What is Currying?

Being a (fairly) little-known concept in programming, currying seems worthy of its own blog post.

In general

Simply put, currying is where:

Methods may define multiple parameter lists. When a method is called
with a fewer number of parameter lists, then this will yield a function
taking the missing parameter lists as its arguments.
— scala-lang.org

In PHP

Currying is not a built-in feature of PHP. However, as outlined in quite some depth in this excellent post over at Zaemis’ blog, it is possible to emulate much of the functionality of currying in PHP (or at least, in PHP v5.3 and above) through use of closures.

An example

My favourite example from Zaemis’ blog post (which I recommend you read) is as below:

  $userPercent = 0.5;   $userList = array_filter($percentVowels,       function($percent) use ($userPercent) {           return ($percent >= $userPercent);       });
It showed an anonymous function being used with array_filter() to filter an array. The array is filtered based on a dynamic value, and a closure is used to “inject” the threshold rather than using a global statement. The same could also be accomplished with currying.

The problem is array_filter() expects a callback function that accepts one argument–the current array element. Currying will allow us to prepare the function with the sorting threshold, and the intermediate function can be used as the callback.

  function callback($userPercent) {       return function($percent) use ($userPercent) {           return ($percent >= $userPercent);       };   }   $userList = array_filter($percentVowels, callback(0.5));