Functional, Declarative, and Imperative Programmin

Question

What do the terms functional, declarative, and imperative programming mean?

Answer 1

Imperative programming: telling the “machine” how to do something, and as a result what you want to happen will happen.

Declarative programming: telling the “machine” what you would like to happen, and letting the computer figure out how to do it.

Example of imperative

function makeWidget(options) {
    const element = document.createElement('div');
    element.style.backgroundColor = options.bgColor;
    element.style.width = options.width;
    element.style.height = options.height;
    element.textContent = options.txt;

    return element;
}

Example of declarative

function makeWidget(type, txt) {
    return new Element(type, txt);
}

Note: The difference is not one of brevity or complexity or abstraction. As stated, the difference is how vs what.

Answer 2

Imperative Programming means any style of programming where your program is structured out of instructions describing how the operations performed by a computer will happen.

Declarative Programming means any style of programming where your program is a description either of the problem or the solution - but doesn't explicitly state how the work will be done.

Functional Programming is programming by evaluating functions and functions of functions... As (strictly defined) functional programming means programming by defining side-effect free mathematical functions so it is a form of declarative programming but it isn't the only kind of declarative programming.

Logic Programming (for example in Prolog) is another form of declarative programming. It involves computing by deciding whether a logical statement is true (or whether it can be satisfied). The program is typically a series of facts and rules - i.e. a description rather than a series of instructions.

Term Rewriting (for example CASL) is another form of declarative programming. It involves symbolic transformation of algebraic terms. It's completely distinct from logic programming and functional programming.