transducers-js: transducers

Given a transformer return a concatenating transformer

• xf <com.cognitect.transducers.ITransformer>

  a transformer

Function composition. Take N function and return their composition.

• varArgs <Function>

  N functions

Example

var t = transducers;
var inc = function(n) { return n + 1 };
var double = function(n) { return n * 2 };
var incDouble = t.comp(double, inc);
incDouble(3); // 8

Take a predicate function and return its complement.

• a <Function>

  predicate function

Example

var isEven = function(n) { return n % 2 == 0; };
var isOdd = transducers.complement(isEven);

A completing transducer constructor. Useful to provide cleanup logic at the end of a reduction/transduction.

• xf <com.cognitect.transducers.ITransformer>

  a transducer

• cf <Function>

  a function to apply at the end of the reduction/transduction

A dropping transducer constructor

• n <Number>

  an integer, the number of inputs to drop.

Example

var t = transducers;
var xf = t.drop(3);
t.into([], xf, [0,1,2,3,4,5]); // [3,4,5];

A dropping transducer that drop inputs as long as pred is true.

• pred <Function>

  a predicate function

Example

var t = transducers;
var xf = t.dropWhile(function(n) { return n < 3; });
t.into([], xf, [0,1,2,3,4,5]); // [3,4,5];

Ensure that a value is reduced. If already reduced will not re-wrap.

• x <Object>

  any JavaScript value

Example

var t = transducers;
var x = t.ensureReduced(1);
var y = t.ensureReduced(x);
x === y; // true

Filtering transducer constructor

• pred <Function>

  a predicate function

Example

var t = transducers;
var isEven = function(n) { return n % 2 == 0; };
var xf = t.filter(isEven);
t.into([], xf, [0,1,2,3,4]); // [0,2,4];

A transformer which simply returns the first input.

Identity function.

• x <Object>

  any JavaScript value

Example

transducers.identity(1); // 1

Reduce a value into the given empty value using a transducer.

• empty <String | Array | Object>

  a JavaScript collection

• xf <com.cognitect.transducers.ITransformer>

  a transducer

• coll <Object>

  any iterable JavaScript value: array, string, object, or iterable.

Example

var t = transducers;
var inc = function(n) { return n+1; };
var isEven = function(n) { return n % 2 == 0; };
var apush = function(arr,x) { arr.push(x); return arr; };
var xf = t.comp(t.map(inc),t.filter(isEven));
t.into([], xf, [1,2,3,4]); // [2,4]

Check if a value is reduced.

• x <Object>

  any JavaScript value

Example

var t = transducers;
t.isReduced(1); // false
t.isReduced(t.reduced(1)); // true

A keeping transducer. Keep inputs as long as the provided function does not return null or undefined.

• f <Function>

  a function

Example

var t = transducers;
var xf = t.keep(function(x) { if(typeof x == "string") return "cool"; });
t.into([], xf, [0,1,"foo",3,4,"bar"]); // ["foo","bar"]

Like keep but the provided function will be passed the index as the second argument.

• f <Function>

  a function

Example

var t = transducers;
var xf = t.keepIndexed(function(i, x) { if(typeof x == "string") return "cool"; });
t.into([], xf, [0,1,"foo",3,4,"bar"]); // ["foo","bar"]

Mapping transducer constructor

• f <Function>

  the mapping operation

Example

var t = transducers;
var inc = function(n) { return n+1; };
var xf = t.map(inc);
t.into([], xf, [1,2,3]); // [2,3,4]

A mapping concatenating transformer

• f <Function>

  the mapping function

Example

var t = transducers;
var reverse = function(arr) { var arr = Array.prototype.slice.call(arr, 0); arr.reverse(); return arr; }
var xf = t.mapcat(reverse);
t.into([], xf, [[3,2,1],[6,5,4]]); // [1,2,3,4,5,6]

A partitioning transducer. Collects inputs into arrays of size N.

• n <Number>

  an integer

Example

var t = transducers;
var xf = t.partitionAll(3);
t.into([], xf, [0,1,2,3,4,5]); // [[0,1,2],[3,4,5]]

A partitioning transducer. Collects inputs into arrays as long as predicate remains true for contiguous inputs.

• f <Function>

  a partition function. When the result for an input changes from the previous result will create a partition.

Example

var t = transducers;
var xf = t.partitionBy(function(x) { return typeof x == "string"; });
t.into([], xf, [0,1,"foo","bar",2,3,"bar","baz"]); // [[0,1],["foo","bar"],[2,3],["bar","baz"]];

Given a transformer returns a transformer which preserves reduced by wrapping one more time. See cat.

• xf <com.cognitect.transducers.ITransformer>

  a transformer

Given a transducer, an intial value and a collection - returns the reduction.

• xf <com.cognitect.transducers.ITransformer | Function>

  a transducer or two-arity function

• init <Object>

  any JavaScript value

• coll <String | Array | Object>

  any iterable JavaScript value

Return a reduced value. Reduced values short circuit transduce.

• x <Object>

  any JavaScript value

Example

var reduced = transducers.reduced(1);

Similar to filter except the predicate is used to eliminate values.

• pred <Function>

  a predicate function

Example

var t = transducers;
var isEven = function(n) { return n % 2 == 0; };
var xf = t.remove(isEven);
t.into([], xf, [0,1,2,3,4]); // [1,3];

A take transducer constructor. Will take n values before returning a reduced result.

• n <Number>

  the number of inputs to receive.

Example

var t = transducers;
var xf = t.take(3);
t.into([], xf, [0,1,2,3,4,5]); // [0,1,2];

A transducer that takes every Nth input

• n <Number>

  an integer

Example

var t = transducers;
var xf = t.takeNth(3);
t.into([], xf, [0,1,2,3,4,5]); // [2,5];

Like the take transducer except takes as long as the pred return true for inputs.

• pred <Function>

  a predicate function

Example

var t = transducers;
var xf = t.takeWhile(function(n) { return n < 3; });
t.into([], xf, [0,1,2,3,4,5]); // [0,1,2];

Convert a transducer transformer object into a function so that it can be used with existing reduce implementation i.e. native, Underscore, lodash

• xf <com.cognitect.transducers.ITransformer>

  a transducer

• builder <Function>

  a function which take the accumulator and the the next input and return a new accumulator value.

Example

var t = transducers;
var arr = [0,1,2,3,4,5],
var apush = function(arr, x) { arr.push(x); return arr; },
var xf = t.comp(t.map(inc),t.filter(isEven));
arr.reduce(t.toFn(xf, apush), []); // [2,4,6]

Given a transducer, a builder function, an initial value and a iterable collection - returns the reduction. collection - returns the reduction.

• xf <com.cognitect.transducers.ITransformer>

  a transducer

• f <com.cognitect.transducers.ITransformer | Function>

  a transducer or two-arity function

• init <Object=>

  any JavaScript value

• coll <String | Array | Object>

  any iterable JavaScript value

Example

var t = transducers;
var inc = function(n) { return n+1; };
var isEven = function(n) { return n % 2 == 0; };
var apush = function(arr,x) { arr.push(x); return arr; };
var xf = t.comp(t.map(inc),t.filter(isEven));
t.transduce(xf, apush, [], [1,2,3,4]); // [2,4]

Ensure a value is not reduced. Unwraps if reduced.

• x <Object>

  any JavaScript value

Example

var t = transducers;
var x = t.reduced(1);
t.unreduced(x); // 1
t.unreduced(t.unreduced(x)); // 1

Take a two-arity reducing function where the first argument is the accumluation and the second argument is the next input and convert it into a transducer transformer object.

• stepFn <Function>

  a two-arity reducing function

Example

var t = transducers;
var arrayPush = t.wrap(function(arr, x) { arr.push(x); return arr; });