I have very mixed feelings about using parser generators instead of hand written parsers. I've contributed to GHC's parser grammar and spent a lot of time reading Rust's hand-written parser.
On one hand, a grammar is an invaluable tool. It helps keep you honest about ambiguities when you start thinking about adding language features. I think languages should always keep some reference grammar up to date.
That said, I don't think it is a good idea for compilers to use parser generators. A hand-written parser can have better error recovery and more helpful error messages. GHC's parser has some pretty bad parse error message.
Some work was merged into Happy a while ago to support better messages (enabling you to see the expected tokens along with the bad token), but using that in GHC itself is blocked by how complex GHC's tokenizer has become.
On one hand, a grammar is an invaluable tool. It helps keep you honest about ambiguities when you start thinking about adding language features. I think languages should always keep some reference grammar up to date.
That said, I don't think it is a good idea for compilers to use parser generators. A hand-written parser can have better error recovery and more helpful error messages. GHC's parser has some pretty bad parse error message.
Some work was merged into Happy a while ago to support better messages (enabling you to see the expected tokens along with the bad token), but using that in GHC itself is blocked by how complex GHC's tokenizer has become.