Wednesday, August 23, 2006

The Partial function in MochiKit

MochiKit is awesome. I’ve only begun to scratch the surface of the power in this framework.

During my demo I had a hard time explaining what the partial function does. I have never been a Python developer, so MochiKit’s influence is foreign to me. I do appreciate the simplicity of the syntax.

Partial is aptly named. It is basically a partial function call, wrapped in an object. Why would you want this, I hear you say? Consider the following:

// $('ctrlid') is MochiKits alias of document.getElementById('ctrlid')
 
$('txtbox').onchange = filterText('some input'); //wrong!  the function will be called immediately!
 
$('txtbox').onchange = function() //need an inline function wrapper to make the function call
            {                
                filterText('some input');
            }        
$('txtbox').onchange = partial(filterText, 'some input'); //partial separates the function from the argument(s)
 

In this example, the top line is setting the onchange event handler of a textbox to a function call. But Javascript doesn’t know that you want a function pointer here, it looks like you want to call the method and return something. So the solution that you can do is write a single function that wraps the function call that you want to make. This can be done inline like I do above. Partial comes in and cleans up those 3 lines and pushes them into 1 line.

Next take a look at this:

 function startsWith(character, target)
 {
    //searches target for character, if found... returns true
 }
 
 var startsWithCapitalB = partial(startsWith, 'B');
 
 alert(startsWithCapitalB('Ben')); //true
 alert(startsWithCapitalB('Henry')); //false