Type: [[Principle]], [[That]], [[Verb]], [[Should]], [[Not]], [[Know]], [[Where]], [[You]], [[Do]], [[Verb]] - Functions are verbs. - Where they happen is a preposition. - Verbs should not attempt to hard-code the place they will happen. - IO is about where things come from and where they go. - The i in io means "where data comes from" - this isn't a normal function input. - The o in io means "where data goes to" - this isn't a normal function output. - Try to decouple your functions from assumptions about where data came from.