parsley.readthedocs.io/en/latest/tutorial.html https://en.wikipedia.org/wiki/OMeta
Absolutely! Furthermore, https://github.com/harc/ohm is the successor to OMeta.
I've been looking at parser generators recently in an effort to begin writing my own simple interpreted language. My end goal is a self hosted language. My first attempt was writing a recursive descent parser by hand: https://gist.github.com/cmcarey/eee1571721141c356d4f61b453a6... This didn't work so well (badly structured as well as finding out my grammar was ambiguous after I'd written 600 lines of parser code). Lo…
FWIW here is a small, self-contained recursive-descent parser I recently wrote in Python (that recognizes certain C data definitions).
https://github.com/oilshell/oil/blob/master/build/cpython_de...
I think your structure is bit odd because Nodes are "smart". That is, the style is more object oriented than functional.
If you clearly separate out the input data (lexer), code for the parser, and the resulting data it will be better. Pretty much all compilers and interpreters follow this structure, even ones in Java (e.g. in Terrence Parr's books). AST nodes should be dumb (no methods related except trivial ones for pretty printing).
I think you might be getting confused between clauses of the grammar and nodes in an AST. They are similar but they don't have a one-to-one correspondence. (Generating a heterogeneous AST is generally more useful than generating a homogeneous parse tree.)
In my case I return tuples; in bigger projects I use Zephyr ASDL. (https://news.ycombinator.com/item?id=17852049)
Parser() is a class because it holds the state of the current token. That is necessary for LL(1) parser. It feels a bit redundant sometimes, but it is a straightforward and useful structure once you get used to it.
Writing an LL(1) recursive descent parser by hand is a good exercise even if you plan to use a parser generator eventually. IME it helps to write out the grammar in comments next to the code.
On the other hand, I have found that what parsing technique works best is very closely related to the actual language. It could be that something like Owl works for you language, but it's not obvious and would require some justification IMO.
I've been looking at parser generators recently in an effort to begin writing my own simple interpreted language. My end goal is a self hosted language. My first attempt was writing a recursive descent parser by hand: https://gist.github.com/cmcarey/eee1571721141c356d4f61b453a6... This didn't work so well (badly structured as well as finding out my grammar was ambiguous after I'd written 600 lines of parser code). Lo…
OTOH, the parser generators will do a good job of making sure your basic grammar hangs together. Here is what I have been doing lately:
1) Use ply (Python LALR parser generator by Dave Beazly) to create an accepter (just parse, no code gen). That will help you chase out all the grammar inconsistencies.
2) For production, write a hand-generated recursive descent parser, and use "precedence climbing" for expressions. This gives you a nice table-driven way to handle expressions with a couple of mutually recursive functions, thus eliminating the main PITA of recursive descent: those darn arithmetic expressions.
I've been looking at parser generators recently in an effort to begin writing my own simple interpreted language. My end goal is a self hosted language. My first attempt was writing a recursive descent parser by hand: https://gist.github.com/cmcarey/eee1571721141c356d4f61b453a6... This didn't work so well (badly structured as well as finding out my grammar was ambiguous after I'd written 600 lines of parser code). Lo…
I'm in the same boat. I've found https://github.com/harc/ohm to be a pretty pleasant grammar and parser to use. Ohm also has a nice interactive editor: https://ohmlang.github.io/editor/
The base performance however very low, as in two orders of magnitude lower than most other parsing libraries in JavaScript.
See benchmark at: https://sap.github.io/chevrotain/performance/ that I have created.
Is there an example of such a language?
> This is what guarantees the language is visibly pushdown: all recursion is explicitly delineated by special symbols.
> Plain recursion isn't allowed. Only two restricted kinds of recursion are available: guarded recursion and expression recursion.
So the Owl grammar itself qualifies:
https://github.com/ianh/owl/blob/master/grammar.owl
JSON certainly qualifies:
https://github.com/ianh/owl/blob/master/test/json.owl
I think the full JavaScript language would have qualified in the past, when anonymous functions could only be the explicitly delimited "function() {}"... but now with ES6 we have fat arrow functions (for example "(x,y) => x+y") so I think modern JavaScript can no longer be parsed with Owl. That's because you can write something like "f = x => y => z => x+y+z" which has no guard nor can it be parsed with expression recursion.
I do not think you can do a C-like language because of function types. But you can do a simpler, C-like language, for which they have an good example in the test directory:
I've been looking at parser generators recently in an effort to begin writing my own simple interpreted language. My end goal is a self hosted language. My first attempt was writing a recursive descent parser by hand: https://gist.github.com/cmcarey/eee1571721141c356d4f61b453a6... This didn't work so well (badly structured as well as finding out my grammar was ambiguous after I'd written 600 lines of parser code). Lo…
Instead of using an abstraction of a declarative grammar definition and code generation to provide an alternative to writing a parser by hand, Chevrotain simplifies the process of hand crafting a parser.
Meaning no code generation and you can still directly debug the code you have written (unlike most parser combinators).
I've been looking at parser generators recently in an effort to begin writing my own simple interpreted language. My end goal is a self hosted language. My first attempt was writing a recursive descent parser by hand: https://gist.github.com/cmcarey/eee1571721141c356d4f61b453a6... This didn't work so well (badly structured as well as finding out my grammar was ambiguous after I'd written 600 lines of parser code). Lo…
The problem with yacc/bison-style LALR parser generators is that you end up with an LALR parser. Which works great on syntactically correct code, but trying to get a reasonable error message out of one is about as much fun as repeatedly poking yourself in the eye with a sharp stick. Also, by the time you add the empty productions that you want to help with code generation, the grammar gets delicate and brittle. OTOH,…
Is the problem that these specific tools (yacc/bison) don't report errors well? Or is the issue you're talking about that LR grammars will fail later on than LL grammars in general? Or the greater ambiguity (e.g. more possible lookaheads due to the greater parsing power)?
Earlier quoted context omitted.
The problem with yacc/bison-style LALR parser generators is that you end up with an LALR parser. Which works great on syntactically correct code, but trying to get a reasonable error message out of one is about as much fun as repeatedly poking yourself in the eye with a sharp stick. Also, by the time you add the empty productions that you want to help with code generation, the grammar gets delicate and brittle. OTOH,…
Could you explain in more detail what you mean about LALR error messages? I haven't used the parser generators you mention but I've implemented LALR parsing before, the error reporting situation doesn't seem drastically different from recursive descent to me. You can report the location where the parse failed, what was recognized before that point, what tokens would have been allowed to follow. Is the problem that th…
This is a garbage 'computer says no' kind of error message.
It doesn't tell you why those tokens would have been allowed to follow, why it is that those are the only tokens allowed to follow, why the token you used is not one of the allowed one, or what you could have more likely been intending to write to make it correct.
In a recursive descent parser you have the freedom to use more context and logic to generate your error messages.
Earlier quoted context omitted.
The problem with yacc/bison-style LALR parser generators is that you end up with an LALR parser. Which works great on syntactically correct code, but trying to get a reasonable error message out of one is about as much fun as repeatedly poking yourself in the eye with a sharp stick. Also, by the time you add the empty productions that you want to help with code generation, the grammar gets delicate and brittle. OTOH,…
Could you explain in more detail what you mean about LALR error messages? I haven't used the parser generators you mention but I've implemented LALR parsing before, the error reporting situation doesn't seem drastically different from recursive descent to me. You can report the location where the parse failed, what was recognized before that point, what tokens would have been allowed to follow. Is the problem that th…