6.4 Imports: require
The require form imports from another module. A require form can appear within a module, in which case it introduces bindings from the specified module into importing module. A require form can also appear at the top level, in which case it both imports bindings and instantiates the specified module; that is, it evaluates the body definitions and expressions of the specified module, if they have not been evaluated already.
A single require can specify multiple imports at once:
(require require-spec ...)
Specifying multiple require-specs in a single require is essentially the same as using multiple requires, each with a single require-spec. The difference is minor, and confined to the top-level: a single require can import a given identifier at most once, whereas a separate require can replace the bindings of a previous require (both only at the top level, outside of a module).
The allowed shape of a require-spec is defined recursively:
module-path In its simplest form, a require-spec is a module-path (as defined in the previous section, Module Paths). In this case, the bindings introduced by require are determined by provide declarations within each module referenced by each module-path.
Examples:
> (module m racket (provide color) (define color "blue"))
> (module n racket (provide size) (define size 17)) > (require 'm 'n) > (list color size) '("blue" 17)
(only-in require-spec id-maybe-renamed ...)
id-maybe-renamed = id | [orig-id bind-id] An only-in form limits the set of bindings that would be introduced by a base require-spec. Also, only-in optionally renames each binding that is preserved: in a [orig-id bind-id] form, the orig-id refers to a binding implied by require-spec, and bind-id is the name that will be bound in the importing context instead of bind-id.
Examples:
> (module m (lib "racket") (provide tastes-great? less-filling?) (define tastes-great? #t) (define less-filling? #t)) > (require (only-in 'm tastes-great?)) > tastes-great? #t
> less-filling? reference to undefined identifier: less-filling?
> (require (only-in 'm [less-filling? lite?])) > lite? #t
(except-in require-spec id ...) This form is the complement of only: it excludes specific bindings from the set specified by require-spec.
(rename-in require-spec [orig-id bind-id] ...) This form supports renaming like only-in, but leaving alone identifiers from require-spec that are not mentioned as an orig-id.
(prefix-in prefix-id require-spec) This is a shorthand for renaming, where prefix-id is added to the front of each identifier specified by require-spec.
The only-in, except-in, rename-in, and prefix-in forms can be nested to implement more complex manipulations of imported bindings. For example,
(require (prefix-in m: (except-in 'm ghost)))
imports all bindings that m exports, except for the ghost binding, and with local names that are prefixed with m:.
Equivalently, the prefix-in could be applied before except-in, as long as the omission with except-in is specified using the m: prefix: