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Higher-order utilities for use with async functions.

Designed for use with async. Allows you to write async code in a more point-free style.


### constant(value)

Returns a function that calls-back with the value provided. Useful in waterfalls.

  function (value, next) {
    // value === 42 
], callback)
### asyncify(func)

Take a sync function and make it async. Useful for plugging sync functions into a waterfall or series. Will catch errors and pass them to the callback.

  function (data, next) {
    // data is the result of parsing the text. 
    // If there was a parsing error, it would have been caught. 
], callback)
### flip(function)

Take a function and move the last argument to the front. Useful for plugging "normal" async functions into

function getUrl(options, callback) {
  // .... 
  url: acomb.constant("")
  data: ["url", acomb.flip(getUrl)],
}, callback)
### partialRight(func, args...)

Like _.partialRight, except it leaves space for a callback at the end. Useful for getting args in the right order when passing functions to async functions.
  acomb.partialRight(fs.readFile, "utf8"),
  function (err, files) {
    // files is an array of strings, rather than buffers 
### spreadOptions(func, option1, option2, ...)

Takes a function of the form function(object, callback) {} and converts it to the form function(option1, option2, ... callback) {} based on the strings passed. The strings passed will pick properties from the object and turn them in to direct arguments. Useful in in conjunction with flip for destructuring the results. You can also pass an array of strings as the second arg.

function doFoo(bar baz, callback) {
  // .... 
  bar: getBar,
  baz: getBaz
  foo: ["bar", "baz", acomb.flip(acomb.spreadOptions(doFoo, "bar", "baz"))],
}, callback)
### before(func, asyncFunc)

Run a synchronous function before an async function. The synchronous function will be called with the arguments passed (without the callback), and the async function will be called with the return value of the sync function.

function trim (str) { return str.trim(); }
  acomb.before(trim, function parseData(str, next) {
    // `str` has its whitespace trimmed 
], callback)
### after(asyncFunc, func)

Run a synchronous function after an async function, with the results of the async function as arguments. The return value of the sync function will be passed to the original callback.

var getTheDataIWant = acomb.after(getData, function (data) {
  return _.pick(data, ["foo", "bar", "baz"])

note: If you want to run an async function before or after another, just use async.seq or async.compose

### provided(predicate, func)

Conditionally run an async func based on the results of a predicate. The predicate function will be passed the same args as the async function. If the predicate returns false, the args will be passed to the async function's callback.
  acomb.provided(_.identity, acomb.partialRight(fs.readFile, "utf8"))
  function (err, results) {
    // results will be an array containing the data of the file names 
    // that actually existed -- no errors due to invalid file names. 

You can also pass a boolean directly as the first arg.

  async.provided(NODE_ENV === "dev", func3)
], done);
### ensureAsync(func)

Ensure that an async function will always call its callback on a later tick in the event loop. No extra deferrals are added if the function passed does indeed callback asynchronously. This is useful for preventing stack overflows in things like async.each.
  acomb.ensureAsync(function (value, cb) {
    if (value < 50000) {
      return callback(null, value); // this function sometimes is synchronous! 
    doSomethingAsync(value, callback);
); // no stack overflows!