[bobbylarrybobby]

> you (and the compiler) will know and agree upon whether the same input will yield the same output

What exactly does this mean? Haskell has plenty of non-deterministic functions — everything involving IO, for instance. I know that IO is non-deterministic, but how is that expressed within the language?

[mrkeen]

Functions which use IO are tagged as such in the type system. IO can call non-IO, but not vice-versa.