On Wed, Dec 28, 2011 at 2:44 PM, Heinrich Apfelmus
The beauty of the IO monad is that it doesn't change anything about purity. Applying the function
bar :: Int -> IO Int
to the value 2 will always give the same result:
bar 2 = bar (1+1) = bar (5-3)
Strictly speaking, that doesn't sound right. The "result" of an IO operation is outside of the control (and semantics) of the Haskell program, so Haskell has no idea what it will be. Within the program, there is no result. So Int -> IO Int is not really a function - it does not map a determinate input to a determinate output. The IO monad just makes it look and act like a function, sort of, but what it really does is provide reliable ordering of non-functional operations - invariant order, not invariant results. To respond to original post: no language that supports IO can be purely functional in fact, but with clever design it can mimic a purely functional language. Cheers Gregg