>>value <- readIORef aaa >>writeIORef aaa (f value) then aaa will *point to* a new value. The original value will be Garbage Collected, right ? BTW, Is [(1,1),(2,2),(3,3)] been regarded as a hash ? If not, what is the best way to change it to [(1,1),(2,2222),(3,3)] in function `f` ?
Bulat Ziganshin-2 wrote: > > 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.zigans...@gmail.com > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > -- View this message in context: http://www.nabble.com/How-can-i-safely-change-the-value-of-specified-key---tp26005244p26006471.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe