Transducers are a type of reducing function that take in a reducing function and give back another reducing function. They allow you to compose functions together in a chain or pipeline structure to quickly, easily and efficiently transform data. In PHP, we have the mtdowling/transducers library, built off the basis of the idea of Clojure's transducer library.

1. The Art of
Transduction
ZendCon 2016 - Oct 19

2. PHP Toolbox

3. The Art of Foreach

4. A Silly Example
$grades = [98, 77, 100, 62, 90, 95, 82, 68];
$sum = 0;
$count = 0;
foreach ($grades as $grade) {
$sum += $grade;
$count ++;
}
echo "Avg: " . $sum / $count . "n";

5. A Less Silly Example
$grades = [98, 77, 100, 62, 90, 95, 82, 68];
$sum = 0;
foreach ($grades as $grade) {
$sum += $grade;
}
echo "Avg: " . $sum / count($grades) . "n";

6. Less Sillier
$grades = [98, 77, 100, 62, 90, 95, 82, 68];
$sum = array_sum($grades);
echo "Avg: " . $sum / count($grades) . "n";

7. Grade Buckets
$grades = [98, 77, 100, 62, 90, 95, 82, 68];
$gradeBuckets = ["A" => 0, "B" => 0, "C" => 0, "F" => 0];
foreach ($grades as $grade) {
switch (true) {
case $grade >= 90:
$gradeBuckets['A']++;
break;
case $grade >= 80:
$gradeBuckets['B']++;
break;
case $grade >= 70:
$gradeBuckets['C']++;
break;
default:
$gradeBuckets['F']++;
}
}

8. Phone Tree
function getNonManagerNumbers(Employee ...$employees)
{
$phoneNumbers = [];
foreach ($employees as $employee) {
if ($employee->isManager()) {
continue;
}
$phoneNumbers[] = $employee->getPhoneNumber();
}
return $phoneNumbers;
}

9. Nav Builder
function buildNav($links)
{
$html = '<ul>';
foreach ($links as $link) {
$html .= '<li><a href="' .
$link->getUrl() . '">' .
$link->getTitle() . '</a></li>';
}
$html .= '</ul>';
return $html;
}

10. Reduce
function reduce(array $items, callable $callback,
$initial)
{
$carryOver = $initial;
foreach ($items as $item) {
$carryOver = $callback(
$carryOver,
$item
);
}
return $carryOver;
}

11. Functional Programming
• Map
• Filter
• Reduce

12. Grades as Reduce
$avg = reduce(
$grades,
function ($carry, $item) {
$total = $carry['count'] * $carry['avg'] + $item;
$carry['count']++;
$carry['avg'] = $total / $carry['count'];
return $carry;
},
['count' => 0, 'avg' => 0]
)['avg'];

13. Phone Tree as Reduce
function getNonManagerNumbers($employees)
{
return reduce(
$employees,
function ($numbers, $employee) {
return $employee->isManager() ?
$numbers :
array_merge(
$numbers,
[$employee->getPhoneNumber()]
);
},
[]
);
}

14. Nav Builder as Reduce
function buildNav($links)
{
return '<ul>' .
reduce(
$links,
function ($html, $link) {
return $html . '<li><a href="' .
$link->getUrl() .
'">' . $link->getTitle() .
'</a></li>';
}
)
. '</ul>';
}

15. What About
Transducers?

17. What are
Transducers?

18. Collection Pipeline
$numbers = collect($employeeService->getAllEmployees())
->filter(function ($employee) {
return ! $employee->isManager();
})->map(function ($employee) {
return $employee->getPhoneNumber();
});

19. Installation
composer require mtdowling/transducers

20. Phone Tree as Transducer
use Transducers as t;
$employees = (new EmployeeService)->getAllEmployees();
$getNonManagerPhones = tcomp(
tfilter(function ($employee) {
return ! $employee->isManager();
}),
tmap(function ($employee) {
return $employee->getPhoneNumber();
})
);
$numbers = tto_array($getNonManagerPhones, $employees);

21. The Data
Name Number Manager
Bob 303-555-1212 Yes
Sue 303-555-1234 No
Barb 303-555-1111 No
Spongebob 303-555-1001 Yes
Arnold 303-555-1313 No

22. Collection Data Pipeline
Name Number Manager
Bob 303-555-1212 Yes
Sue 303-555-1234 No
Barb 303-555-1111 No
Spongebob 303-555-1001 Yes
Arnold 303-555-1313 No
Filter
Name Number Manager
Sue 303-555-1234 No
Barb 303-555-1111 No
Arnold 303-555-1313 No
Map
Number
303-555-1234
303-555-1111
303-555-1313

23. Transducer Data Flow
Name Number Manager
Bob 303-555-1212 Yes
Sue 303-555-1234 No
Barb 303-555-1111 No
Spongebob 303-555-1001 Yes
Arnold 303-555-1313 No
Number
303-555-1234
303-555-1111
303-555-1313
filter
map
NO

24. Transducer Data Sources
• Anything that you can use foreach on
• Arrays
• Iterators
• Traversables
• Generators

25. Transducer Output
• Eager
• transduce()
• into()
• to_array()
• to_assoc()
• to_string()

26. Transducer Output
• Lazy
• to_iter()
• xform()
• stream filters

27. A Bigger Example
• Incoming TSV, but should be CSV
• Date format is wrong
• Names are not capitalized
• We need days from or until birthdate, for reasons

28. Transformer
use transducers as t;
/* SNIP Definition of the functions used below */
$transformer = tcomp(
tdrop(1), // Get rid of the header
tmap($convertToArray), // Turn TSV to Array
tmap($convertToDate), // Change to DateTimeImmutable Object
tmap($addDaysFromBirthday), // Date math
tmap($fixDateFormat), // Format DateTimeImmutable
// to Y-m-d string
$fixNames, // Capitalize names
);

29. Convert TSV to Array
$convertToArray = function ($tsvRow) {
$arrayRow = explode("t", $tsvRow);
$columns = ['id', 'first', 'last', 'dob'];
return array_combine($columns, $arrayRow);
};

30. What it does
42 t david t stockton t 1/1/1999
[
'id' => 42,
'first' => 'david',
'last' => 'stockton',
'dob' => '1/1/1999'
]

31. Convert Date to Object
$convertToDate = function ($row) {
$date =
DateTimeImmutable::createFromFormat(
'm/d/Y',
trim($row['dob']
)
);
$row['dob'] = $date;
return $row;
};

32. Add Days from Birthday
$now = new DateTimeImmutable();
$thisYear = $now->format('Y');
$addDaysFromBirthday = function($row) use ($now, $thisYear) {
$dob = $row['dob'];
$birthday = DateTimeImmutable::createFromFormat(
'Y-m-d',
$dob->format("$thisYear-m-d")
);
$timeUntilBirthday = $now->diff($birthday);
$row['time_until_bday'] = $timeUntilBirthday->invert
? $timeUntilBirthday->format('%m months, %d days ago')
: $timeUntilBirthday->format('%m months, %d days');
return $row;
};

33. Fix Date Formatting
$fixDateFormat = function ($row) {
$row['dob'] = $row['dob']->format('Y-m-d');
return $row;
};

34. Uppercasing Names
$capFirst = function ($row) {
$row['first'] = ucfirst($row['first']);
return $row;
};
$capLast = function ($row) {
$row['last'] = ucfirst($row['last']);
return $row;
};

35. Function to Build a Function
// Function to return a function
$ucField = function($field) {
return function ($row) use ($field) {
$row[$field] = ucfirst($row[$field]);
return $row;
};
};

36. Functionally Functional
$mungeField = function ($field, $munger) {
return function ($row) use ($field, $munger) {
$row[$field] = $munger($row[$field]);
return $row;
};
};

37. Name Capitalization
$fixNames = tcomp(
tmap($ucField('first')),
tmap($ucField('last'))
);
$fixNamesMunge = tcomp(
tmap($mungeField('first', 'ucfirst')),
tmap($mungeField('last', 'ucfirst'))
);

38. Revisit Transformer
use transducers as t;
/* SNIP Definition of the functions used below */
$transformer = tcomp(
tdrop(1), // Get rid of the header
tmap($convertToArray), // Turn TSV to Array
tmap($convertToDate), // Change to DateTimeImmutable Object
tmap($addDaysFromBirthday), // Date math
tmap($fixDateFormat), // Format DateTimeImmutable
// to Y-m-d string
$fixNames, // Capitalize names
);

39. Where We Are
Data converted from TSV to Array

40. Where We Are
function array_to_csv($data)
{
$fh = fopen('php://temp', 'rw');
fputcsv($fh, $data);
rewind($fh);
$csv = stream_get_contents($fh);
fclose($fh);
return $csv;
}

41. Reuse
$transformToCsv = tcomp(
$transformer,
tmap('array_to_csv')
);

42. Data Source
$fh = fopen(__DIR__ . '/ZendconData.tsv', 'r');
$reader = function () use ($fh) {
while ($row = fgets($fh)) {
yield $row;
}
};

43. Output
$write = fopen(__DIR__ . '/../data/Zendcon.csv', 'w');

44. TRANSFORM!!!11!
tinto($write, $reader(), $transformToCsv);

45. Included Transducer
Functions
• map($f) - Apply $f function to each value in a
collection
• filter($predicate) - If predicate returns true, retain
the value, otherwise discard
• remove($predicate) - Removes items that satisfy the
predicate function
• cat() - Concatenates items from nested lists

46. More Included Functions
• partition($size) - Splits the source into arrays of the
specified size
• partition_by($predicate) - Splits input into arrays
when value returned by $predicate changes
• take($n) - Takes $n items from the collection
• take_while($predicate) - Takes items from the
collection while the $predicate is true

47. Even Moar!
• take_nth($n) - Takes every $n values from the
collection
• drop($n) - Drops $n items from the start of a
sequence
• drop_while($predicate) - Drops items from the
collection as long as $predicate returns true
• replace(array $map) - Replaces values in the
sequence according to the $map

48. Ermegerhd, even more?!
• keep($f) - Keeps items when $f does not
return null
• keep_indexed($f) - Returns the non-null
results of calling $f($index, $value)
• dedupe - Removes values that are the same
as the previous value in an ordered
sequence
• interpose($separator) - Adds the separator
between each value in a sequence

49. Last list, I promise
• tap($interceptor) - "Taps" into the chain, in
order to do something with the intermediate
result. Does not change the sequence
• compact() - Trims out all "falsey" values from
the sequence
• words() - Splits input into words
• lines() - Splits input by lines

50. Transducers
• Compose powerful data processing functions
• Interact with streams of data
• Easy to understand
• Simple to test

51. Questions?