On Sat, 29 Apr 2000, Fergus Henderson wrote:
> On 28-Apr-2000, Jan Skibinski <[EMAIL PROTECTED]> wrote:
> >
> > When can I safely cheat haskell compiler/interpreter
> > by pretending that I perform pure computations,
> > when in fact they are not?
>
> That depends on what degree of safety and portability you want.
> If you want the greatest degree of both of those, then currently
> the only safe answer is "never". The Haskell 98 Report does not
> standardize `unsafePerformIO', and so there are no guarantees
> about whether future implementations will have such a function,
> or what it would do, or when it would be safe.
That's the best answer I got. It should be framed.
If my implementation currently cheats a bit but
it works in Hugs, that's my responsibility and
my possible future headaches. Period.
Lennart wrote:
> So, currentSecond is not safe to the kind of compiler optimizations
> that a good Haskell compiler can do.
> You can't make a working currentSecond if you don't involve IO in the
> type, that's just the way it is.
Just out of curiosity: Is your compiler clever enough
to do just what you said? Another words, would this
attached code fail to produce random nonce string (
the idea apparently criticized by Erik, but I do not care
where this came from. It works fine in Hugs-98,
February 2000 release). Humor me please :-)
nonce :: Int -> String
nonce size
= take size (filter isAlpha
(randoms $ mkStdGen (fst $ unsafePerformIO timeFrom1970)))
timeFrom1970 :: IO (Int, Int)
-- you can simulate it somehow, but
-- source code is available to all
-- at www.numeric-quest.com/haskell/bridge/
------------------------------------------------------------
Erik, Frank and Nigel:
I appreciate your answers too. Please try to understand
that I am searching for clear answers about the limits
of usage of "unsafePerformIO". I do not try to
"outsmart compilers".
I often do just that in my other life - rocking a boat
a little. Really! When I take a dinghy for my
first sail, when the weather is warm, wind gusty,
no baggage and children on boat then I drive her
to her limits to gain some confidence of what can
I do with her.
Jan Skibinski
