Generating HTML pages ¶
Ocsigen provides several ways to generate and type HTML pages.
- The default technique to produce HTML pages using Eliom are the Eliom_content.Html.F, Eliom_content.Html.D and Eliom_content.Html.R modules. It is the only one supported for client-server Eliom programs. This module provides a typing based on OCaml's polymorphic variants, which ensures at compile time, that the pages you will generate will respect the recommendations of the W3C (or be very close).
- It is also possible to use a syntax extension to write your pages with the usual HTML syntax. This solution is also typed with polymorphic variants and is compatible with the previous one.
- You can also choose to generate untyped html as text.
The types in OCaml closest to XML types are polymorphic variants. Ocsigen uses them to provide a module with very good HTML typing. The full documentation is available in the TyXML documentation.
Table of contents
Generating HTML for Eliom applications
The TyXML library vs. the DOM API
On client side there are two kinds of HTML representations: one is based on the TyXML library and the other one is the browser DOM tree accessible through Js_of_ocaml modules Js_of_ocaml.Dom and Js_of_ocaml.Dom_html. The TyXML representation is a OCaml immutable typed tree. The DOM tree is mutable structure manipulated using the browser API which permit the modification of the displayed page. In the DOM represention adding a node as a child to an other node removes it from its previous ancessor.
Since those representation does not behave at all the same way, they are not used for the same thing.
- It is far easier and safer to describe content using TyXML, but it is not possible to add a TyXML element to the page without explicit conversion to the DOM representation.
- The TyXML representation has the same interface on client and server side. This allows share code between server and client.
- Dom manipulation is heavy: to build some part of a tree, one needs to create each node separately then append them to their parents.
For example, here is a div element built with TyXML and then converted to the DOM representation using the module
open%client Eliom_content.Html.D
let%client n = div ~a:[a_id "some div id"]
[ txt "some text";
br ();
txt "some other text"; ]
let%client b = Eliom_content.Html.To_dom.of_div nAnd here the same built using the DOM API:
open%client Dom
open%client Dom_html
let%client d =
let d = createDiv document in
let t1 = document##createTextNode(Js.string "some text") in
let t2 = document##createTextNode(Js.string "some other text") in
let b = createB document in
appendChild d t1;
appendChild d b;
appendChild d t2;
d##.id := (Js.string "some div id");
dTo ease the DOM manipulation on the client, the usual DOM manipulation function are also available on TyXML elements. See section the next section for HTML element manipulation, by value and by reference.
HTML element manipulation, by value and by reference ¶
There are four modules to create typed HTML: Eliom_content.Html.F, Eliom_content.Html.D, Eliom_content.Html.C and Eliom_content.Html.R. The last one is for reactive elements and is addressed in another section .
It is possible to mix the four kinds of nodes in the same page.
Elements built with Html.F are sent by value, while elements built with Html.D are sent to the client by reference. Eliom adds an identifier as attribute of D elements to make it possible to find them back in the page from client side.
Sending elements by reference allows easy manipulation of elements included in the initial html document from event handlers, as the input element in the following example.
let%server main_service =
My_app.register_service
~path:(Eliom_service.Path [""])
~meth:(Eliom_service.Get Eliom_parameter.unit)
(fun () () ->
let open Eliom_content.Html.D in
let input = input ~a:[a_input_type `Text] () in
let onclick_handler = [%client (fun _ ->
let v =
Js.to_string
(Eliom_content.Html.To_dom.of_input ~%input)##.value
in
Dom_html.window##alert(Js.string ("Input value :" ^ v)))
] in
let button =
button ~a:[a_onclick onclick_handler] [txt "Read value"]
in
Lwt.return
(html
(head (title (txt "Test")) [])
(body [input; button])))In this example, if the input button would have been incorrectly sent by value, two different input fields would have been created: one displayed in the document and one referenced from the event handler. The latter will always contains an empty value.
There are still two situations where sending elements by value is still required:
- one want to have multiple occurences of the same elements in the
document. Indeed, elements sent by reference follow the DOM
semantics where an element have only one instance in current
document. For example, the following list will contains a single
element:
let li = li [txt "Shared item"] in ul [li; li; li;] .
- one have a large page with a lot elements. Handling elements by references add a small overhead while loading the page, around 50ms per 1000 elements on a not so fast computer.
In any case, it is possible to mix elements sent by references and elements sent by value in the same document.
By default, a reference on an element is only valid in the current HTTP request: hence, sending an element built with Html.D in two different pages will produce two distinct nodes. If you want to define a element reference that is preserved accross the different page of an application, you must explicitely name this element with the function Eliom_content.Html.Id.create_named_elt, that takes as parameters an element identifier and a non named element. Element identifiers are created with the function Eliom_content.Html.Id.new_elt_id. See also section Global elements of an application.
The module Eliom_content.Html.Manip allows using the classical DOM manipulation functions (e.g. appendChild, addEventlistener, ...) directly on the identifier of an HTML elements.
Reactive DOM ¶
Eliom_content.Html.R allows one to insert time-varying values into the DOM tree. It relies on React's signal 'a React.signal. More information about react can be found on the homepage. The react nodes also use ReactiveData, which allows to manipulate lists of nodes in a reactive way.
When dealing with dynamic content, one usally ends up with a lot of imperative DOM manipulations: replacing, appending, removing DOM elements, updating attributes, etc. Html.R hides most of those imperative DOM operations. Every time a signal changes, the corresponding DOM tree updates itself.
Usage on client side
To insert reactive DOM elements, just use module Html.R instead of Html.D or Html.F for these elements. Html.R makes also possible to define reactive attributes.
Use function Html.R.node : 'a elt React.signal -> 'a elt to insert a reactive node in a page.
Example
open%shared Eliom_lib
open%shared Eliom_content
open%shared Html
open%shared Fmodule%server Reactivenodes_app =
Eliom_registration.App (
struct
let application_name = "reactivenodes"
let global_data_path = None
end)open%client Eliom_content.Html
let%client split s =
let len = String.length s in
let rec aux acc = function
| 0 -> acc
| n -> aux (s.[n - 1] :: acc) (pred n)
in aux [] len
let%client value_signal, set_value = React.S.create "initial"
let%client value_len = React.S.map String.length value_signal
let%client content_signal : Html_types.div_content_fun elt React.signal =
React.S.map
(fun value ->
let l = split value in
F.div (
List.map (fun c -> F.p [F.txt (Printf.sprintf "%c" c)]) l
))
value_signal
let%client make_color len =
let d = (len * 10) mod 255 in
Printf.sprintf "color: rgb(%d,%d,%d)" d d d
let%client make_client_nodes () =
[
D.p [R.txt value_signal];
D.p ~a:[ R.a_style (React.S.map make_color value_len)]
[R.txt value_signal];
R.node content_signal
]let%server make_input () =
let inp = D.Raw.input ~a:[a_input_type `Text] () in
let _ = [%client
(Lwt_js_events.(async (fun () ->
let inp = To_dom.of_input ~%inp in
keyups inp (fun _ _ ->
let s = Js.to_string (inp##.value) in
set_value s;
Lwt.return ())))
: unit)
] in
inp
let%server main_service =
Eliom_service.create
~path:(Eliom_service.Path [])
~meth:(Eliom_service.Get Eliom_parameter.unit)
()
let%server () =
Reactivenodes_app.register
~service:main_service
(fun () () ->
let inp = make_input () in
let cldiv = C.node [%client D.div (make_client_nodes ())] in
Lwt.return
(Eliom_tools.F.html
~title:"reactivenodes"
~css:[["css"; "reactivenodes.css"]]
(body [F.h1 [txt "Reactive DOM"];
inp;
F.h2 [txt "Client side reactive nodes:"];
cldiv;
])
))Dom & Client-values ¶
Eliom_content.Html.C allows one to insert client-side content into server-side HTML-trees. This makes possible, for example, to insert reactive nodes in a server-side generated page.
Example
open%shared Eliom_lib
open%shared Eliom_content
open%shared Html
open%shared Fmodule%server Testnodes_app =
Eliom_registration.App (
struct
let application_name = "testnodes"
let global_data_path = None
end)
let%server main_service =
Eliom_service.create
~path:(Eliom_service.Path [])
~meth:(Eliom_service.Get Eliom_parameter.unit)
()open%client Eliom_content.Html
let%client
(value_signal : string React.signal), set_value = React.S.create "initial"let%server client_reactive_attrib () = [%client
R.a_style value_signal
]
let%server client_reactive_title () = [%client
F.h1 [txt "I'm a client node"]
]
let%server () =
Testnodes_app.register
~service:main_service
(fun () () ->
Lwt.return
(Eliom_tools.F.html
~title:"testnodes"
~css:[["css"; "testnodes.css"]]
(body [
F.div ~a:[C.attr (client_reactive_attrib ())] [
C.node (client_reactive_title ())
]
])))Module C is also available on client-side, to make it possible to use it in shared sections.
Global elements of an application ¶
Sometimes you may want to modify the content of an HTML element and to keep the element and its modified content when changing page. For example a div element which contains a chat box or a music player should be preserved while browsing across the different pages of your site. For this purpose, Eliom provides a notion of global element. Such elements are instantied only once for an application and that unique instance is used in every page that references the element.
To create a global element, use function Eliom_content.Html.Id.create_global_elt.
val create_global_elt: 'a elt -> 'a eltIn the following example, the content of global_list will be preserved when you click on the "reload page" link.
open%shared Eliom_content.Html.Dmodule%server My_app =
Eliom_registration.App (
struct
let application_name = "myo"
let global_data_path = None
end)
let%server global_list = Eliom_content.Html.Id.create_global_elt (ul [])
let%server cpt = ref 0
let%server main_service =
Eliom_service.create
~path:(Eliom_service.Path [""])
~meth:(Eliom_service.Get Eliom_parameter.unit)
()
let%server reload_link =
a ~service:main_service [txt "reload page"] ()
let%server _ =
My_app.register ~service:main_service
(fun () () ->
let page_number = incr cpt; string_of_int !cpt in
let append_item = [%client
let item_text = "item inserted in page #" ^ ~%page_number in
let item = Eliom_client.Html.of_li (li [txt item_text]) in
Dom.appendChild (Eliom_client.Html.of_ul ~%global_list) item
]
in
let append_link =
a ~a:[a_onclick append_item] [txt "append item"]
in
Lwt.return
(html
(head (title (txt "Test")) [])
(body [h1 [txt ("Page #" ^ page_number)];
p [append_link];
p [reload_link];
global_list]) ) )Another use of global element is for external javascript that should be included in every page but must be executed only once in an application. In the following code snippet, the alert "global script" is displayed only once, while the alert "non global script" is display every time you click on the "reload page" link.
open%server Eliom_content.Html.D
let%server global_script =
Eliom_content.Html.Id.create_global_elt
(script (cdata_script "alert(\"global script\")"))
let%server simple_script =
script (cdata_script "alert(\"non global script\")")
let%server main_service =
Eliom_service.create
~path:(Eliom_service.Path [])
~meth:(Eliom_service.Get Eliom_parameter.unit)
()
let%server reload_link =
a ~service:main_service [txt "reload page"] ()
let%server _ =
My_app.register ~service:main_service
(fun () () ->
Lwt.return
(html
(head
(title (txt "Global script example"))
[global_script; simple_script])
(body [p [reload_link]])))HTML syntax extension
It is possible to use regular HTML syntax using Tyxml's PPX syntax extension.
See Tyxml's manual.
Text HTML ¶
The last possibility is to use untyped HTML. Just build strings containing your pages. Here is an example:
let%server hello =
Eliom_registration.Html_text.create
~path:(Eliom_service.Path ["hello"])
~meth:(Eliom_service.Get Eliom_parameter.unit)
(fun () () -> Lwt.return "<html>Hello</html>")Writing HTML as text makes applications much more difficult to maintain. We do not recommend this.
Custom data for HTML ¶
Eliom provides a type-safe interface for create new attributes of the form data-*, using Eliom_content.Html.Custom_data.
Creation
Custom data may be created either from string-conversation functions by Eliom_content.Html.Custom_data.create
open%server Eliom_content
let%server my_int_data =
Html.Custom_data.create ~name:"my_int"
~of_string:int_of_string
~to_string:string_of_int ()or by a Json-deriving type Eliom_content.Html.Custom_data.create_json
type%shared coord = { x : int; y : int; } [@@deriving json]
let%shared coord_data =
Html.Custom_data.create_json ~name:"coord" [%derive.json: coord]Injecting
Custom data can be injected into HTML-trees of type Eliom_content.Html.elt by the function Eliom_content.Html.Custom_data.attrib:
div ~a:[Html.Custom_data.attrib coord_data {x = 100; y = 200}] []Reading/writing the DOM
On the client side, custom data can be read from and written to JavaScript DOM elements of type Js_of_ocaml.Dom_html.element.
Custom data can be read from a DOM-element with the function Eliom_content.Html.Custom_data.get_dom. If no respective custom data attribute can be found in the element
- the default value from creating the custom data is returned, if any, or
- an exception Not_found is raised, otherwise.
The custom data of a DOM-element can be set with the function Eliom_content.Html.Custom_data.set_dom.
[%client
fun (div : Dom_html.element Js.t) ->
let i = Html.Custom_data.get_dom div coord_data in
debug "{x=%d; y=%d}" i.x i.y;
Html.Custom_data.set_dom div coord_data { x = i.x + 1; y = i.y - 1 }
]Default value
If a custom data is created with the optional argument default, calls to Eliom_content.Html.Custom_data.get_dom return that instead of throwing an exception [Not_found].
let coord_data' =
Html.Custom_data.create_json
~name:"coord" default:{x=0;y=0;} [%derive.json: my_data]