Re: [Haskell-cafe] On the purity of Haskell

On Thu, Dec 29, 2011 at 07:19:17AM -0600, Gregg Reynolds wrote:

On Wed, Dec 28, 2011 at 2:44 PM, Heinrich Apfelmus wrote:

...

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.

Not only strictly speaking. In practice too: bar _ = do s <- readFile "/tmp/x.txt" return (read s) Once you're in a monad that has 'state', the return value doesn't strictly depend anymore on the function arguments. At least that's my understanding. regards, iustin