> I'm not sure how the expansion would be usable in the module it is
> defined?  It seems I could manage to get 'manifest->friendly-name' to be
> a procedure returning a gexp, but that gexp wouldn't be readily usable
> in that module (it could only be used when gexp-unquote from inside
> another G-Exp), and the expansion in the macro above doesn't bind any
> identifier, unless I'm missing something?

The macro does two things: define a procedure manifest->friendly-name
that returns a string.

(define (manifest->friendly-name manifest)
  "Return a friendly name computed from the entries in MANIFEST, a
  <manifest> object."
  (let loop ((names (map manifest-entry-name
                      (manifest-entries manifest))))
    (define str (string-join names "-"))
      (if (< (string-length str) 40)
          str
          (match names
            ((_) str)
            ((names ... _) (loop names))))))) ;drop one entry

and also define a G-exp define-manifest->friendly-name

(define define-manifest->friendly-nam
  #~(define (manifest->friendly-name manifes)
      "Return a friendly name [...]"
      [...])

Testing from a REPL:

$ guix repl
(use-modules (guix gexp) (ice-9 match) (guix profiles))

(define-syntax define-gexp-and-expand
  (syntax-rules ()
    ((_ variable code) ; code* ... turned out to be unnecessary
     (begin (define variable #~code)
            code))))

(define-gexp-and-expand define-manifest->friendly-name
  (define (manifest->friendly-name manifest)
    "Return a friendly name computed from the entries in MANIFEST, a
    <manifest> object."
    (let loop ((names (map manifest-entry-name
                           (manifest-entries manifest))))
      (define str (string-join names "-"))
        (if (< (string-length str) 40)
            str
            (match names
              ((_) str)
              ((names ... _) (loop names))))))) ;drop one entry
$ define-manifest->friendly-name
$3 = #<gexp (define (manifest->friendly-name manifest) "Return a friendly name computed from the entries in MANIFEST, a\n    <manifest> object." (let loop ((names (map manifest-entry-name (manifest-
entries manifest)))) (define str (string-join names "-")) (if (< (string-length str) 40) str (match names ((_) str) ((names ... _) (loop names)))))) 7f4b3c5ee5a0>
$ (manifest->friendly-name (specifications->manifest '("guile")))
$8 = "guile"

Seems to work.

Greetings,
Maxime.