Haskell in web browser/Haskell web toolkit
Haskell Web Toolkit (further referred to as HsWTK) is a thin layer built on top of DOM interfaces. It provides program interfaces to compose static layout of a web application page, and to hook up visual elements of an application to event handlers and XML HTTP communication means. HsWTK hides the low-level DOM APIs where possible; however their knowledge may be necessary to develop certain types of visual components and event handlers.
To build a web-based GUI, HsWTK defines the following type:
type Widget = THTMLDocument -> THTMLElement -> Bool
That is, Widget is a function, or, to be a more precise, an action (as evaluation of this function does assume side effects). This action's first argument, of type THTMLDocument, refers to the HTML document containing the GUI elements. The action's second argument, of type THTMLElement, refers to a parent HTML element. This makes perfect sense from the DOM standpoint, as in order to create a visible element on a Web page, it is at least necessary to create an element by calling
Document.createElement method (which needs a Document), and next to insert the newly created element into some (parent) node by calling
HTML elements as widgets
So, inside the Widget action, some work may be done to create a HTML element, and make it a child of some other Widget. This is generalized by HsWTK as Element Creation Function, and defined as
type ECRF n = (THTMLDocument -> CPS Bool n)
Now, if we refer to Maker functions defined for HTML-tagged Elements we may see some similarity:
mkDiv :: CHTMLDocument a => a -> CPS c THTMLDivElement
If we substitute n in ECRF definition with THTMLDivElement, and remember that THTMLDocument is an instance of CHTMLDocument, we get a perfect match. From this, it may be concluded that Maker functions may serve as Element creation functions.
The simpliest form of Widget is passive widget. Passive widgets only display themselves as part of Web GUI, but are not capable of nesting other widgets. A good example of such passive widget is text label or non-clickable image.
HsWTK defines a function
passive which given an Element Creation Function, returns a Widget:
passive :: CNode n => ECRF n -> Widget
Thus, given a Maker function (e. g. mkImg), applying passive to it creates an image which will appear in the proper place of a Web page.
For a text element itself, there is a Maker function
mkText which produces a text node to be inserted into a < DIV > or < SPAN > or any other element with closing tag. mkText is not a pure Maker function though; due to its type signature,
flip has to be applied to it.
One important passive widget not based on a HTML element is
nowidget. It may be used as a placeholder for any passive widget, and does not produce any kind of effects.
HTML elements with closing tags contain other elements in between. To reflect this, HsWTK defines another function,
container. This function, applied to a Maker function, produces a widget capable of nesting other widgets.
For the purpose of sequencing widgets within a container, or nesting widgets in containers, HsWTK defines combinators
++|. Please refer to the appropriate section of the Graphics.UI.HsWTK module documentation.
+++ combinator sequences two widgets inside a container. Widgets (
a +++ b) appear second (b) at the right of the first (a). Result of such composition is also a widget, and it may be sequenced with other widgets.
(a +++ b) +++ c is same as
a +++ b +++ c.
<< combinator nests a widget (or composition of widgets) in a container.
So, the following fragment of code creates a widget consisting of a < DIV > with a text inside:
mkTextFl = flip mkText container mkDiv << passive (mkTextFl "Hello World")
To simplity the code, HsWTK defines these two functions:
textPwhich is equivalent to
passive (flip mkText txt)
|<<which is defined as
c |<< d = container c << d, that is the word
containermay be omitted.
The toplevel widget (one that is not nested in any other widget) has to be inserted into the HTML document body. This is acieved by applying the function
docBodyC to the toplevel widget. This application often becomes the
main function of a Web application. Please note that document body is not a Widget.
Code example - Hello Web
At this point, we are able to code our first HsWTK example. By tradition, this is a Hello ... program. Paste the code below into Yhc Web Service New Entry Form and press the Submit button (don't forget to fill out the Author and Title field).
After the compilation is finished, load the generated Web page. Haskell says Hello to the new environment it just has started to explore.
The example shows all the facilities of HsWTK we recently discussed:
- widgets sequencing (+++ combines two text elements one after another)
- creation of a HTML Element based widget (mkSpan)
- nesting composition of widgets in a container (|<<)
- inserting the toplevel widget into the document body (docBodyC)
Decorators are not Widgets: they are tools to change properties of a widget they are applied to, at the moment of that widget creation.
Earlier we discussed setters defined for HTML Elements' writable attributes. HsWTK defines a function
decorate which when applied to a setter, produces a decorator. An example of such a decorator is
withTitle defined as shown below:
withTitle :: (CHTMLElement b) => ((a -> ((b -> (CPS c b)) -> d)) -> (String -> (a -> d))) withTitle = decorate set'title
set'title function is a setter for the title attribute of HTMLElement.
Decorators are binary functions taking the Widget to decorate as the first argument, and the value used to decorate as the second. In our case, this looks like this:
... mkDiv `withTitle` "This div has a title" |<< textP "titled div"
A special decorator,
withStyle, represents a more complicated case. It does not alter attributes/properties of a widget element it is used with; instead it affects inline style properties of the widget element.
While arttribute-based decorators take simple values (strings, less often numbers) to decorate widget elements, withStyle takes a list of name-value pairs. Both names and values are strings, and should conform to the Cascading Style Sheets Specification.
Property names and values are paired using the
:= data constructor. The latter is just a binary infix data constructor, and it is used just to improve the code appearance, so CSS property assignments look more natural.
Our next pasteable examples deal with widget elements' inline style manipulations. The following example displays a centered white bold text on green background spanning over 45% of browser window width floating to the right side of browser window.
Our next example is more contrived. It displays a line of text with each character colored differently, and colors are taken from an infinitely repeating sequence. Note the use of nowidget: it is used with foldr as a "starting value".
At this point, we are able to create static layout of Widgets. No way has been shown yet how to make GUI elements exchange information among themselves.
Activators serve exactly this purpose. Activators are functions, usually endlessly looped which are executed each in its own thread. These functions repeatedly receive messages from Message Boxes, and also intercept events induced by user. Activators also are able to modify on-the-fly properties of Widgets to which they are "attached". Some Activators also perform data exchange over XMLHTTP.
Method of inter-widget communication described here is derived from one discussed in the following paper:
HsWTK defines a type for these functions:
type ACTF = THTMLElement -> Bool
HsWTK also defines a function
active which being applied to an Activator turns it into a Widget, that is, an Activator may be nested within a container: that's what was meant above for an Activator "attached" to a Widget.
Our next example consists of two <INPUT> elements: when something is typed in the left one, it is repeated in the right one:
Let's go line by line. As new things are introduced, they will be discussed in appropriate sections.
Message boxes and activators
Message boxes were discussed earlier as means for inter-thread communications. Each Activator is represented by a separate thread. Unlike previous code examples, the main Widget code contains two calls to the
msgBox function. The
msgBox function passes reference to newly created Message box to its continuation. Thus, after the following code:
msgBox $ \ibx2 -> msgBox $ \dummy ->
is executed, two Message boxes are created.
Wiring the static layout
The following code of the main Widget would have produced the same static layout of two input elements:
mainW = inputI |<< nowidget +++ inputI |<< nowidget
Note the use of
nowidget here: the
inputI function produces an <INPUT> element, and the
|<< combinator assumes that the <INPUT> element is a container, that is, should have something nested.
This approach may be used during Web GUI design: make Widgets containers where necessary, but nest
nowidget's instead of Activators.
Compare the code of
mainW above with the code from our example:
mainW = ... inputI |<< active (evtBCastA "keyup" rtgt [ibx2]) +++ inputI |<< active (fwdValueA ibx2 dummy)
Each <INPUT> element got an Activator. As it may be guessable, Activator attached to the first declared (showing at the left of Web page) <INPUT> element, transmits its value (whatever is typed in), and Activator attached to the <INPUT> element declared second (showing right of the first <INPUT>) receives the value.
Mapping and broadcasting events
HsWTK defines the following Activator function:
evtBCastA :: CEvent e => String -> (e -> CPS Bool v) -> [MSGBOX Bool v] -> ACTF
The first argument of
String type is the type of event this Activator responds to. Only one event type may be specified. Event types are derived from the on- attribute names, e. g. keyup corresponds to onkeyup event, etc.
The second argument is a CPS-style function mapping an event to some value of type v. Such a function may retrieve certain parameters of an event (such as codeof key pressed), or event produce some side effects, although this is not recommended.
The third argument is list of Message boxes where the mapped value will be broadcast over. Broadcast will be performed using the
sendMsg_ function, so there is possibility of non-delivery.