What is the difference between declarative and imperative language?

Answer 1

Imperative versus Declarative Programming

Assuming that the reader has an intuitive understanding of what an imperative

(or procedural) program is, we approach the topic from the declarative angle.

According to Lloyd "declarative programming involves stating what is to be

computed, but not necessarily how it is to be computed"[32]. Equivalently, in the

terminology of Kowalski's equation [33] 'algorithm = logic + control', it involves

stating the logic of an algorithm (i.e. the knowledge to be used in problem

solving), but not necessarily the control (i.e. the problem-solving strategies).

While the logic component determines the meaning of an algorithm, the control

component only affects its efficiency [33].

Roy and Haridi [34] suggest to use the concept of a state for defining the line

between the two approaches more precisely. Declarative programming is often

referred to as stateless programming as an evaluation works on partial data struc-

tures. In contrast to that, imperative programming is characterized as stateful

programming [34]: a component's result not only depends on its arguments, but

also on an internal parameter, which is called its "state". A state is a collection of

values being intermediate results of a desired computation (at a specific point in

time). Roy and Haridi [34] differentiate between implicit (declarative) state and

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explicit state. Implicit states only exist in the mind of the programmer without

requiring any support from the computation model. An explicit state in a pro-

cedure, in turn, is a state whose lifetime extends over more than one procedure

call without being present in the procedure's arguments. Explicit state is visible

in both the program and the computation model.

Answer 2

In declarative programming, you specify what you want to achieve without explicitly providing the steps to reach that goal, while in imperative programming, you provide step-by-step instructions on how to achieve the desired outcome. Declarative languages focus on the "what" whereas imperative languages focus on the "how".