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Comment by Animats | Hacker News Reader

What they were thinking can be see in K&R C - there is no "typedef" in early C. Without typedef, the syntax of C is context-independent and LALR-1. You don't have to know if a name is a type to parse the syntax. Then came "typedef", which broke parsing. C parsing became context-dependent. To parse C with "typedef", and especially C++, you must read all the header files first.

With name-first declaration syntax (Pascal, Modula, Go, Rust), parsing is context-independent again. Readability improves. Error messages improve. Syntax-coloring editors with a single-file view don't get lost.

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That's interesting -- I was wondering in which cases typedef changes the parse tree, and came across a few [1]:

    a (b);      /* function call or declaration */
    a * b;      /* multiplication or declaration */
    f((a) * b); /* multiplication or deref and cast */

> With one further change, namely deleting the production typedef-name: identifier and making typedef-name a terminal symbol, this grammar is acceptable to the YACC parser-generator.

[1] http://eli.thegreenplace.net/2007/11/24/the-context-sensitiv...

С++ takes it all the way to 11 with templates. Here's a program that is parsed differently depending on whether pointers are 32-bit or 64-bit:

    template<size_t N = sizeof(void*)> struct a;

    template<> struct a<4> {
        enum { b };
    };

    template<> struct a<8> {
        template<int> struct b {};
    };

    enum { c, d };

    int main() {
        a<>::b<c>d;
        d;
    }

Depending on which instantiation is used, the first line of main is either a variable declaration, or two operators < and > applied in sequence.

This is especially fun to deal with for C++ IDEs that support semantic highlighting (i.e. typenames are in a different color etc). If I remember correctly, the first one that could handle this right was VS 2012 - it only took 14 years after ISO C++ standard was released...

That explains how they ended up with context-dependent syntax - you're right, in the absence of typedefs, it's context-free to parse because of tags. But it can still be a pain to read, if you run into things like arrays of pointers to functions.

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