These tools basically input a grammar and output code which processes a series of tokens into something more useful, like a syntax tree. But could these tools be written in the form of a library instead? What is the reason for generating source code as output? Is there a performance gain? Is it more flexible for the end user? Easier to implement for the authors of yacc and ANTLR?
Sorry if the question is too vague, I'm just curious about the historical reasons behind the decisions the authors made, and what purpose auto-generated code has in today's environment.
There's a big performance advantage achieved by the parser generator working out the interactions of the grammar rules with respect to one another, and compiling the result to code.
One could build interpreters that simply accepted grammars and did the parsing; there are parser types (Earley) that would actually be relatively good at that, and one could compute the grammar interactions at runtime (Earley parsers kind of do this anyway) rather than offline and then execute the parsing algorithm.
But you would pay a parsing performance penalty of 10 to 100x slowdown, and probably a big storage demand.
If you are parsing using only very small grammars, or you are parsing only very small documents, this might not matter. But the grammars that many parser generators get applied too end up being fairly big (people keep wanting to add things to what you can say in a language), and they often end up processing pretty big documents. So performance now matters, and viola, people build code-generating parser generators.
Once you have a tool, it is often easier to use even in simple cases. So now that you have parser generators, you can even apply them to little grammars or to parsing little documents.
EDIT: Addendum. The historical reason is probably driven by space and time demands. Earlier systems had not a lot of room (32Kb in 1975), didn't run very fast (1 MIPS same time frame), and people had big source files already. Parser generators tended to help with this set of problems; interpreted grammars would have had intolerably bad performance.