[Haskell-cafe] Re: I just don't get it (data structures and OO)

[Christopher Lane Hinson]

> Let's say i have a deep nested data structure.
> Universe containing galaxies, containing solar systems, containing
> planets, containing countries, containing inhabitants, containing
> ...whatever.

Oh.  I had /exactly/ this problem.  If you use separate types (i.e. a 
newtyped integer, acting kindof like a pointer) to represent (1) the 
identity and (2) the state of the object, you can use a separate data 
structure that remembers which object is inside which other object.

http://www.downstairspeople.org/darcs/unstable/roguestar-engine/src/InsidenessMap.hs

Where "InsidenessMap a b c" represents a relationship where b's are inside 
a's, and b's have a state of c.  Then, you need to declare a separate 
InsidenessMap for each possible relationship, but this ensures that you'll 
never put a galaxy inside a solar system.  Or you can make 'a' be a 
reference to any type of object; there are options.

But, you just update this structure once, with no thought of recursively 
updating a heterogenous tree of data.

It may not be the best solution, but if I knew of something better I
would be using the better thing instead.

--Lane