config.rkt

;; <base-name> is used both (1) to define a parameter named
;; current-<base-name> and (2) to get an environment variable named
;; "WEB_<BASE_NAME>" (note conversion of kebab-case to SNAKE_CASE).
(define-syntax-parser dp
  [(_ base-name:id
      default:expr
      contract:expr
      (~optional (~seq #:coerce coerce:expr)      #:defaults ([coerce #'values]))
      (~optional (~seq #:secret? secret?:boolean) #:defaults ([secret? #'#f])))
   (with-syntax ([id-name  (format-id #'base-name
                                      "current-~a"
                                      (syntax-e #'base-name))]
                 [env-name (string-upcase
                            (regexp-replace* #px"-"
                                             (format "WEB_~a"
                                                     (syntax-e #'base-name))
                                             "_"))])
     #'(begin
         (define/contract id-name (parameter/c contract)
           (make-parameter (match (getenv env-name)
                             [#f default]
                             [v  (coerce v)])))
         (provide id-name)
         (unless secret?
           (hash-set! non-secret 'id-name id-name))))])

;; e.g.
(dp external-port 8080 port-num? #:coerce string->number)
;; defines `current-external-port` defaulting to env var WEB_EXTERNAL_PORT