A pure function does not need to be idempotent. For example f(x) = x * 2 is a pure function, but applying it twice multiplies by four instead of two. In comput

A pure function does not need to be idempotent. For example f(x) = x * 2 is a pure function, but applying it twice multiplies by four instead of two. In computer science, another sense of the word idempotent has arisen, which relates to operations on a mutable identity. In this sense, you might say "UPDATE table SET col1=0 WHERE id=25" is an idempotent operation on databases: applying it to a database in some state results in a new database state, but applying it again to that new database state results in that same database state again (this is the sense in which HTTP GET requests are idempotent, for example). An IMMUTABLE function is a pure function, at least with respect to the database, since it cannot mutate the database which is the side effect we care about. It is not necessarily idempotent: given that f is an IMMUTABLE function, and f(2) == 4, I can't say for sure that f(4) == 4.