Editing WxHaskell/Quick start (section)

==== Types ====

A typical wxHaskell program imports the <tt>Graphics.UI.WX</tt> library. If you need to access specific wxWidgets functionality, you would also import the lower level <tt>Graphics.UI.WXCore</tt> library. The <tt>main</tt> function uses <tt>start</tt> to start our GUI. The function <tt>start</tt> initializes the GUI framework with the provided argument and starts the window event loop until the application quits or when all top-level windows are closed. The GUI itself is described with the following functions:

<pre>frame&#160;&#160;::&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;[Prop&#160;(Frame&#160;())]&#160;-&gt;&#160;IO&#160;(Frame&#160;())
button&#160;::&#160;Window&#160;a&#160;-&gt;&#160;[Prop&#160;(Button&#160;())]&#160;-&gt;&#160;IO&#160;(Button&#160;())

text&#160;&#160;&#160;::&#160;Attr&#160;(Window&#160;a)&#160;String
layout&#160;::&#160;Attr&#160;(Frame&#160;a)&#160;&#160;Layout

(:=)&#160;&#160;&#160;::&#160;Attr&#160;w&#160;a&#160;-&gt;&#160;a&#160;-&gt;&#160;Prop&#160;w
set&#160;&#160;&#160;&#160;::&#160;w&#160;-&gt;&#160;[Prop&#160;w]&#160;-&gt;&#160;IO&#160;()

command::&#160;Event&#160;(Control&#160;a)&#160;(IO&#160;())
on&#160;&#160;&#160;&#160;&#160;::&#160;Event&#160;w&#160;a&#160;-&gt;&#160;Attr&#160;w&#160;a

widget&#160;::&#160;Window&#160;a&#160;-&gt;&#160;Layout</pre>
Actually, some of these functions have (even) more general types – you can use the <tt>:t</tt> command in GHCi to see them.

The types <tt>Frame ()</tt> and <tt>Button ()</tt> denote graphical objects. These objects can have ''properties''. When an object is created we can supply an initial list of properties but we can also set them later using <tt>set</tt>. The type of properties for frames are <tt>Prop (Frame ())</tt> and for buttons <tt>Prop (Button ())</tt>.

Properties are created by combining ''attributes'' with values. Examples of attributes are <tt>text</tt> and <tt>layout</tt>. An attribute of type <tt>Attr w a</tt> applies to objects of type <tt>w</tt> and values of type <tt>a</tt>. Values can be assigned to attributes using the <tt>(:=)</tt> operator. You can find out more about attributes in the haddock documentation for the modules [http://wxhaskell.sourceforge.net/doc/Graphics-UI-WX-Attributes.html WX.Attributes] and [http://wxhaskell.sourceforge.net/doc/Graphics-UI-WX-Classes.html WX.Classes].

Somewhat special attributes are ''events''. An event of type <tt>Event w a</tt> can be transformed into an attribute <tt>Attr w a</tt> using <tt>on</tt>. The value of an event attribute is normally an <tt>IO</tt> action that is executed when the event happens. Find out more about events in the haddock documentation for [http://hackage.haskell.org/packages/archive/wx/latest/doc/html/Graphics-UI-WX-Events.html WX.Events] and the lower level [http://hackage.haskell.org/packages/archive/wxcore/latest/doc/html/Graphics-UI-WXCore-Events.html WXCore.Events]

Since wxHaskell is based on an object-oriented framework, we also encode inheritance. The extra type parameter of objects encodes the inheritance relationship. When the parameter of an object is unit <tt>()</tt>, it denotes an object of that exact class. When the parameter is a type variable <tt>a</tt>, it denotes any object that is instance of that class. For example, both the <tt>frame</tt> and <tt>button</tt> functions return precisely a frame or button and use a <tt>()</tt> type parameter. However, the <tt>text</tt> attribute applies to any kind of window, including frames and buttons, and has a <tt>Window a</tt> as its argument. We can now use the <tt>text</tt> attribute for example for both frames and buttons. In wxHaskell, this works since a <tt>Frame&#160;()</tt> is actually a type synonym for <tt>Window&#160;(CFrame&#160;())</tt> and can thus be passed where a <tt>Window a</tt> is expected. The same hold for a <tt>Button ()</tt> that is a synonym for <tt>Control&#160;(CButton&#160;())</tt> that is again a synonym for <tt>Window&#160;(CControl&#160;(CButton&#160;()))</tt>.