[Haskell-beginners] Question about data structures

[Daniel Fischer]

On Wednesday 24 November 2010 22:12:37, Russ Abbott wrote:
> Cool. I wasn't aware of that notation.  It doesn't quite get to the
> issue though.
>
> The problem I'm concerned about is the need to define y in the first
> place. If one is chasing through a data structure and finds a need to
> change something buried within it, it seems necessary to rebuild
> everything that includes the changed thing.

In general, values are immutable, so you can't "change something buried 
within it". You have to build a new value containing some of the old stuff 
and a new part. Building the new value usually consists mostly of copying a 
couple of pointers (plus building the new part of course), so isn't too 
expensive normally.

You can have mutable values in the IO or (ST s) monads, if you need them.

> That is, I can't change a
> component of somethingNew without creating y. The point is there's
> nothing about x that changed,

The thing with the changed component is not x anymore.

> and there may be nothing about (var1 x)
> that changed, and there may be nothing about var11 . var1 $ x that
> changed, etc. Yet one is apparently forced to keep track of and
> reconstruct all those elements.

The compiler takes care of that.

>
> Another example is to imagine a Tree in which the leaves contain
> "objects." If I want to change a property of one of those leaf objects,

You can't in general, the thing with a different property is a different 
object.

> I am forced to rebuild all the ancestor nodes of that leaf down to
> rebuilding the root.

Yes (well, not you, the compiler does it), except if your tree contains 
mutable objects (IORefs/STRefs for example).

>
> One way to avoid that is for the leaves to refer to their objects
> through a Map. Then changing a leaf object requires only that the value
> associated with key represented by the leaf be (re-)inserted into the
> Map.  The Tree itself need not change at all.

Oh, it will. If you change a Map, you get a new one, thus you get a new 
tree containing the new Map.

>
> But that trick isn't always available.  In the example we are talking
> about we can't make a Map where they keys are the instance variable
> names and the values are their values.  That would seem to do the job,
> but since the values are of different types, we can't create such a Map.
>
> So now what?

Well, what's the problem with the compiler copying some nodes?
Normally, that doesn't cost very much performance, if it does in your case, 
we'd need to know more to suggest the best way to go.

> *
> -- Russ *
>