How do I set the meta-language in the Racket REPL? - racket

Question

I have a program that uses the at-exp meta-language:
#lang at-exp racket
(define v '#f{str})
(displayln v)
I want to manually type it line-by-line into the Racket REPL. Unfortunately, #lang at-exp racket is not valid in the REPL:
$ racket
Welcome to Racket v6.11.
> #lang at-exp racket
; readline-input:1:0: read: #lang not enabled in the current context [,bt for
; context]
; at-exp: undefined;
; cannot reference undefined identifier
; [,bt for context]
; racket: undefined;
; cannot reference undefined identifier
; [,bt for context]
>
How do I set the meta-language in the Racket REPL?

Answer 1

For the beginners who have ended up here, the short answer is no. The #lang shorthand should only be used in files. If you want to run your example program in the REPL, you could load it using this command.
racket --repl --eval '(enter! (file "your-file.rkt"))'
The more detailed answer is that the #lang shorthand gets re-written behind the scenes into module form.
So for example something like this...
#lang racket
(define v "Hello!")
(displayln v))
Is re-written to what you see below and you could copy and paste this into the repl and then require the module, e.g. in the repl type...
> (module the-hello-module racket
(define v "Hello!")
(displayln v))
> (require 'the-hello-module)
Hello!
But from what I can tell, that re-write is dependent on the languages you're using, so the rewrite is probably not something you want to do manually and I don't think the at-exp metalanguage is something you would be add manually to module form.

Answer 2

I don't know why they're different, but the REPL inside DrRacket can use the #lang shorthand:
Unlike the command-line REPL, the DrRacket REPL adopts the language spec­i­fied by the #lang line at the top of the defi­n­i­tions panel. DrRacket also uses the REPL itself to print output from the program.
https://beautifulracket.com/explainer/repl.html#a_Rv5YS
So if you have #lang at-exp racket/base in the definitions panel and click "Run" the bottom REPL panel will show Language: at-exp racket/base, etc., and this will work:
> #string-append{this is a test}
"this is a test"
That page also says:
More intri­cate REPL hacking is possible. For instance, the REPL can be config­ured to use a custom reader (other than Racket’s default S-expres­sion reader).
https://beautifulracket.com/basic-3/the-repl.html
However, the steps described there are much more involved than I care to get into. I don't know what DrRacket is doing in the background to make #lang at-exp work.