Hello zaxis, Thursday, October 22, 2009, 11:28:14 AM, you wrote:
aaa <- newIORef ([]::[(Int,Int)]) writeIORef aaa [(1,1),(2,2),(3,3)]
then if i want to change aaa to [(1,1),(2,222),(3,3)] , what's the best way ? re-write aaa is not permitted.
it's the only way. in Haskell, you have *immutable* values. aaa is a reference to immutable value. you can mutate reference so it will point to another immutable value but you cannot change this value. there are two ways to make aaa==[(1,1),(2,222),(3,3)]. first, you can apply function to whole value: value <- readIORef aaa writeIORef aaa (f value) second, you may create list of IORefs, tuple of IORefs and so: a <- newIORef (1,1) ... let aaa = [a,b,c] now aaa is a immutable list of mutable IORefs. of course, you can create IORef pointing to list of IORefs too -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com