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 scheme       (provide color)       (define color "blue"))

> (module n scheme       (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 "scheme")       (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:

  (require (except-in (prefix m: 'm) m:ghost))