Lately I'm learning Racket, and I'm having some difficulties with understanding the "define-type" syntax. I've tried the following code:
#lang racket
(define-type Num Number)
but it outputs the following error message:
define-type: unbound identifier in module in: define-type
May anyone help me dealing this error? I have read all possible documentation and it seems like it should be working.
There is a reason every Racket program starts with a line beginning with #lang: Racket is not just a programing language, but an ecosystem of programming languages. Every file (and more specifically, every module) can be in its own programming language, and they can all talk to each other with ease.
By default, Dr. Racket creates new files with a #lang racket line at the top. This is the “Racket language”, but it is not the only language provided by the “Racket system”, which actually includes dozens of languages, some not too different from #lang racket, others almost entirely unrelated.
When you want to use Typed Racket, you need to opt in to using that language instead of ordinary #lang racket, which is dynamically typed. You can do this by writing #lang typed/racket at the top of your program.
#lang typed/racket
(define-type Num Number)
Now all the features of Typed Racket will be available to you within that module.
