How do you decide whether to use a library or write your own implementation

Question

Inspired by this question which started out innocently but is turning into a major flame war.

Let\'s say you need to a utility method - reasonably straightforward but no

Answer 1

If it's a trivial function, it's not worth pulling in an entire library.

If it's a non-trivial function, then it may be worth it.

If it's multiple functions which can all be handled by pulling in a single library, it's almost definitely worth it.