[Haskell-cafe] On the purity of Haskell

[Gregg Reynolds]

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.

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