JavaScript: Reduce, an example. Thanks to Christian Johansen.

reduce.js

// We want to compute the total length of a buffer
var totalLength = 0;
for (var i = 0; i < buffers.length; i++) {
    totalLength += buffers[i].length;
}

// We can do this with map-reduce
var totalLength = buffers.
    map(function (chunk) { return chunk.length; }).
    reduce(function (prev, curr) { return prev + curr; }, 0);

// But we're using this simple operation
function add(a, b) {
    return a + b;
}

// Why not use it and our own 'prop' function?
// ...this one-liner is much more readable!
var totalLength = buffers.map(prop("length")).reduce(add);