> Parser/lexer generators are really easy to use for DSLs but for real languages and tools, they're too heavyweight and constraining.

Haskell and OCaml are examples of real languages that use parser generators.

GHC: https://github.com/ghc/ghc/blob/master/compiler/parser/Parse...

Ocaml: https://github.com/ocaml/ocaml/blob/trunk/parsing/parser.mly