An interface can be thought of as the type of a structure. In its basic form it is just a list of variable names, written (export name ). However, in place of a name one may write (name type), indicating the type of name's binding. Currently the type field is ignored, except that exported macros must be indicated with type :syntax.
Interfaces may be either anonymous, as in the example in the introduction, or they may be given names by a define-interface form, for example =0pt=0pt=0pt =0pt (define-interface foo-interface (export a c cons)) (define-structure foo foo-interface )
In principle, interfaces needn't ever be named. If an interface had to be given at the point of a structure's use as well as at the point of its definition, it would be important to name interfaces in order to avoid having to write them out twice, with risk of mismatch should the interface ever change. But they don't.
Still, there are several reasons to use define-interface:
The compound-interface operator forms an interface that is the union of two or more component interfaces. For example, =0pt=0pt=0pt =0pt (define-interface bar-interface (compound-interface foo-interface (export mumble)))
defines bar-interface to be foo-interface with the name mumble added.