Basically no serious programming language uses bison or flex, and everything that does (awk is YACC based) has terrible error messages. People totally do handwrite lexers and parsers, if you use some language in earnest the chances it has a handwritten parser or lexer or both are far greater than that it has been developed with bison/flex.
Can we stop with this "duh, real programmers write their own handwritten parsers"? Parser generators are awesome! They force you to write a declarative grammar which a human can understand. If you're designing a language this is very useful. With a handwritten parser you're very likely to encode some unintended behaviour in edge cases; using a parser generator early in this process will very quickly reveal the ambiguities.
That doesn't mean that parser generators are always the best. There are many use cases where a handwritten parser is better. But the picture is way more nuanced than "no serious programming language uses bison or flex".
The current go compiler uses a hand-written parser, judging from it's source.
The benefit of having a BNF grammar for your language is rightfully enormous, and all modern languages definitely lay out their lexing grammar and BNFs explicitly in their specification. But there are downsides. Debugging LALR grammars is painful; error conditions are almost uniformly painful. Many languages make tokenization dependent on the current context of a parse for language ergonomic reasons [1], and you might not be able to specify the action in the right place in your automated parser generator. Hell, the language might not even conform to the LL(k) or LALR(1) your parser generator translates.
[1] I'd say this is context-sensitive lexing, but the resulting grammar isn't context-sensitive. Just somewhat more complicated than a naive translation of the grammar would indicate.
I can name just as many "serious programming language" implementations that use recursive descent or migrated to it because a parser generator turned out to be a bad idea after all:
They force you to write a declarative grammar which a human can understand
It's a separate language that also has to be learnt, an extra step in the compilation process, and the difficulty of debugging parser-generator-generated code cannot be overstated. A recursive-descent parser written in a programming language is easily understood, debuggable, and does not incur any extra complexity.
Perhaps that is where the "no serious programming language" argument comes from --- the migration away from parser generators is a sign that the language has received enough attention such that the limitations of a generated parser become prominent.
The problem with LALR parser generators is that you end up with an LALR parser. Which is fine as long as the input is error-free, but trying to get a reasonable error message out of any LALR parser is about as much fun as repeatedly poking yourself in the eye with a sharp stick.
FLEX, on the other hand, is perfect for the job. I can't imagine why anyone would hand-write a lexer.
> FLEX, on the other hand, is perfect for the job. I can't imagine why anyone would hand-write a lexer.
Flex introduces a new dependency to your project, which involves generating source files at build time. This is a lot of complexity in your build system if this is the first time you need it. And it also by default uses a style that isn't really modern--it's more annoying to use if you want to use it in a multithreaded programming, or if you want to have multiple lexers in one program. (Or you're not even using C/C++ in the first place!).
Furthermore, lexers are actually really easy to write. Most tokens [1] are recognizable within a character or two, and usually amount to a regex on the order of [a-z][a-z0-9_]+ or "([^\"]|\.)*" -- the sorts of regexes that have very trivial implementations to handcode. The lexer I wrote for a JSON-ish file format is 128 lines of code. Sure, it'd be more like 30 lines of code if I used an automatic generator, but running into any issues integrating the automatic generator into my workflow would toss out the benefits of saving 100 lines of code.
[1] Excluding keywords, but keywords are usually implemented by parsing it as an identifier and looking for the identifier in a hashtable of known keywords.
In my experience, re2c is an improvement on lex/flex in most ways, but I agree - flex is indeed nicer than hand-rolled in every way (except if you are Arthur Whitney - his hand rolled lexers are a marvel)
I'm guilty of being overly hyperbolic, and you are right to correct me. But if your poster boys to illustrate the benefits of parser generators forcing you to write a declarative grammar which a human can understand are perl, bash or even postgresql, I'm not sure what to say ;)
Apart from Go, (and maybe PHP, which I don't know well enough to judge) all the languages you list have pretty horrible syntax that probably few (ruby?) to basically none of its users (bash, perl, C++, postgresql) really understand.
> if your poster boys to illustrate the benefits of parser generators forcing you to write a declarative grammar which a human can understand are perl, bash or even postgresql, I'm not sure what to say ;)
And that is a straw man argument because, of course, no one is saying that.
> They force you to write a declarative grammar which a human can understand.
The problem is many languages' grammars are not declarative. They're full of completely uncontrolled side effects. So why are we trying to use a declarative tool for them? It's the wrong tool for the job.
Yes! We should have both handwritten parsers and parser generators in our toolkit and then depending on the language we want to parse we should choose the best tool for the job.
My point about a declarative grammar was more that if the language permits it (e.g. is simple enough for Bison), it's so valuable to have a declarative grammar which serves as both specification and parser. It will be way less bug-prone than any handwritten parser. (From my experience handwritten parsers tends to have both weird crashes and unexpected handling of edge cases. But maybe I just suck at programming?)
I've seen people using Bison on one complicated languages, given up, and then they bring the negative bias to other languages they want to parse — even when the other languages are very simple.
How is that relevant to this thread in which I am saying that the parsing/lexing as described in the link is very outdated? Maybe you should start a separate discussion.
Hmm, I wrote a VHDL (a superset of ADA used for hardware design) front end using bison and flex. That's a serious language.
Regarding error messages, if one implements the grammar as specified by the language spec, yes, confusing and useless error messages will result. That's because the language spec grammar includes restrictions based on semantics. To get good error messages, one has to remove/relax the spec grammar and then use semantic checking. Parsing is an art; not a science.
Right - no 'serious language' is hyperbole, and another counter example is Ruby.
But... if I was asked to implement a language tomorrow I wouldn't even consider using Bison and Flex. Why? Because they're context-free tools, and almost every industrial language is context-sensitive. Square pegs and round holes.
These tools are a great example of over-complicating and over-theorising programming languages. We invented a theory that doesn't even apply to the problem we have, and then created crazy tools to solve the wrong problem, and gloss over the fact that they don't apply with terrible hacks and side-effects. We made a mistake with that phase in programming language history...
Interesting. I've always thought that there's no real reason why e.g. Ruby has to be context-sensitive. Yes, I am aware of the fact that local variables are resolved statically, but do they have to be resolved during parsing? Couldn't they just be parsed as a method_call_or_var_lookup, and then later resolved? The main "feature" that Ruby's context-sensitive parser has is the ability to parse differently depending on what precedes it is a local variable or not, and I'm not sure if that's a good feature or not.
Are there other parts of Ruby's grammars that are also context-sensitive?
I'd say that Bison's biggest issue is that it's limited to LALR; if it actually supported full context-free grammars it would be far more useful.
> Yes, I am aware of the fact that local variables are resolved statically
That's the one that comes to my mind yes. I'd bet that there are others, but couldn't list them to you now.
> do they have to be resolved during parsing?
I guess not in practice?
But why are we doing this to ourselves, right? These parsing tools work great! (As long as you only do half do the job you were trying to do and then sort of hack it back together afterward.)
> But why are we doing this to ourselves, right? These parsing tools work great! (As long as you only do half do the job you were trying to do and then sort of hack it back together afterward.)
I agree with you in practice: The current parser generators are not as good as I'd like them to be. They are too restrictive and too locked into a "let's parse this subset of grammars mathematically optimally" instead of "let's make it easy for programmers to declare grammars".
(Disclaimer: I started working on a completely new parser compiler https://www.sanity.io/blog/why-we-wrote-yet-another-parser-c... which is capable of parsing full context-free grammars. I personally believe this is part of the puzzle of making them more usable.)
However, since we're talking about Ruby's syntax: I don't think Ruby's parser would be any nicer if it was handwritten. The problem here is that they discovered the power of "lexer tweaks parser state" and let it drive the design of the language. This has caused the language itself to be complicated (both for humans and for computers!) Unfortunately I don't have time for it, but I'd love to dive into the changes required in Ruby's syntax to make it context-free (e.g. pushing the local variable lookup into the bytecode generator).
Is there any grammar for a modern language that is context sensitive and expresses the constraints better than a context free grammar?
It’s true almost no usable language is really CF, but a CF grammar with a few semantic patches still seems to be the better way to describe them.
It’s been a while since I needed to parse a grammar, but if I had to I’d probably use re2c+lemon, Ometa/ohm, or a hand-rolled Pratt parser. (Or an Earley parser if the grammar was ambiguous and it made sense)
That's more hyperbole. You're putting a lot of responsibility on some simple tools. Programming language design complexity came first and not as a result of yacc or lex.
> Programming language design complexity came first and not as a result of yacc or lex.
I'm really not sure where you got the impression I was blaming Yacc or Lex for the complexity of the languages - I think you've imagined that.
The situation we're in in practice is that our language grammars are complex and are context-sensitive. So why are we trying to use context-free tools? It's madness - we're trying to hammer a nail using a screwdriver.
I got that impression from this sentence of yours:
> These tools are a great example of over-complicating and over-theorising programming languages.
If programming languages are over-complicated, it's not the fault of these tools.
> So why are we trying to use context-free tools?
I no longer keep up with compilers (or languages), but when I was, there was no other tools. Either go through the tedium and effort of hand-rolling a lexer/parser or munge the language grammar to fit.
I see why you'd think that, but I meant over-complicating the implementation of languages, not their design.
> there was no other tools
I don't think you need tools. Just write your parser as a normal program directly in a general-purpose programming language. I'm not sure that parsing is so special as to require an external DSL.
Defining a variable before use makes a language context sensitive. So does e.g. typedef in C - is (a)*b multiplying a by b or typecasting what b points to? Depends on the types.
We usually semantically patch that at the right stage through the lexer or the AST so things still work in a CF setting, but almost no modern language is CF.
Use before declaration is resolved at the type checker level, not during parsing. C-style casting isn’t context sensitive if you don’t mind treating the casting type has an expression and then pulling a type out of it in the type checker (or evaluator if dynamic).
Type checking is where all the complexity really is. Parsing and lexing are just easy things you do before the hard stuff.
C has a lot of things which are context sensitive, and you can't treat the casting type as an expression:
c*(a)-b
parses under (double a, b, c;) as ((ca)-b) ; but under (typedef double a; double b, c) it parses as c-b, (with a type conversion applied after the negation) which is a different parse, and which can be more complicated than that to the point that fixing the AST is equivalent to reparsing the source.
Use before declaration can only be resolved at the typechecker if you assume every name is a variable, but the introduction of typedefs makes that assumption wrong, so you can't wait for the type checker - every C parser I know patches that information back into the lexer so that it would parse correctly. Indeed, the "typename" keyword was added to C++ because you can't leave that to the typechecker if you want a unique parse before instantiation.
Parsing with type information is not the norm. It was only done in C because no one knew any better back then. Later language that adopt C-style casting (C#, Java) do not need to know if something is a type to deal with parens, they look for the follow on and evaluate the tree accordingly. That is why in C#:
var x = (Foo) - foo;
Will be rejected with a type error (must enclose -foo in parentheses because Foo is a type) rather than a parse error.
I'm reminded of another comment from some time ago [0] that recommended 'permissive' parsers for a different reason: you can build a list of syntax errors for the user, which isn't possible if you explode immediately, and you may be able to generate better syntax errors than your parser would have.
