[Haskell-cafe] On the purity of Haskell

[Iustin Pop]

On Thu, Dec 29, 2011 at 07:19:17AM -0600, Gregg Reynolds wrote:
> On Wed, Dec 28, 2011 at 2:44 PM, Heinrich Apfelmus
> <apfelmus at quantentunnel.de> 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