First Class Functions

In most programming languages, type, object, entity, or value is considered first-class object and hence can be returned from a function, modified or assigned to a variable. If a programming language considers functions as first-class object, then the programming language is said to have first-class functions. Hence, in such programming languages (like Python, JavaScript and others), a function can be assigned to variable, modified or returned from a higher order function. Usually such languages also supports anonymous functions.

First-class function is one of the necessity for functional programming; hence all functional programming languages (such as Haskell, Scala, Clojure etc.) have first-class functions.

Example

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p = print
p("Hi! Check out this text.")
p.__call__("Hi! Check out this text.")

Here print function is assigned to variable p and then called using syntax p(...) similar to print(...). It should be noted that the last line is specific to python only. In python almost every thing is a object. Functions are special object that __call__ data structure function (or member) defined for them. This __call__ member function have all the logic for the parent function; hence calling print(...) is same as print.__call__(...).