File content as of revision 2:b70964f8a427:

#include "mbed.h"
#include "neopixel.h"

// This must be an SPI MOSI pin.
#define DATA_PIN p5

void generate(neopixel::Pixel * out, uint32_t index, uintptr_t extra)
{
    uint32_t brightness = (index + extra) >> 3;
    out->red   = ((index + extra) & 0x1) ? brightness : 0;
    out->green = ((index + extra) & 0x2) ? brightness : 0;
    out->blue  = ((index + extra) & 0x4) ? brightness : 0;
}

int main()
{
    // Create a temporary DigitalIn so we can configure the pull-down resistor.
    // (The mbed API doesn't provide any other way to do this.)
    // An alternative is to connect an external pull-down resistor.
    DigitalIn(DATA_PIN, PullDown);

    // The pixel array control class.
    neopixel::PixelArray array(DATA_PIN);

    uint32_t offset = 0;
    while (1) {
        array.update(generate, 100, offset++);
        wait_ms(250);
    }
}