Write a recursive descent parser generator

Details about the "incremental" mode are listed in the documentation PDF[0] at section 9. The parser does not have access to the lexer. Instead, when the parser needs the next token, it stops and returns its current state to the user.

Write a recursive descent parser generator

Details about the "incremental" mode are listed in the documentation PDF[0] at section 9. The parser does not have access to the lexer. Instead, when the parser needs the next token, it stops and returns its current state to the user.

The user is then responsible for obtaining this token typically by invoking the lexer and resuming the parser from that state. Assuming that semantic values are immutable, a parser state is a persistent data structure: The parser can be re-started in the middle of the buffer whenever the user edits a character.

Because two successive parser states share most of their data in memory, a list of n successive parser states occupies only O n space in memory. I can try to explain a bit. My goal was Merlin https: Also as of today only incrementality and error message generation are part of upstream version of Menhir, but the rest should come soon.

Incrementality, part I The notion of incrementality that comes builtin with Menhir is slightly weaker than what you are looking for. With Menhir, the parser state is reified and control of the parsing is given back to the user. The important point here is the departure from a Bison-like interface.

The user of the parser is handled a pure abstract object that represents the state of the parsing. In regular parsing, this means we can store a snapshot of the parsing for each token, and resume from the first token that has changed effectively sharing the prefix.

But on the side, we can also run arbitrary analysis on a parser for error message generation, recovery, syntactic completion, or more incrementality Incrementality, part II Sharing prefix was good enough for our use case parsing is not a bottleneck in the pipeline.

But it turns out that a trivial extension to the parser can also solve your case. Using the token stream and the LR automaton, you can structure the tokens as a tree: In a later parse, whenever you identify a known state number, prefix pair, you can short-circuit the parser and directly reuse the subtree of the previous parse.

If you were to write the parser by hand, this is simply memoization done on the parsing function which is defunctionalized to a state number by the parser generator and the prefix of token stream that is consumed by a call.

In your handwritten parser, reusing the objects from the previous parsetree amounts to memoizing a single run and forgetting older parses.

write a recursive descent parser generator

Here you are free to choose the strategy: So with part I and II, you get sharing of subtrees for free. Indeed, absolutely no work from the grammar writer has been required so far: A last kind of sharing you might want is sharing the spine of the tree by mutating older objects.

Error messages The error message generation is part of the released Menhir version. It is described in the manual and papers by F.Another fun way to write a recursive descent parser is to abstract and parametrize the recursive descent algorithm.

Best done in dynamic languages. I wrote an abstract recursive descent parser in JS which accepts an array of terminal (regexp) and non-terminal definitions (array of terminal/non-terminal names + action callback), and .

So there's really nothing stopping you from implementing a lexer as a recursive descent parser or using a parser generator to write a lexer.

It's just not usually as convenient as using a more specialized tool. Rewriting parts of recursive descent parser, when language or format it conforms to has changed a bit, may be a lot easier, then modifying parser generator file. But it 1) you have to totally rewrite your parser every day (as a planned part of developing process) 2) grammar changes in layout, not in complexity, it may be far easier to rewrite.

I need to write a lexer and a parser for a given grammar (I need to handcraft it not with generators). I have done a lot of research but I still can't figure out how to code it.

Back when I tried to learn how to write a recursive descent parser, the examples I found either ignored correct expression parsing or wrote an additional parse method for each precedence level.

Recursive descent parser - Wikipedia

Writing a parser by hand seemed just too much work. Recursive descent parser generator is a draft programming task.

So there's really nothing stopping you from implementing a lexer as a recursive descent parser or using a parser generator to write a lexer. It's just not usually as convenient as using a more specialized tool. Rewriting parts of recursive descent parser, when language or format it conforms to has changed a bit, may be a lot easier, then modifying parser generator file. But it 1) you have to totally rewrite your parser every day (as a planned part of developing process) 2) grammar changes in layout, not in complexity, it may be far easier to rewrite. This will teach you how a recursive descent parser works, but it is completely impractical to write a full programming language parser by hand. You should look into some tools to generate the code for you - if you are determined to write a classical recursive descent parser (TinyPG, Coco/R, Irony).

It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page. Write a recursive descent parser generator that takes a description of a grammar as input and outputs the source code for a parser in the same language as the.

parsing - Lexer and parser in C++ from EBNF - Stack Overflow