macro for defining constant / immutable variables in Racket

README.md

A macro that creates constants in Racket

Inspired by this Racket discourse post I tried writing a macro that defines bindings that work like constants in Racket.

Note that the solution here handles using set!; you can't prevent shadowing or redefining these variables -- see the Discourse thread.

constants-macro.rkt

#lang racket

(require (for-syntax syntax/parse))

(define-syntax (define/constant stx)
  (syntax-parse stx
    [(_ constant:id value:expr)
     #:with module-path (datum->syntax stx 'racket/base)
     #:attr module-name (datum->syntax stx (gensym))
     #:with require-expr
     (datum->syntax stx
                    `(require (prefix-in constants:
                                         (quote ,(attribute module-name)))))
     #`(begin
         (module module-name module-path
           (provide (all-defined-out))
           (define constant value))
         require-expr)]))

(define/constant foo "foo via define/constant")
(define/constant bar "bar via the macro")

constants:bar
constants:foo

;; since : has no special meaning for Racket identifiers, you can
;; do a kind of namespacing:
(define/constant db:thing1 "something related to the database")
(define/constant db:thing2 "another thing related to the database")
(define/constant ui:thing1 "some UI-related constant")

(list constants:db:thing1 constants:db:thing2 constants:ui:thing1)