Setting language with #lang in the REPL

Question

I would like to set the language in the REPL on the fly, with #lang, not using \"-I\" command-line argument. But this gives me the error \"read: #lang not enabled i

Answer 1

[Edit]

I can't get C-x C-b to work with #lang either.

But a buffer containing #lang can be sent to a REPL started from Geiser with C-c C-a. This is Switch to REPL and Enter Module from the Geiser drop down menu. If I have a buffer for bugsy.rkt:

;; bugsy.rkt
#lang racket
(define k 6)
(define j 7)
(define (f lhs rhs)
   (+ lhs rhs))

Typing C-c C-a gives me this in the REPL:

racket@> ,enter "/bugsy.rkt"
racket@bugsy.rkt>

I can then access the module in the REPL:

racket@bugsy.rkt> k
6
racket@bugsy.rkt> (f 3 4)
7

If I want to switch to a different module [or buffer of a file] I can use the ,enter command in the REPL:

racket@bugsy.rkt> ,enter "clyde.rkt"
racket@clyde.rkt> ,enter "bonny.rkt"
racket@bonny.rkt>

There is an example of the ,enter command in the documentation. Look above the Dinosaur.

[Original]

According to the Racket documentation #lang has very simple syntax, the reader essentially bootstraps a language syntax from whatever follows the space character after #lang. This means in some sense that #lang is not in Racket's [or any other language's] syntax. Instead it is a implementation feature of the reader which forms part of the larger "Racket" development ecosystem.

Geiser [and presumably Quack and racket-mode] handle this by parsing #lang in elsip before passing code to the Racket REPL. In Geiser, the work is done in geiser-racket.el.

The parsing function is at line 132:

(defun geiser-racket--language ()
  (or (cdr (geiser-racket--explicit-module))
      (save-excursion
        (goto-char (point-min))
        (if (re-search-forward "^#lang +\\([^ ]+\\)" nil t)
            (geiser-syntax--form-from-string (match-string-no-properties 1))))
      "#f"))

And it is called by geiser-racket--geiser-procedure on line 166.

(defun geiser-racket--geiser-procedure (proc &rest args)
  (case proc
    ((eval compile)
     (format ",geiser-eval %s %s %s"
             (or (car args) "#f")
             (geiser-racket--language)
             (mapconcat 'identity (cdr args) " ")))
    ((load-file compile-file)
     (format ",geiser-load %S" (geiser-racket--find-module)))
    ((no-values) ",geiser-no-values")
    (t (format ",apply geiser:%s (%s)" proc (mapconcat 'identity args " ")))))

That may give you a starting point for rolling your own code if one of the existing Emacs modes does not meet your needs.