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Transducers are composable algorithmic transformations. They are independent from the context of their input and output sources and specify only the essence of the transformation in terms of an individual element. Because transducers are decoupled from input or output sources, they can be used in many different processes - collections, streams, channels, observables, etc. Transducers compose directly, without awareness of input or creation of intermediate aggregates.

For more information about Clojure transducers and transducer semantics see the introductory blog post and this video.

You can transduce anything that you can iterate over in a foreach-loop (e.g., arrays, \Iterator, Traversable, Generator, etc.). Transducers can be applied eagerly using transduce(), into(), to_array(), to_assoc(), to_string(); and lazily using to_iter(), xform(), or by applying a transducer stream filter.

composer.phar require mtdowling/transducers

Defining Transformations With Transducers

Transducers compose with ordinary function composition. A transducer performs its operation before deciding whether and how many times to call the transducer it wraps. You can easily compose transducers to create transducer pipelines. The recommended way to compose transducers is with the transducers\comp() function:

use Transducers as t;

$xf = t\comp(
    t\map(function ($x) { return $x + 1; }),
    t\filter(function ($x) { return $x % 2; }),

The above composed transducer is a function that creates a pipeline for transforming data: it skips the first two elements of a collection, adds 1 to each value, filters out even numbers, then takes 3 elements from the collection. This new transformation function can be used with various transducer application functions, including xform().

$data = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
$result = t\xform($data, $xf);

// Contains: [5, 7, 9]


Transducers are functions that return a function that accept a reducing function array $xf and return a new reducing function array that wraps the provided $xf.

Here's how to create a transducer that adds $n to each value:

$inc = function ($n = 1) {
    // Return a function that accepts a reducing function array $xf.
    return function (array $xf) use ($n) {
        // Return a new reducing function array that wraps $xf.
        return [
            'init'   => $xf['init'],
            'result' => $xf['result'],
            'step'   => function ($result, $input) use ($xf, $n) {
                return $xf['step']($result, $input + $n);

$result = t\xform([1, 2, 3], $inc(1));
// Contains: 2, 3, 4

Reducing Function Array

Reducing function arrays are PHP associative arrays that contain a 'init', 'step', and 'result' key that maps to a function.

key arguments Description
init none Invoked to initialize a transducer. This function should call the 'init' function on the nested reducing function array $xf, which will eventually call out to the transducing process. This function is only called when an initial value is not provided while transducing.
step $result, $input This is a standard reduction function but it is expected to call the $xf['step'] function 0 or more times as appropriate in the transducer. For example, filter will choose (based on the predicate) whether to call $xf or not. map will always call it exactly once. cat may call it many times depending on the inputs.
result $result Some processes will not end, but for those that do (like transduce), the 'result' function is used to produce a final value and/or flush state. This function must call the $xf['result'] function exactly once.

Using Transducers

Transducers can be used in any number of ways. This library provides several methods that can be used to apply transducers.


function transduce(callable $xf, array $step, $coll, $init = null)

Transform and reduce $coll by applying $xf($step)['step'] to each value.

  • callable $xf: Transducer function to apply.
  • array $step: Transformer array that has 'init', 'result', and 'step' keys that map to a callable.
  • $coll: Data to transform. Can be an array, iterator, or PHP stream resource.
  • $init: Optional first initialization value of the reduction. If this value is not provided, the $step['init']() function will be called to provide a default value.
use Transducers as t;

$data = [[1, 2], [3, 4]];
$xf = t\comp(
    t\filter(function ($value) { return $value % 2; }),
$result = t\transduce($xf, t\array_reducer(), $data);

// Contains: [1, 3]

When using this function, you can use any of the built-in reducing function arrays as the $step argument:

  • transducers\array_reducer(): Creates a reducing function array that appends values to an array.

    $data = [[1, 2], [3, 4]];
    $result = t\transduce(t\flatten(), t\array_reducer(), $data);
    // Results contains [1, 2, 3, 4]
  • transducers\stream_reducer(): Creates a reducing function array that writes values to a stream resource. If no $init value is provided when transducing then a PHP temp stream will be used.

    $data = [[1, 2], [3, 4]];
    $result = t\transduce(t\flatten(), t\stream_reducer(), $data);
    fseek($result, 0);
    echo stream_get_contents($result);
    // Outputs: 1234
  • transducers\string_reducer(): Creates a reducing function array that concatenates each value to a string.

    $xf = t\flatten();
    // use an optional joiner on the string reducer.
    $reducer = t\string_reducer('|');
    $data = [[1, 2], [3, 4]];
    $result = t\transduce($xf, $reducer, $data);
    // Result is '1|2|3|4'
  • transducers\assoc_reducer(): Creates a reducing function array that adds key value pairs to an associative array. Each value must be an array that contains the array key in the first element and the array value in the second element.

  • transducers\create_reducer(): Convenience function that can be used to quickly create reducing function arrays. The first and only required argument is a step function that takes the accumulated result and the new value and returns a single result. The next, optional, argument is the init function that takes no arguments an returns an initialized result. The next, optional, argument is the result function which takes a single result argument and is expected to return a final result.

    $result = t\transduce(
        t\create_reducer(function ($r, $x) { return $r + $x; }),
        [[1, 2], [3, 4]]
    // Result is equal to 10
  • transducers\operator_reducer(): Creates a reducing function array that uses the provided infix operator to reduce the collection (i.e., $result <operator> $input). Supports: '.', '+', '-', '*', and '/' operators.

    $result = t\transduce(
        [[1, 2], [[3], 4]]
    // Result is equal to 10


function xform($coll, callable $xf)

Returns the same data type passed in as $coll with $xf applied.

xform() using the following logic when returning values:

  • array: Returns an array using the provided array.
  • associative array: Turn the provided array into an indexed array, meaning that each value passed to the step reduce function is an array where the first element is the key and the second element is the value. When completed, xform() returns an associative array.
  • \Iterator: Returns an iterator in which $xf is applied lazily.
  • resource: Reads single bytes from the provided value and returns a new fopen resource that contains the bytes from the input resource after applying $xf.
  • string: Passes each character from the string through to each step function and returns a string.
// Give an array and get back an array
$result = t\xform([1, false, 3], t\compact());
assert($result === [1, 3]);

// Give an iterator and get back an iterator
$result = t\xform(new ArrayIterator([1, false, 3]), t\compact());
assert($result instanceof \Iterator);

// Give a stream and get back a stream.
$stream = fopen('php://temp', 'w+');
fwrite($stream, '012304');
$result = t\xform($stream, t\compact());
assert($result == '1234');

// Give a string and get back a string
$result = t\xform('abc', t\map(function ($v) { return strtoupper($v); }));
assert($result === 'abc');

// Give an associative array and get back an associative array.
$data = ['a' => 1, 'b' => 2];
$result = t\xform('abc', t\map(function ($v) {
    return [strtoupper($v[0]), $v[1]];
assert($result === ['A' => 1, 'B' => 2]);


function into($target, $coll, callable $xf)

Transduces items from $coll into the given $target, in essence "pouring" transformed data from one source into another data type.

This function does not attempt to discern between arrays and associative arrays. Any array or ArrayAccess object provided will be treated as an indexed array. When a string is provided, each value will be concatenated to the end of the string with no separator. When an fopen resource is provided, data will be written to the end of the stream with no separator between writes.

use Transducers as t;

// Compose a transducer function.
$transducer = t\comp(
    // Remove a single level of nesting.
    // Filter out even values.
    t\filter(function ($value) { return $value % 2; }),
    // Multiply each value by 2
    t\map(function ($value) { return $value * 2; }),
    // Immediately stop when the value is >= 15.
    t\take_while(function($value) { return $value < 15; })

$data = [[1, 2, 3], [4, 5], [6], [], [7], [8, 9, 10, 11]];

// Eagerly pour the transformed data, [2, 6, 10, 14], into an array.
$result = t\into([], $data, $transducer);


function to_iter($coll, callable $xf)

Creates an iterator that lazily applies the transducer $xf to the $input iterator. Use this function when dealing with large amounts of data or when you want operations to occur only as needed.

// Generator that yields incrementing numbers.
$forever = function () {
    $i = 0;
    while (true) {
        yield $i++;

// Create a transducer that multiplies each value by two and takes
// ony 100 values.
$xf = t\comp(
    t\map(function ($value) { return $value * 2; }),

foreach (t\to_iter($forever(), $xf) as $value) {
    echo $value;


function to_array($iterable, callable $xf)

Converts a value to an array and applies a transducer function. $iterable is passed through to_traversable() in order to convert the input value into an array.

.. code-block:: php

$result = t\to_array(
    t\map(function ($v) { return strtoupper($v); })

// Contains: ['A', 'B', 'C']


function to_assoc($iterable, callable $xf)

Creates an associative array using the provided input while applying $xf to each value. Values are converted to arrays that contain the array key in the first element and the array value in the second.

$result = t\to_assoc(
    ['a' => 1, 'b' => 2],
    t\map(function ($v) { return [$v[0], $v[1] + 1]; })

assert($result == ['a' => 2, 'b' => 3]);


function to_string($iterable, callable $xf)

Converts a value to a string and applies a transducer function to each character. $iterable is passed through to_traversable() in order to convert the input value into an array.

echo t\to_string(
    ['a', 'b', 'c'],
    t\map(function ($v) { return strtoupper($v); })

// Outputs: ABC


function to_fn(callable $xf, callable|array $builder = null)

Convert a transducer into a function that can be used with existing reduce implementations (e.g., array_reduce).

$xf = t\map(function ($x) { return $x + 1; });
$fn = t\to_fn($xf); // $builder is optional
$result = array_reduce([1, 2, 3], $fn);
assert($result == [2, 3, 4]);

$fn = t\to_fn($xf, t\string_reducer());
$result = array_reduce([1, 2, 3], $fn);
assert($result == '234');

Stream Filter

You can apply transducers to PHP streams using a stream filter. This library registers a transducers stream filter that can be appended or prepended to a PHP stream using the transducers\append_stream_filter() or transducers\prepend_stream_filter() functions.

use transducers as t;

$f = fopen('php://temp', 'w+');
fwrite($f, 'testing. Can you hear me?');

$xf = t\comp(
    // Split by words
    // Uppercase/lowercase every other word.
    t\keep_indexed(function ($i, $v) {
        return $i % 2 ? strtoupper($v) : strtolower($v);
    // Combine words back together into a string separated by ' '.
    t\interpose(' ')

// Apply a transducer stream filter.
$filter = t\append_stream_filter($f, $xf, STREAM_FILTER_READ);
echo stream_get_contents($f);
// Be sure to remove the filter to flush out any buffers.
echo stream_get_contents($f);


// Echoes: "testing. CAN you HEAR me?"

Available Transducers


function map(callable $f)

Applies a map function $f to each value in a collection.

$data = ['a', 'b', 'c'];
$xf = t\map(function ($value) { return strtoupper($value); });
assert(t\xform($data, $xf) == ['A', 'B', 'C']);


function filter(callable $pred)

Filters values that do not satisfy the predicate function $pred.

$data = [1, 2, 3, 4];
$odd = function ($value) { return $value % 2; };
$result = t\xform($data, t\filter($odd));
assert($result == [1, 3]);


function remove(callable $pred)

Removes anything from a sequence that satisfied $pred.

$data = [1, 2, 3, 4];
$odd = function ($value) { return $value % 2; };
$result = t\xform($data, t\remove($odd));
assert($result == [2, 4]);


function cat()

Transducer that concatenates items from nested lists. Note that cat() is used differently than other transducers: you use cat using the string value of the function name (i.e., 'transducers\cat');

$xf = 'transducers\cat';
$data = [[1, 2], [3], [], [4, 5]];
$result = t\xform($data, $xf);
assert($result == [1, 2, 3, 4, 5]);


function mapcat(callable $f)

Applies a map function to a collection and concats them into one less level of nesting.

$data = [[1, 2], [3], [], [4, 5]];
$xf = t\mapcat(function ($value) { return array_sum($value); });
$result = t\xform($data, $xf);
assert($result == [3, 3, 0, 9]);


function flatten()

Takes any nested combination of sequential things and returns their contents as a single, flat sequence.

$data = [[1, 2], 3, [4, new ArrayObject([5, 6])]];
$xf = t\flatten();
$result = t\to_array($data, $xf);
assert($result == [1, 2, 3, 4, 5, 6]);


function partition($size)

Partitions the source into arrays of size $size. When the reducing function array completes, the array will be stepped with any remaining items.

$data = [1, 2, 3, 4, 5];
$result = t\xform($data, t\partition(2));
assert($result == [[1, 2], [3, 4], [5]]);


function partition_by(callable $pred)

Split inputs into lists by starting a new list each time the predicate passed in evaluates to a different condition (true/false) than what holds for the present list.

$data = [['a', 1], ['a', 2], [2, 3], ['c', 4]];
$xf = t\partition_by(function ($v) { return is_string($v[0]); });
$result = t\into([], $data, $xf);

assert($result == [
    [['a', 1], ['a', 2]],
    [[2, 3]],
    [['c', 4]]


function take($n);

Takes $n number of values from a collection.

$data = [1, 2, 3, 4, 5];
$result = t\xform($data, t\take(2));
assert($result == [1, 2]);


function take_while(callable $pred)

Takes from a collection while the predicate function $pred returns true.

$data = [1, 2, 3, 4, 5];
$xf = t\take_while(function ($value) { return $value < 4; });
$result = t\xform($data, $xf);
assert($result == [1, 2, 3]);


function take_nth($nth)

Takes every nth item from a sequence of values.

$data = [1, 2, 3, 4, 5, 6];
$result = t\xform($data, t\take_nth(2));
assert($result == [1, 3, 5]);


function drop($n)

Drops $n items from the beginning of the input sequence.

$data = [1, 2, 3, 4, 5];
$result = t\xform($data, t\drop(2));
assert($result == [3, 4, 5]);


function drop_while(callable $pred)

Drops values from a sequence so long as the predicate function $pred returns true.

$data = [1, 2, 3, 4, 5];
$xf = t\drop_while(function ($value) { return $value < 3; });
$result = t\xform($data, $xf);
assert($result == [3, 4, 5]);


function replace(array $smap)

Given a map of replacement pairs and a collection, returns a sequence where any elements equal to a key in $smap are replaced with the corresponding $smap value.

$data = ['hi', 'there', 'guy', '!'];
$xf = t\replace(['hi' => 'You', '!' => '?']);
$result = t\xform($data, $xf);
assert($result == ['You', 'there', 'guy', '?']);


function keep(callable $f)

Keeps $f items for which $f does not return null.

$result = t\xform(
    [0, false, null, true],
    t\keep(function ($value) { return $value; })

assert($result == [0, false, true]);


function keep_indexed(callable $f)

Returns a sequence of the non-null results of $f($index, $input).

$result = t\xform(
    [0, false, null, true],
    t\keep_indexed(function ($index, $input) {
        echo $index . ':' . json_encode($input) . ', ';
        return $input;

assert($result == [0, false, true]);

// Will echo: 0:0, 1:false, 2:null, 3:true,


function dedupe()

Removes duplicates that occur in order (keeping the first in a sequence of duplicate values).

$result = t\xform(
    ['a', 'b', 'b', 'c', 'c', 'c', 'b'],

assert($result == ['a', 'b', 'c', 'b']);


function interpose($separator)

Adds a separator between each item in the sequence.

$result = t\xform(['a', 'b', 'c'], t\interpose('-'));
assert($result == ['a', '-', 'b', '-', 'c']);


function tap(callable $interceptor)

Invokes interceptor with each result and item, and then steps through unchanged.

The primary purpose of this method is to "tap into" a method chain, in order to perform operations on intermediate results within the chain. Executes interceptor with current result and item.

// echo each value as it passes through the tap function.
$tap = t\tap(function ($r, $x) { echo $x . ', '; });

    ['a', 'b', 'c'],
        t\map(function ($v) { return strtoupper($v); }),

// Prints: a, A, b, B, c, C,


function compact()

Trim out all falsey values.

$result = t\xform(['a', true, false, 'b', 0], t\compact());
assert($result == ['a', true, 'b']);


function words($maxBuffer = 4096)

Splits the input by words. You can provide an optional max buffer length that will ensure the buffer size used to find words is never exceeded. The default max buffer length is 4096. To use an unbounded buffer, provide INF.

$xf = t\words();
$data = ['Hi. This is a test.'];
$result = t\xform($data, $xf);
assert($result == ['Hi.', 'This', 'is', 'a', 'test.']);

$data = ['Hi. ', 'This is',  ' a test.'];
$result = t\xform($data, $xf);
assert($result == ['Hi.', 'This', 'is', 'a', 'test.']);


function lines($maxBuffer = 10240000)

Splits the input by lines. You can provide an optional max buffer length that will ensure the buffer size used to find lines is never exceeded. The default max buffer length is 10MB. To use an unbounded buffer, provide INF.

$xf = t\lines();
$data = ["Hi.\nThis is a test."];
$result = t\xform($data, $xf);
assert($result == ['Hi.', 'This is a test.']);

$data = ["Hi.\n", 'This is',  ' a test.', "\nHear me?"];
$result = t\xform($data, $xf);
assert($result == ['Hi.', 'This is a test.', 'Hear me?']);

Utility Functions


function indentity($value)

Returns the provided value. This is useful for writing reducing function arrays that do not need to modify an 'init' or 'result' function. In these cases, you can simply use the string 'transducers\identity' as the 'init' or 'result' function to continue to proxy to further reducers.


function assoc_iter($iterable)

Converts an iterable into an indexed array iterator where each value yielded is an array containing the key followed by the value.

$data = ['a' => 1, 'b' => 2];
assert(t\assoc_iter($data) == [['a', 1], ['b', 2]];

This can be combined with the assoc_reducer() to generate associative arrays.

$result = t\transduce(
    t\map(function ($v) { return [$v[0], $v[1] + 1]; }),
    t\assoc_iter(['a' => 1, 'b' => 2])

assert($result == ['a' => 2, 'b' => 3]);

You should really just use the t\to_assoc() function if you know you're reducing an associative array.

$result = t\to_assoc(
    ['a' => 1, 'b' => 2],
    t\map(function ($v) { return [$v[0], $v[1] + 1]; })

assert($result == ['a' => 2, 'b' => 3]);


function stream_iter($stream, $size = 1)

Creates an iterator that reads from a stream using the given $size argument.

$s = fopen('php://temp', 'w+');
fwrite($s, 'foo');

// outputs: foo
foreach (t\stream_iter($s) as $char) {
    echo $char;


// outputs: fo-o
foreach (t\stream_iter($s, 2) as $char) {
    echo $char . '-';


function to_traversable($value)

Converts an input value into something this is traversable (e.g., an array or \Iterator). This function accepts arrays, \Traversable, PHP streams, and strings. Arrays pass through unchanged. Associative arrays are returned as iterators that yield arrays where each value is an array that contains the key of the array in the first element and the value of the array in the second element. Iterators are returned as-is. Strings are split by character using str_split(). PHP streams are converted into iterators that yield a single byte at a time.


function is_traversable($coll)

Returns true if the provided $coll is something that can be traversed in a foreach loop. This function treats arrays, instances of \Traversable, and stdClass as iterable.


function reduce(callable $fn, $coll, $accum = null)

Reduces the given iterable using the provided reduce function $fn. The reduction is short-circuited if $fn returns an instance of Reduced.


Composable algorithmic transformations in PHP







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