Interfaces are a new type of Scheme component
distinct from programs, libraries, and REPL scripts.
They consist solely of a define-interface declaration.
An interface specifies identifiers
that can be exported from libraries as a group.
Identifiers provide a higher-level representation of the export-import graph of a program or REPL script and its libraries. If a component specifies the interfaces that it requires, then any imported libraries that provide those interfaces can be replaced as long as the new imports also provide those interfaces.
If an interface is named but no definition is available, or an interface is required but not implemented, an expand-time error is signaled.
If an interfaces is required by a component but not implemented anywhere in the import-export graph, an expand-time error is signaled.
This SRFI recommends but does not require
that a source file containing an interface definition
be given the extension .sif.
This extension is used in a number of other contexts,
but those uses would be unlikely to collide with this one.
(define-interface interface-name identifier-or-interface-name ...)
Defines an interface, a collection of identifiers. Interface names are syntactically like library names, but belong to a separate namespace.
It is an error for a define-identifier to appear
within a library, program, or REPL script; it is a fourth
type of top-level component.
The identifiers may be mentioned individually or by mentioning the name of another interface, which implicitly mentions all the identifiers and interfaces mentioned directly or indirectly in that interface. An expand-time error is signaled if an interface is mentioned but no definition is available.
It is not an error to mention an identifier both explicitly and through an interface, or through two or more different interfaces. Loops in an interface graph are resolved by flattening them, so that:
(define-interface (a) foo (b))
(define-interface (b) bar (a))
simply causes the interfaces (a) and (b) to mention both foo and bar.
(implement-interface interface-name ...)
If any of the identifiers mentioned directly or indirectly are not exported by the library, an expand-time error is signaled. Otherwise, no action is taken.
It is not an error for an identifier to be mentioned directly or indirectly in more than one of the named interfaces.
(require-interface interface-name ...)
If the identifiers mentioned directly or indirectly in the named interfaces are not imported into it, an expand-time error is signaled. Otherwise, no action is taken.
It is not an error for an identifier to be mentioned directly or indirectly in more than one of the named interfaces.
This SRFI requires an additional type of feature requirement
in cond-expand declarations and expressions, the
interface feature. It is exactly equivalent
syntactically and semantically
to the library feature, except that it checks for the
existence of an interface rather than a library.
Chicken provides define-interface and export-interface
with different syntax: see
Chicken modules
for details.