The #lang at the start of a module file begins a shorthand for a module form, much like ' is a shorthand for a quote form. Unlike ', the #lang shorthand does not work well in a REPL, in part because it must be terminated by an end-of-file, but also because the longhand expansion of #lang depends on the name of the enclosing file.
6.2.1 The module Form
The longhand form of a module declaration, which works in a REPL as well as a file, is
(module name-id initial-module-path decl ...)
where the name-id is a name for the module, initial-module-path is an initial import, and each decl is an import, export, definition, or expression. In the case of a file, name-id must match the name of the containing file, minus its directory path or file extension.
The initial-module-path is needed because even the require form must be imported for further use in the module body. In other words, the initial-module-path import bootstraps the syntax available in the body. The most commonly used initial-module-path is racket, which supplies most of the bindings described in this guide, including require, define, and provide. Another commonly used initial-module-path is racket/base, which provides less functionality, but still much of the most commonly needed functions and syntax.
For example, the "cake.rkt" example of the previous section could be written as
(module cake racket (provide print-cake) (define (print-cake n) (printf " ~a \n" (make-string n #\.)) (printf " .-~a-.\n" (make-string n #\|)) (printf " | ~a |\n" (make-string n #\space)) (printf "---~a---\n" (make-string n #\-))))
Declaring a module does not immediately evaluate the body definitions and expressions of the module. The module must be explicitly required at the top level to trigger evaluation. After evaluation is triggered once, later requires do not re-evaluate the module body.
The body of a #lang shorthand has no specific syntax, because the syntax is determined by the language name that follows #lang.
In the case of #lang racket, the syntax is
which reads the same as
where name is derived from the name of the file that contains the #lang form.
The #lang racket/base form has the same syntax as #lang racket, except that the longhand expansion uses racket/base instead of racket. The #lang honu form, in contrast, has a completely different syntax that doesn’t even look like Racket, and which we do not attempt to describe in this guide.
Unless otherwise specified, a module that is documented as a “language” using the #lang notation will expand to module in the same way as #lang racket. The documented language name can be used directly with module or require, too.