On 22 Apr 2009, at 10:38, Daniel K. wrote:
Hello,
imagine the following situation: You want to implement e.g.
Dijkstra's algorithm to find a shortest path between nodes u and v
in a graph. This algorithm relies heavily on mutating arrays, so the
type signature would look something like
getDistance :: Graph -> Node -> Node -> IO Int
Not mutating the underlying arrays would probably result in poor
performance. BUT: For a constant graph, the distance between two
nodes stays the same all the time, so in fact getDistance should be
a pure function!
So here is my question: Is there a way to write functions in Haskell
that do some IO internally, but that are guaranteed to be side-
effect free? Of course one would have to make sure that the array
that is mutated inside getDistance must not be accessible from
outside the function.
Is that possible? If not, wouldn't that be desirable? If not, why not?
Either, as Eugene suggested, use the ST monad, as is possible in this
case (and much better than the solution I'm proposing), or if you
*really* can't get out of using IO, use unsafePerformIO. You will
though have to provide several guarantees yourself that the type
system would normally provide for you (hence the unsafe part - it
should really be verifyItsSafeYourselfPerformIO).
Bob
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
