[Haskell-cafe] I just don't get it (data structures and OO)
Donald Bruce Stewart
dons at cse.unsw.edu.au
Sun Jun 3 03:34:18 EDT 2007
> Hello all,
> I'm coming from the OO world, and there's something i don't quite
> understand in the way haskellers manipulate data (as all functional
> programmers i guess).
> Let's say i have a deep nested data structure.
> Universe containing galaxies, containing solar systems, containing
> planets, containing countries, containing inhabitants, containing
> Using the OO paradigm, once i get a reference to an inhabitant, i can
> update it quite easily (say by changing it's age), and that's the end of it.
> On the other side, using the functional paradigm, it seems to me that
> the function i use in order to create a _new_ inhabitant with a
> different age will need to have knowledge of the country over it, the
> planet ..and so on up to the universe...as i need to update all these
> structures to reflect the change. This is pretty bad and most probably
> doesn't need to be like this.
> So here I am hoping for you all to give me some pointers on how this is
> done "the functional way".
Nope, its not done like that. You share as much of the original
structure as you can, as a general principle.
Imagine updating a node in a tree by just detaching and reattaching a
/ \ / \
  update node 5  
/ \ with value 7 / \
   *
and share the rest of the structure. Since the rest isn't mutable
anyway, you can share all over.
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