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).

Thanks I'll read that article.

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.

No tool is ever necessary. But not using yacc is tedious, slow, error-prone, uninteresting, and utterly no fun.

What production languages are context sensitive? C++ surely, Java and C# only when dealing with angle brackets for generics, what else?

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.

[0] https://news.ycombinator.com/item?id=17072670