If you call 'next' on an IO stream, you always get the same answer, an IO effect which, when executed, would destructively get the next item: next :: Stream

If you call 'next' on an IO stream, you always get the same answer, an IO effect which, when executed, would destructively get the next item: next :: Stream a -> IO a The function part is pure. The `execution` of the resulting `IO a` action (which is completely separate from its pure evaluation) is not.