2.23 Limiting Scope: define-package, open-package, ...

The define-package form is similar to module, except that it can appear in any definition context. The forms within a define-package form can be definitions or expressions; definitions are not visible outside the define-package form, but exports determines a subset of the bindings that can be made visible outside the package using the definition form (open-package package-id).

The (id ...) and #:only (id ...) exports forms are equivalent: exactly the listed ids are exported. The #:all-defined form exports all definitions from the package body, and #:all-defined-except (id ...) exports all definitions except the listed ids.

All of the usual definition forms work within a define-package body, and such definitions are visible to all expressions within the body (and, in particular, the definitions can refer to each other). However, define-package handles define*, define*-syntax, define*-values, define*-syntaxes, and open*-package specially: the bindings introduced by those forms within a define-package body are visible only to forms that appear later in the body, and they can shadow any binding from preceding forms (even if the preceding binding did not use one of the special * definition forms). If an exported identifier is defined multiple times, the last definition is the exported one.

Examples:

> (define-package presents (doll)       (define doll "Molly Coddle")       (define robot "Destructo"))

> doll

reference to undefined identifier: doll

> robot

reference to undefined identifier: robot

> (open-package presents)

> doll

"Molly Coddle"

> robot

reference to undefined identifier: robot

> (define-package big-russian-doll (middle-russian-doll)       (define-package middle-russian-doll (little-russian-doll)         (define little-russian-doll "Anastasia")))

> (open-package big-russian-doll)

> (open-package middle-russian-doll)

> little-russian-doll

"Anastasia"

A package-begin form can be used as an expression, but if it is used in a context where definitions are allowed, then the definitions are essentially spliced into the enclosing context (though the defined bindings remain hidden outside the package-begin).

Examples:

> (package-begin       (define secret "mimi")       (list secret))

'("mimi")

> secret

reference to undefined identifier: secret

Examples:

> (define-package mail (cookies)       (define* cookies (list 'sugar))       (define* cookies (cons 'chocolate-chip cookies)))

> (open-package mail)

> cookies

'(chocolate-chip sugar)

> (define-syntax-rule (define-seven id) (define id 7))

> (define-syntax-rule (define*-seven id)       (begin         (define-package p (id) (define-seven id))         (open*-package p)))

> (package-begin       (define vii 8)       (define*-seven vii)       vii)

7

The package? predicate returns #t if v is a package value as obtained by syntax-local-value on an identifier that is bound to a package.

Given such an identifier, the package-exported-identifiers function returns a list of identifiers that correspond to the bindings that would be introduced by opening the package in the lexical context being expanded. The package-original-identifiers function returns a parallel list of identifiers for existing bindings of package’s exports.