Bill Farmer

Random thoughts on random subjects

Lambda Expressions

by Bill Farmer. Categories: Hacking .

Lambda expressions or anonymous functions can be quite useful. For example using jQuery in JavaScript all the work gets done in anonymous functions unless you particularly want to extract them and give them names…

jQuery(document).ready(function($) {
    // ...
    // Process the start button
    $("#start").click(function() {
        question = 0;
        $("#data").css("display", "none");
        $(".intro").fadeOut(function() {
        // Do stuff
        });
    // ...
    });
});

In C++ the syntax is curious in that there are a pair of square brackets enclosing ‘captured’ variables…

    // Must have at least one lambda expression
    gdk_threads_add_idle([](gpointer widget) -> gboolean
    {
        gtk_widget_queue_draw(GTK_WIDGET(widget));
        return G_SOURCE_REMOVE;
    }, widget);

In Objective C the syntax is even more peculiar. The ProcessAudio function is called a block. I tried to make it a lambda expression but had to give up. Whether this is an Objective C or C construct I don’t know…

// Input proc
OSStatus InputProc(void *inRefCon, AudioUnitRenderActionFlags *ioActionFlags,
		   const AudioTimeStamp *inTimeStamp, UInt32 inBusNumber,
		   UInt32 inNumberFrames, AudioBufferList *ioData)
{
    static AudioBufferList abl =
	{1, {1, 0, nil}};
    // ...
    // Run in main queue
    dispatch_async(dispatch_get_main_queue(), ProcessAudio);

    return noErr;
}

// Process audio
void (^ProcessAudio)() = ^
{
    enum
    {kTimerCount = 16};
    // ...
};

In java they can be used for callbacks or listeners. The syntax is simpler, you don’t even need the brackets if there’s only one argument…

                showEdit = () ->
                {
                    startAnimation(edit, R.anim.fade_in, View.VISIBLE);
                    scrollUp = false;
                    scrollDn = false;
                };
                // ...
                        // Show button delayed
                        markdownView.removeCallbacks(showEdit);
                        markdownView.postDelayed(showEdit, VISIBLE_DELAY);

            // On click
            accept.setOnClickListener(v ->
            {
                // Get text
                if (display)
                    loadMarkdown();
                // ...
            });

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