Programming in XPCE/Prolog: Section 2.5

## 2.5 Argument conversion

Arguments to XPCE methods are typed and variables are dynamically typed. This combination is used for two purposes: automatic conversion and raising exceptions at the earliest possible point if conversion is not defined.

For example, the send-method `colour' on class graphical specifies accepts a single argument of type `colour'. A colour in XPCE is represented by a colour object. Colour objects may be created from their name. The natural way to specify a box should be coloured `red' is:

    ...,
    send(Box, colour(colour(red))),
    ...

Because the `->`colour method knows that it expects an instance of class colour, and because class colour defines the conversion from a name to a colour (see section 7.3.1), the following message achieves the same goal:

    ...,
    send(Box, colour(red)),
    ...

Some other examples of classes defining type conversion are font, image, name, string, real and the non-object data item int. The following messages are thus valid:

    ...,
    send(Box, x('10')),             % atom --> int
    send(Box, selected(true)),      % atom --> boolean
    send(Text, font(bold)),         % atom --> font
    send(Text, string(10)),         % int  --> string
    ...