I've seen two approaches to parsing:
Use a parser generator like happy. This allows you to specify your language in BNF, and not worry about the intricacies of parsing. However, since it's a preprocessor you have to write your whole parse tree textually.
Use a parser directly like megaparsec. With this approach you have direct access to your code so you can generate your parser programatically, but you haven't got the convenience of happy's simple BNF specification with precedence annotations etc. Also it seems non trivial to print out a BNF tree for documentation from your parsing code unless this is considered during it's construction.
What I'd like to do is something like this:
Generate a data structure programatically that represents BNF.
Feed this through to a "happy like" parser generator to generate a parser.
Feed this through a pretty printer to generate actual BNF documentation.
The reason I want to do this is that the grammar I'm working on has grown quite large and has a lot of repetition, as a lot of it's constructs are similar to others but slightly different. It would improve maintenence effort if it could be generated programmatically instead of modifying happy BNF spec directly, but I'd rather not have to develop my own parser from scratch.
Any ideas about a good approach here. It would be great if I could just generate a data structure and force it into happy (as it presumably generates it's own internal structure after parsing the BNF feed to it) but happy doesn't seem to have a library interface.
I guess I could generate attonated BNF, and feed that through to happy, but it seems like a messy process of converting back and forth. A cleaner approach would be better. Perhaps even a BNF style extension to parsec or megaparsec?
The simplest thing to do would to make some data type representing the relevant grammar, and then convert it to a parser using some parser combinators as a (run-time) "compile" step. Unfortunately, most parser combinators are less efficient and/or less flexible (in some ways) than the parser generators, so this would be a bit of a lowest common denominator approach. That said, the grammar-combinators library may be useful, though it doesn't appear to be maintained.
There are libraries that can generate parsers at run-time. One I found just now is Grempa, which doesn't appear to be maintained but that may not be a problem. Another option (by the same person who made Grempa but maintained) is Earley which, due to the way Earley parsers are made, it makes sense to have an explicit grammar that gets processed into a parser. Earley parsing is certainly flexible, but may be overpowered for you (or maybe not).
I am trying to write a simple YAML parser, I read the spec from yaml.org,
before I start, I was wondering if it is better to write a hand-rolled parser, or
use lex (flex/bison). I looked at the libyaml (C library) -
doesn't seem to use lex/yacc.
YAML (excluding the flow styles), seems to be more line-oriented, so, is it
easier to write a hand-rolled parser, or use flex/bison
Thanks.
This answer is basically an answer to the question: "Should I roll my own parser or use parser generator?" and has not much to do with YAML. But nevertheless it will "answer" your question.
The question you need to ask is not "does this work with this given language/grammar", but "do I feel confident to implement this". The truth of the matter is that most formats you want to parse will just work with a generated parser. The other truth is that it is feasible to parse even complex languages with a simple hand written recursive descent parser.
I have written among others, a recursive descent parser for EDDL (C and structured elements) and a bison/flex parser for INI. I picked these examples, because they go against intuition and exterior requirements dictated the decision.
Since I established on a technical level it is possible, why would you pick one over the other? This is really hard question to answer, here are some thoughts on the subject:
Writing a good lexer is really hard. In most cases it makes sense to use flex to generate the lexer. There is little use of hand-rolling your own lexer, unless you have really exotic input formats.
Using bison or similar generators make the grammar used for parsing explicitly visible. The primary gain here is that the developer maintaining your parser in five years will immediately see the grammar used and can compare it with any specs.
Using a recursive descent parser makes is quite clear what happens in the parser. This provides the easy means to gracefully handle harry conflicts. You can write a simple if, instead of rearranging the entire grammar to be LALR1.
While developing the parser you can "gloss over details" with a hand written parser, using bison this is almost impossible. In bison the grammar must work or the generator will not do anything.
Bison is awesome at pointing out formal flaws in the grammar. Unfortunately you are left alone to fix them. When hand-rolling a parser you will only find the flaws when the parser reads nonsense.
This is not a definite answer for one or the other, but it points you in the right direction. Since it appears that you are writing the parser for fun, I think you should have written both types of parser.
To learn how to write and parse a context-free grammar I want to choose a tool. For Haskell, there are two big options: Happy, which generates a parser from a grammar description and *Parsec, which allows you to directly code a parser in Haskell.
What are the (dis)advantages of either approach?
External vs internal DSL
The parser specification format for Happy is an external DSL, whereas with Parsec you have the full power of Haskell available when defining your parsers. This means that you can for example write functions to generate parsers, use Template Haskell and so on.
Precedence rules
With Happy, you can use precedences to simplify your grammar, whereas with Parsec you have to nest the grammar rules correctly yourself. Changing the precedence of an operator is therefore much more tedious in Parsec.
Static checking
Happy will warn you about ambiguities in your grammar at compile time. (Though it's not great at telling you where they are.) With Parsec, you get no warning until your parser fails at run time.
This is the traditional decision: do I use lex/yacc (happy) or do I write my own (mostly recursive descent) parser, only that the parsec library is like a DSL for doing it right.
If one has experience with the yacc/lex approach, using happy will be a smaller learning curve.
In my opinion Parsec hides most of the nasty grammar details and lets you write your parsers more intuitively. If you want to learn this stuff in the first place go with some parser-generator like Happy (or even try to implement one yourself).
I'm used to the parser combinator library uu-parsinglib from utrecht university. One can have error correcting and permutations for free, and also the things that parsec has. I also like it because my implemented grammar looks like an EBNF grammar, without so much monadic stuff, and is easy to read.
Naive parser combinators do not allow left-recursion in grammar rules and I haven't found a library that does.
Happy does allow full BNF in language spec, and some useful staff like priority rules. So, for complicated cases Happy and parser generators in general are much better. However, in case of simple, stupid languages with LL(k) parseable grammars, I would use a parser combinator library as more maintainer-friendly.
I need to parse a simple DSL which looks like this:
funcA Type1 a (funcB Type1 b) ReturnType c
As I have no experience with grammar parsing tools, I thought it would be quicker to write a basic parser myself (in Java).
Would it be better, even for a simple DSL, for me to use something like ANTLR and construct a proper grammar definition?
Simple answer: when it is easier to write the rules describing your grammar than to write code that accepts the language described by your grammar.
If the only thing you need to parse looks exactly like what you've written above, then I would say you could just write it by hand.
More generally speaking, I would say that most regular languages could be parsed more quickly by hand (using a regular expression).
If you are parsing a context-free language with lots of rules and productions, ANTLR (or other parser generators) can make life much easier.
Also, if you have a simple language that you expect to grow more complicated in the future, it will be easier to add rule descriptions to an ANTLR grammar than to build them into a hand-coded parser.
Grammars tend to evolve, (as do requirements). Home brew parsers are difficult to maintain and lead to re-inventing the wheel example. If you think you can write a quick parser in java, you should know that it would be quicker to use any of the lex/yacc/compiler-compiler solutions. Lexers are easier to write, then you would want your own rule precedence semantics which are not easy to test or maintain. ANTLR also provides an ide for visualising AST, can you beat that mate. Added advantage is the ability to generate intermediate code using string templates, which is a different aspect altogether.
It's better to use an off-the-shelf parser (generator) such as ANTLR when you want to develop and use a custom language. It's better to write your own parser when your objective is to write a parser.
UNLESS you have a lot of experience writing parsers and can get a working parser that way more quickly than using ANTLR. But I surmise from your asking the question that this get-out clause does not apply.
PS.Where to read about parsing theory?
Summary: the shortest is probably Antlr.
Its tempting to go to the Dragon Book to learn about parsing theory. But I don't think the Dragon Book and you have the same idea of what "theory" means. The Dragon Book describes how to built hand-written parsers, parser generators, etc, but you almost certainly want to use a parser-generation tool instead.
A few people have suggested Bison and Flex (or their older versions Yacc and Lex).
Those are the old stalwarts, but they are not very usable tools.
Their documentation is not poor per se, its just that it doesn't quite help in getting dealing with the accidental complexity of using them.
Their internal data is not well encapsulated, and it is very hard to do anything advanced with them. As an example, in phc we still do not have correct line numbers because it is very difficult. They got better when we modified out grammar to include No-op statements, but that is an incredible hack which should not be necessary.
Ostensibly, Bison and Flex work together, but the interface is awkward. Worse, there are many versions of each, which only play nicely with some specific versions of the other. And, last I checked at least, the documentation of which versions went with which was pretty poor.
Writing a recursive descent parser is straightforward, but can be tedious. Antlr can do that for you, and it seems to be a pretty good toolset, with the benefit that what you learn on this project can be applied to lots of other languages and platforms (Antlr is very portable). There are also lots of existing grammars to learn from.
Its not clear what language you're working in, but some languages have excellent parsing frameworks. In particular, the Haskell Parsec Library seems very elegant. If you use C++ you might be tempted to use Spirit. I found it very easy to get started with, and difficult--but still possible--to do advanced things with it. This matches my experience of C++ in general. I say I found it easy to start, but then I had already written a couple of parsers, and studied parsing in compiler class.
Long story short: Antlr, unless you've a very good reason.
It's always a good idea to read the Dragon Book. But be aware that if your language is not trivial, there's not really a "short" way to do it.
It rather depends on your language. Some very simple languages take very little parsing so can be hand-coded; other languages use PEG generators such as Rats! ( PEG is parser expression grammar, which sits between a Regex and a LR parser ) or conventional parser generators such as Antlr and Yacc. Less formal languages require probabilistic techniques such as link grammars.
Write a Recursive Descent Parser. This is sometimes easier than YACC/BISON, and usually more intuitive.
Douglas Crockford has an approachable example of a parser written in JavaScript.
YACC, there are various implementation for different languages.
Good luck with your language ;-)
I used the GOLD Parsing System, because it seemed easier to use than ANTLR for a novice like me, while still being sufficiently-fully-featured for my needs. The web site includes documentation (including an instructions on Writing Grammars, which is half the work) as well as software.
Try Bison for parsing and Flex for lexing
The bison definition of your language is in the form of a context-free grammar. The wikipedia artcile on this topic is quite good, and is probably a good place to start.
Using a parser generator for your host language is the fastest way, combined with parsing theory from a book such as the Dragon Book or the Modern Compiler Construction in {C,ML} series.
If you use C, yacc and the GNU version bison are the standard generators. Antlr is widely used in many languages, supporting Java, C#, and C++ as far as I know. There are also many others in almost any language.
My personal favorite at present is Menhir, an excellent parser generator for OCaml. ML-style languages (Ocaml, Standard ML, etc.) dialects in general are very good for building compilers and interpreters.
ANTLR is the easiest for someone without compiler theory background because of:
ANTLRWORKS (visual parsing and AST debugging)
The ANTLR book (no compiler theory background required)
Just 1 syntax for lexer and parser.
If you are happy with parsing expression grammars, writing your own parsers can be incredibly short. Here is a simple Packrat parser that takes a reasonable subset of PEG:
import functools
class peg_parse:
def __init__(self, grammar):
self.grammar = {k:[tuple(l) for l in rules] for k,rules in grammar.items()}
#functools.lru_cache(maxsize=None)
def unify_key(self, key, text, at=0):
if key not in self.grammar:
return (at + len(key), (key, [])) if text[at:].startswith(key) \
else (at, None)
rules = self.grammar[key]
for rule in rules:
l, res = self.unify_rule(rule, text, at)
if res is not None: return l, (key, res)
return (0, None)
def unify_line(self, parts, text, tfrom):
results = []
for part in parts:
tfrom, res = self.unify_key(part, text, tfrom)
if res is None: return tfrom, None
results.append(res)
return tfrom, results
It accepts grammars of the form of a python dictionary, with nonterminals as keys and alternatives as elements of the array, and each alternative is a sequence of expressions. Below is an example grammar.
term_grammar = {
'expr': [
['term', 'add_op', 'expr'],
['term']],
'term': [
['fact', 'mul_op', 'term'],
['fact']],
'fact': [
['digits'],
['(','expr',')']],
'digits': [
['digit','digits'],
['digit']],
'digit': [[str(i)] for i in list(range(10))],
'add_op': [['+'], ['-']],
'mul_op': [['*'], ['/']]
}
Here is the driver:
import sys
def main(to_parse):
result = peg_parse(term_grammar).unify_key('expr', to_parse)
assert (len(to_parse) - result[0]) == 0
print(result[1])
if __name__ == '__main__': main(sys.argv[1])
Which can be invoked thus:
python3 parser.py '1+2'
('expr',
[('term',
[('fact',
[('digits', [('digit', [('1', [])])])])]),
('add_op', [('+', [])]),
('expr',
[('term', [('fact', [('digits', [('digit', [('2', [])])])])])])])
Parsing Expression Grammars take some care to write: The ordering of alternatives is important (Unlike a Context Free Grammar, the alternatives are an ordered choice, with the first choice being tried first, and second being tried only if the first did not match). However, they can represent all known context free grammars.
If on the other hand, you decide to go with a Context Free Grammar, Earley Parser is one of the simplest.