<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> <HTML> <HEAD> <TITLE>Programming in XPCE/Prolog: Section 2.5</TITLE><LINK REL=home HREF="index.html"> <LINK REL=contents HREF="Contents.html"> <LINK REL=index HREF="DocIndex.html"> <LINK REL=summary HREF="summary.html"> <LINK REL=previous HREF="namedargs.html"> <LINK REL=next HREF="sendmanyargs.html"> <STYLE type="text/css"> /* Style sheet for SWI-Prolog latex2html */ dd.defbody { margin-bottom: 1em; } dt.pubdef { background-color: #c5e1ff; } pre.code { margin-left: 1.5em; margin-right: 1.5em; border: 1px dotted; padding-top: 5px; padding-left: 5px; padding-bottom: 5px; background-color: #f8f8f8; } div.navigate { text-align: center; background-color: #f0f0f0; border: 1px dotted; padding: 5px; } div.title { text-align: center; padding-bottom: 1em; font-size: 200%; font-weight: bold; } div.author { text-align: center; font-style: italic; } div.abstract { margin-top: 2em; background-color: #f0f0f0; border: 1px dotted; padding: 5px; margin-left: 10%; margin-right:10%; } div.abstract-title { text-align: center; padding: 5px; font-size: 120%; font-weight: bold; } div.toc-h1 { font-size: 200%; font-weight: bold; } div.toc-h2 { font-size: 120%; font-weight: bold; margin-left: 2em; } div.toc-h3 { font-size: 100%; font-weight: bold; margin-left: 4em; } div.toc-h4 { font-size: 100%; margin-left: 6em; } span.sec-nr { } span.sec-title { } span.pred-ext { font-weight: bold; } /* Footnotes */ sup.fn { color: blue; text-decoration: underline; } span.fn-text: { display: none; } sup.fn span {display: none;} sup:hover span { display: block !important; position: absolute; top: auto; left: auto; width: 80%; color: #000; background: white; border: 2px solid; padding: 5px; margin: 10px; z-index: 100; font-size: smaller; } </STYLE> </HEAD> <BODY BGCOLOR="white"> <DIV class="navigate"><A class="nav" href="index.html"><IMG SRC="home.gif" BORDER=0 ALT="Home"></A> <A class="nav" href="Contents.html"><IMG SRC="index.gif" BORDER=0 ALT="Contents"></A> <A class="nav" href="DocIndex.html"><IMG SRC="yellow_pages.gif" BORDER=0 ALT="Index"></A> <A class="nav" href="summary.html"><IMG SRC="info.gif" BORDER=0 ALT="Summary"></A> <A class="nav" href="namedargs.html"><IMG SRC="prev.gif" BORDER=0 ALT="Previous"></A> <A class="nav" href="sendmanyargs.html"><IMG SRC="next.gif" BORDER=0 ALT="Next"></A> </DIV> <H2><A NAME="sec:2.5"><SPAN class="sec-nr">2.5</SPAN> <SPAN class="sec-title">Argument conversion</SPAN></A></H2> <P><A NAME="idx:argumentstyped:55"></A><A NAME="idx:type:56"></A><A NAME="idx:typeconversion:57"></A>Arguments to <font size=-1>XPCE</font> methods are <EM>typed</EM> 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. <P>For example, the send-method `colour' on class <A class="" href="summary.html#class:graphical">graphical</A> specifies accepts a single argument of type `colour'. A colour in <font size=-1>XPCE</font> is represented by a colour <EM>object</EM>. Colour objects may be created from their name. The natural way to specify a box should be coloured `red' is: <PRE class="code"> ..., send(Box, colour(colour(red))), ... </PRE> <P>Because the <B><CODE>-></CODE>colour</B> method knows that it expects an instance of class colour, and because class colour defines the conversion from a name to a colour (see <A class="sec" href="sec-7.3.html">section 7.3.1</A>), the following message achieves the same goal: <PRE class="code"> ..., send(Box, colour(red)), ... </PRE> <P>Some other examples of classes defining type conversion are <A class="" href="summary.html#class:font">font</A>, <A class="" href="summary.html#class:image">image</A>, <A class="" href="summary.html#class:name">name</A>, <A class="" href="summary.html#class:string">string</A>, <A class="" href="summary.html#class:real">real</A> and the non-object data item <EM>int</EM>. The following messages are thus valid: <PRE class="code"> ..., send(Box, x('10')), % atom --> int send(Box, selected(true)), % atom --> boolean send(Text, font(bold)), % atom --> font send(Text, string(10)), % int --> string ... </PRE> <P></BODY></HTML>