Kevin Jardine wrote:
I think that we are having a terminology confusion here. For me, a pure function is one that does not operate inside a monad. Eg. ++, map, etc.
Ivan Miljenovic has already given a good response, to which I'll only add this: I suspect that your idea of the meaning of purity came from over-generalization from the IO monad. IO actions may be impure, but that's not true of all other monad types. (Most are actually pure.) Really, the IO monad is a horrible exception to normal monadic behavior, and in an ideal world it should only be introduced as a special case after gaining a good understanding of monads in general. Of course in practice, people like their programs to be able to do I/O, so the IO monad ends up being one of the first things learned. It's a bit like teaching a new carpenter about the concept of "tools", and then starting them out with a chainsaw, leading to the natural conclusion that tools are loud, insanely dangerous things. Anton