transduce.js•2.34 kB
var _reduce = require('./internal/_reduce');
var _xwrap = require('./internal/_xwrap');
var curryN = require('./curryN');
/**
* Initializes a transducer using supplied iterator function. Returns a single
* item by iterating through the list, successively calling the transformed
* iterator function and passing it an accumulator value and the current value
* from the array, and then passing the result to the next call.
*
* The iterator function receives two values: *(acc, value)*. It will be
* wrapped as a transformer to initialize the transducer. A transformer can be
* passed directly in place of an iterator function. In both cases, iteration
* may be stopped early with the `R.reduced` function.
*
* A transducer is a function that accepts a transformer and returns a
* transformer and can be composed directly.
*
* A transformer is an an object that provides a 2-arity reducing iterator
* function, step, 0-arity initial value function, init, and 1-arity result
* extraction function, result. The step function is used as the iterator
* function in reduce. The result function is used to convert the final
* accumulator into the return type and in most cases is R.identity. The init
* function can be used to provide an initial accumulator, but is ignored by
* transduce.
*
* The iteration is performed with R.reduce after initializing the transducer.
*
* @func
* @memberOf R
* @since v0.12.0
* @category List
* @sig (c -> c) -> (a,b -> a) -> a -> [b] -> a
* @param {Function} xf The transducer function. Receives a transformer and returns a transformer.
* @param {Function} fn The iterator function. Receives two values, the accumulator and the
* current element from the array. Wrapped as transformer, if necessary, and used to
* initialize the transducer
* @param {*} acc The initial accumulator value.
* @param {Array} list The list to iterate over.
* @return {*} The final, accumulated value.
* @see R.reduce, R.reduced, R.into
* @example
*
* var numbers = [1, 2, 3, 4];
* var transducer = R.compose(R.map(R.add(1)), R.take(2));
*
* R.transduce(transducer, R.flip(R.append), [], numbers); //=> [2, 3]
*/
module.exports = curryN(4, function transduce(xf, fn, acc, list) {
return _reduce(xf(typeof fn === 'function' ? _xwrap(fn) : fn), acc, list);
});