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gen_js_api: ppx mode

While the primary mode of operation for gen_js_api is to generate an .ml file from an annotated .mli file, it is also possible to use it as a ppx preprocessor on an .ml file directly to insert local JS bindings.

The -ppx command-line option must be the first argument passed to gen_js_api to enable the ppx mode:

  $ ocamlc -c -ppx "gen_js_api -ppx" my_prog.ml

or with findlib:

  $ ocamlfind ocamlc -c -package gen_js_api my_prog.ml

Note: the ppx currently does nothing on .mli files.

Several forms are supported:

  • [%js: <signature>] extension as a module expression. Examples:

      include [%js: <signature>]

  module M = [%js: <signature>]

    The signature is processed as if it were found in an .mli file, and the resulting structure is inserted in place of the [%js: ...] extension. See this page for a list of declarations supported in such interfaces.

  • [@@js] attributes on type declarations.

    Example:

      type t = { x : int; y : int } [@@js]

    This generates the corresponding *_of_js and *_to_js functions. In case of a multi-type declaration, each type must be annotated with [@@js] (if needed). See this page for a description of support forms of type declarations.

  • [%js.to: ty] and [%js.of: ty] extensions on expressions.

    Example:

      let x : Ojs.t = [%js.of: int list]  [ 10; 20; 30 ]

    This form generates the mapping function associated to a JS-able type. See this page for a description of JS-able type.