This isn't a strict definition, but I just think of it as turning the usual model inside out. You always have a state that you're modifying, if you're doing fu

This isn't a strict definition, but I just think of it as turning the usual model inside out. You always have a state that you're modifying, if you're doing functional or imperative programming (or a mix). Usually you're "inside" the state, changing it via pokes and prods from within. (Think global/shared state typical of C/C++ programs) In FP you pass around a big ball of state and no modification happens outside of it (there is no global or shared state, or at least access to it is tightly controlled if it's not a pure language). The functions live 'outside' the state, so to speak. Programming this way was, to me, one of the major revelations of learning the functional style.