4 ms·

eh!? A pure function is an idempotent function which has no side-effects. For example no logging. You can have an impure idempotent function: a function alwa

eh!? A pure function is an idempotent function which has no side-effects. For example no logging. You can have an impure idempotent function: a function always producing the same output for given input but that is impure because it has side-effects, like logging or sending a heartbeat on the network or whatever. But you cannot have a pure non-idempotent function. Any idempotent function --and hence any pure function-- shall always be giving the same output given some input (and hence can be cached etc.) but the idempotent function can have side-effects (but not side-effects that would modify the "idempotentness" of the function itself.

sirclueless 14y ago
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.