Page "Abstract data type" Paragraph 6

In the " imperative " view, which is closer to the philosophy of imperative programming languages, an abstract data structure is conceived as an entity that is mutable — meaning that it may be in different states at different times.

Some operations may change the state of the ADT ; therefore, the order in which operations are evaluated is important, and the same operation on the same entities may have different effects if executed at different times — just like the instructions of a computer, or the commands and procedures of an imperative language.

To underscore this view, it is customary to say that the operations are executed or applied, rather than evaluated.

The imperative style is often used when describing abstract algorithms.

This is described by Donald E. Knuth and can be referenced from here The Art of Computer Programming.