Pretty forms for nonterminals are supported but undocumented

[texdraft]

The define-language macro allows => to follow nonterminal productions as well as terminal definitions, but this is not mentioned in the documentation. It is described in Andy Keep's dissertation, which provides the (here abridged) example

(define-language Lsrc
  …
  (Expr (e body)
    …
    (call e e* ...) => (e e* ...)))

and then states

The => (e e* ...) syntax that follows the production indicates a pretty form for the production. The define-language form defines an unparser for each language, and the unparser uses the pretty form when unparsing this production. This is useful to produce a language form that can be evaluated in the host Scheme system.

The feature is in use in the wild, e.g. in https://github.com/soegaard/urlang/blob/master/compiler-rjs/compiler2.rkt#L301

[spdegabrielle]

I think there is enough there to draft a PR to update the docs at

nanopass-framework-racket/scribblings/nanopass.scrbl

Line 179 in deac3a4

@subsection{The define-language form}

I’d suggest making something up or simply quoting & citing the dissertation.

https://docs.racket-lang.org/nanopass/index.html#(form._((lib._nanopass%2Fbase..rkt)._define-language))