Source: dataflow/dataflow.js

// a dataflow abstraction that is not based on concurrency but on laziness

export { DataFlowVariable, Scheduler }

const DataFlowVariable = howto => {
    let value = undefined;
    return () => {
        if (value !== undefined) { return value }
        value = howto();
        return value;
    }
};

/**
 * execute asynchronous tasks in strict sequence
 * @returns {object} returns the API
 * @constructor
 */
const Scheduler = () => {
    let inProcess = false;
    const tasks = [];
    function process() {
        if (inProcess) return;
        if (tasks.length === 0) return;
        inProcess = true;
        const task = tasks.pop();
        const prom = new Promise( (ok, reject) => task(ok) );
        prom.then( _ => {
            inProcess = false;
            process();
        });
    }
    function add(task) {
        tasks.unshift(task);
        process();
    }
    return {
        add: add,
        addOk: task => add( ok => { task(); ok(); }) // convenience
    }
};