What Makes an Object First-Class in Programming Languages?

First-Class Objects in Programming Languages

Within programming languages, the concept of "first-class objects" encompasses entities that enjoy unrestricted usage and rights. Unlike second-class objects, which have limitations, first-class objects possess the ability to be dynamically created, modified, and passed around as parameters or return values.

First-class objects fulfill a comprehensive set of criteria, including but not limited to:

• Expressible as anonymous literal values
• Storable in variables and data structures
• Possessing an intrinsic identity distinct from their name
• Comparable for equality
• Passable as arguments to functions
• Returnable as function results
• Constructible and destructible at runtime
• Printable and readable
• Transmissible across distributed processes
• Storable external to running processes

First-Class Functions and Objects

In languages such as Python, both functions and objects are considered first-class entities. Functions can be dynamically created, passed as arguments, and returned as results. For example, in JavaScript:

// Function that takes a number and returns an approximate derivative
function makeDerivative(f, deltaX) {
  return function(x) {
    return (f(x + deltaX) - f(x)) / deltaX;
  };
}

In contrast, in C , functions themselves are not first-class objects, although overriding the '()' operator or using function pointers can simulate first-class functions. Classes are not first-class either, while instances of classes are.

First-Class Entities and Everything Being an Object

The statement that "everything is an object" in Python, while partially true, does not necessarily imply that everything is first-class. While most entities in Python are objects, they may not meet the full criteria for first-class objects. Nonetheless, the language strives to make as many entities as possible first-class, giving them a high degree of flexibility and expressiveness.

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