6.1 Creating Units

Each import or export sig-spec ultimately refers to a sig-id, which is an identifier that is bound to a signature by define-signature.

In a specific import or export position, the set of identifiers bound or required by a particular sig-id can be adjusted in a few ways:

• (prefix id sig-spec) as an import binds the same as sig-spec, except that each binding is prefixed with id. As an export, this form causes definitions using the id prefix to satisfy the exports required by sig-spec.

• (rename sig-spec (id id) ...) as an import binds the same as sig-spec, except that the first id is used for the binding instead of the second id (where sig-spec by itself must imply a binding that is bound-identifier=? to second id). As an export, this form causes a definition for the first id to satisfy the export named by the second id in sig-spec.

• (only sig-spec id ...) as an import binds the same as sig-spec, but restricted to just the listed ids (where sig-spec by itself must imply a binding that is bound-identifier=? to each id). This form is not allowed for an export.

• (except sig-spec id ...) as an import binds the same as sig-spec, but excluding all listed ids (where sig-spec by itself must imply a binding that is bound-identifier=? to each id). This form is not allowed for an export.

As suggested by the grammar, these adjustments to a signature can be nested arbitrarily.

A unit’s declared imports are matched with actual supplied imports by signature. That is, the order in which imports are suppplied to a unit when linking is irrelevant; all that matters is the signature implemented by each supplied import. One actual import must be provided for each declared import. Similarly, when a unit implements multiple signatures, the order of the export signatures does not matter.

To support multiple imports or exports for the same signature, an import or export can be tagged using the form (tag id sig-spec). When an import declaration of a unit is tagged, then one actual import must be given the same tag (with the same signature) when the unit is linked. Similarly, when an export declaration is tagged for a unit, then references to that particular export must explicitly use the tag.

A unit is prohibited syntactically from importing two signatures that are not distinct, unless they have different tags; two signatures are distinct only if they share no ancestor through extends. The same syntactic constraint applies to exported signatures. In addition, a unit is prohibited syntactically from importing the same identifier twice (after renaming and other transformations on a sig-spec), exporting the same identifier twice (again, after renaming), or exporting an identifier that is imported.

When units are linked, the bodies of the linked units are executed in an order that is specified at the linking site. An optional (init-depend tagged-sig-id ...) declaration constrains the allowed orders of linking by specifying that the current unit must be initialized after the unit that supplies the corresponding import. Each tagged-sig-id in an init-depend declaration must have a corresponding import in the import clause.

• Each id in a signature declaration means that a unit implementing the signature must supply a variable definition for the id. That is, id is available for use in units importing the signature, and id must be defined by units exporting the signature.

• Each define-syntaxes form in a signature declaration introduces a macro that is available for use in any unit that imports the signature. Free variables in the definition’s expr refer to other identifiers in the signature first, or the context of the define-signature form if the signature does not include the identifier.

• Each define-values form in a signature declaration introduces code that effectively prefixes every unit that imports the signature. Free variables in the definition’s expr are treated the same as for define-syntaxes.

• Each define-values-for-export form in a signature declaration introduces code that effectively suffixes every unit that exports the signature. Free variables in the definition’s expr are treated the same as for define-syntaxes.

• Each contracted form in a signature declaration means that a unit exporting the signature must supply a variable definition for each id in that form. If the signature is imported, then uses of id inside the unit are protected by the appropriate contracts using the unit as the negative blame. If the signature is exported, then the exported values are protected by the appropriate contracts which use the unit as the positive blame, but internal uses of the exported identifiers are not protected. Variables in the contract expressions are treated the same as for define-syntaxes.

• Each (open sig-spec) adds to the signature everything specified by sig-spec.

• Each (struct id (field ...) struct-option ...) adds all of the identifiers that would be bound by (struct id (field ...) field-option ...), where the extra option #:omit-constructor omits the constructor identifier.

• Each (sig-form-id . datum) extends the signature in a way that is defined by sig-form-id, which must be bound by define-signature-form. One such binding is for struct/ctc.

When a define-signature form includes an extends clause, then the define signature automatically includes everything in the extended signature. Furthermore, any implementation of the new signature can be used as an implementation of the extended signature.