5.4 Defining Bindings

(require ffi/unsafe/define)

(define-id id type-expr   bind-option ...)

bind-option   =   #:c-id c-id     |   #:wrap wrap-expr     |   #:make-fail make-fail-expr     |   #:fail fail-expr

A define-id form binds id by extracting a binding with the name c-id from the library produced by ffi-lib-expr, where c-id defaults to id. The other options support further wrapping and configuration:

• Before the extracted result is bound as id, it is passed to the result of wrap-expr, which defaults to values. Expressions such as (allocator delete) or (deallocator) are useful as wrap-exprs.

• The #:make-fail and #:fail options are mutually exclusive; if make-fail-expr is provided, it is applied to 'id to obtain the last argument to get-ffi-obj; if fail-expr is provided, it is supplied directly as the last argument to get-ffi-obj. The make-not-available function is useful as make-fail-expr to cause a use of id to report an error when it is applied if c-id was not found in the foreign library.

If provide-id is provided to define-ffi-definer, then define-id also provides its binding using provide-id. The provide-protected form is usually a good choice for provide-id.

If core-define-id is provided to define-ffi-definer, then code-define-id is used in place of define in the expansion of define-id for each binding.

If default-make-fail-expr is provided to define-ffi-definer, it serves as the default #:make-fail value for define-id.

For example,

  (define-ffi-definer define-gtk gtk-lib)

binds define-gtk to extract FFI bindings from gtk-lib, so that gtk_rc_parse could be bound as

  (define-gtk gtk_rc_parse (_fun _path -> _void))

If gtk_rc_parse is not found, then define-gtk reports an error immediately. If define-gtk is instead defined with

(define-ffi-definer define-gtk gtk-lib

#:default-make-fail make-not-available)

then if gtk_rc_parse is not found in gtk-lib, an error is reported only when gtk_rc_parse is called.