What is meant by 'first class object'?

Question

In a recent question, I received suggestions to talk on, amongst other things, the aspect of JavaScript where functions are \'first class\' objects. What does the \'first c

Answer 1

More complete approval of Strachey-Sussman-Abelson's formulation. So if your language supports such a construct then you've got a function as a first-class language :)

var men = function (objectOfAdmiration) {
  return objectOfAdmiration();
};
men.isSweetHeart = true;

var women = function (objectOfAdmiration) {
  return objectOfAdmiration();
};
women.isSweetHeart = true;

var aliens = function (objectOfAdmiration) {
  return objectOfAdmiration();
};

function like(obj){
  if (obj.isSweetHeart) {
      return function (){ return "Holy TRUE!"}; 
  }
  else {
      return function (){ return "Holy CRAP!"};
  }
}

alert("Men like women is " + men(like(women))); // -> "Holly TRUE!"
alert("Women like men is " + women(like(men))); // -> "Holly TRUE!"

alert("Men like aliens is " + men(like(aliens))); // -> "Holly CRAP!"
alert("Aliens like women is " + aliens(like(women))); // -> "Holly TRUE!" :)

//women(like(aliens)); //  Who knows? Life is sometimes so unpredictable... :)

In short, anything is a first-class object if it acts in the language as a state manipulation sort of object or type of object. Simply something you can operate on and pass around statements and evaluate in expressions at the same time. Or even shorter: when you can think of a function as an object that can be additionally invoked.

Answer 2

To quote Wikipedia:

In computer science, a programming language is said to support first-class functions (or function literal) if it treats functions as first-class objects. Specifically, this means that the language supports constructing new functions during the execution of a program, storing them in data structures, passing them as arguments to other functions, and returning them as the values of other functions.

This page also illustrates it beautifully:

Really, just like any other variable

• A function is an instance of the Object type
• A function can have properties and has a link back to its constructor method
• You can store the function in a variable
• You can pass the function as a parameter to another function
• You can return the function from a function

also read TrayMan's comment, interesting...

Answer 3

JavaScript functions are first-class functions meaning functions and objects are treated as the same thing. Functions can be stored as a variable inside an object or an array as well as it can be passed as an argument or be returned by another function. That makes function "first-class citizens in JavaScript"

JavaScript uses literal notation syntax which makes it hard to fully grasp the fact that in JavaScript functions are objects.

For example..

var youObj1 = new Object();
// or
var youObj1 = {};

both declerations are equivalent. By using new we are calling the constructor function of an Object. Also by using {} (JavaScript shortcut called literals) we are calling the construction function of an Object. {} is just a shorter representation for instantiating the constructor.

Most languages uses new keyword to create an object, so lets create a JavaScript object.

var myFunction = new Function("a",  "b", 'return a_b');

As you see we created an object name function.

Creating same object name function using JavaScript function expression..

var myFunction = function myFunction(a,b) {
    return a+b;
}

Here we go we just created a object name function.

Answer 4

i guess when something is first class in a language, it means that it's supported by its syntax rather than a library or syntactic sugar. for example, classes in C are not first class

Answer 5

It means that functions are objects, with a type and a behaviour. They can be dynamically built, passed around as any other object, and the fact that they can be called is part of their interface.

Answer 6

Simple test. If you can do this in your language (Python as example):

def double(x):
    return x*x

f = double

print f(5) #prints 25

Your language is treating functions as first class objects.