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The functorial interface

TyXML provides a functorial interface to implement HTML and SVG on top of any XML implementation. This is used heavily by Eliom to implement the F and D modules, providing respectively a functional and a DOM version of the HTML implementation.

The Make functors

These interfaces are available in the modules Html_f and Svg_f. They provide a functor Make which takes a concrete implementation of Xml following Xml_sigs.T. A default implementation is provided by the Tyxml.Xml module. The functor Html_f.Make also needs an Svg implementation that one can obtain, for example, with the functor Svg_f.Make. The Xml always needs to provide a module W of type Xml_wrap.T, along with types

type 'a wrap = 'a W.t


type 'a list_wrap = 'a W.tlist

. The purpose of the Wrap module is explained in the next section.

Wrapping up the nodes

The module Xml_sigs.T.W allows to wrap Xml elements in a monad 'a t. A good example of application is the R modules with reactive nodes in Eliom. Here is the simplified signature of the div element:

R.div : 'a elt list t -> div elt

t will wrap the input of every Xml node and be integrated in the resulting node.

The W module needs to implement operations over the type of nodes, and provides a special type for lists of nodes. The W module additionally provides a type (-'a, 'b) ft for (wrapped) functions, whose purpose is explained in the next section. In most cases, it is sufficient to define:

(-'a, 'b) ft = 'a -> 'b

An identity wrapper, Xml_wrap.NoWrap, is provided. Xml_wrap.NoWrap can be used to apply the functor without wrapping the elements.

The Make_with_wrapped_functions functors

The Make_with_wrapped_functions functors (available in the modules Html_f and Svg_f) differ from the Make functors by requiring an additional argument C (of type Html_f.Wrapped_functions and Svg_f.Wrapped_functions respectively).

The C functor argument defines a type (-'a, 'b) ft, and a collection of ft values. For applying the functor, the type constraint

('a, 'b) Xml.W.ft = ('a, 'b) C.ft

needs to be satisfied. The ft values are wrapped functions that the functor uses internally to operate on wrapped elements.

The motivation of providing the Make_with_wrapped_functions is as follows. Certain monads 'a t can only be operated upon by wrapped functions, and not by plain OCaml functions. The wrapped functions cannot be produced internally by TyXML, and thus have to be provided to the functor. Our intended application is with Eliom shared (i.e., client-server) signals. Such signals can only be operated upon with Eliom shared functions.

The Make functors simply apply the Make_with_wrapped_functions functors with TyXML-provided Wrapped_functions modules. The latter modules do not wrap the functions, i.e., they satisfy:

(-'a, 'b) ft = 'a -> 'b


Exporting the correct signature

In order to help export the correct signature after a functor application, two signature functors are provided: Svg_sigs.Make and Html_sigs.Make.

As an example of use, let us look at the module Tyxml.Svg. Here is the definition of the module:

module M = Svg_f.Make(Tyxml_xml)

In this case, the declaration in the interface file should look like this:

module M : Svg_sigs.Make(Tyxml_xml).T

The signature functor Svg_sigs.Make contains only a signature T, which is equal to Svg_sigs.T, but exports various equalities with the module Xml.

You should always use a signature functor to give the type of a module produced by a functor application. It will ensure that exactly the right type equalities are exported and will naturally keep track of changes in TyXML.

There are some important notes about theses signature functors:

  • module M : Svg_sigs.Make(Tyxml_xml).T doesn't mean that M.Xml is a submodule of Xml. It only means that the types uri, event_handler, mouse_event_handler, keyboard_event_handler, touch_event_handler, attrib and elt are the same in both modules. This is useful when not exporting the exact module that was used in the functor, but another (smaller and simpler) module. This is the case in Tyxml_js.R, for example.
  • Html_f and Svg_f functors export two additional equalities, +'a elt = Xml.elt and +'a attrib = Xml.attrib. These equalities should never be exported in a public interface. Exporting them would break HTML typing by allowing to build invalid HTML trees. These equalities are useful internally, for example in Eliom they are used to make F.elt, D.elt and R.elt equals.