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.
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.
ะก++ 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.
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.