Haskell in web browser/Haskell web toolkit

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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.


Widgets are basic building blocks of Graphical User Interface (GUI).

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 Node.appendChild method.

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.

Passive widgets

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.

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.

Composition combinators

For the purpose of sequencing widgets within a container, or nesting widgets in containers, HsWTK defines combinators +++, <<, |<<, and ++|. Please refer to the appropriate section of the Graphics.UI.HsWTK module documentation.