There is a variation on what Matthias said, which is when you need a shared structure whose value changes but whose representation may contain immutable constants.
Consider a queue to which you may need to store references in multiple places (including, say, a hash-table). The queue may itself be a functional value, and the empty queue may well be represented with `empty'. However, empty is immutable, so you can't store the empty in the hash-table, change the content of the queue, and expect the hash-table to notice the change. In such cases, you would use the box to represent the queue, with the box's content representing the queue's content. The hash table refers to the box, so updates to the box are reflected in the hash table. If you work it out, you realize that at some point this involves the by-reference pattern that Matthias referred to. However, its point is not really to implement by-reference, but a different concept (that of shared references to changing structures that may contain immutable and transient values). In the above example, you could instead make the queue itself be a mutable structure, and store a special kind of value that reflects emptiness. This has two downsides: 1. You've made your queue mutable, which may pollute code, weaken invariants, and reduce performance. 2. Now you have a special kind of queue that says "I am not a queue". Traditional programming textbooks for languages like Pascal and C++ are full of these kinds of monstrosities and the very tricky problems of programming correctly with them. Crack open any "circular queue" chapter and feel the pain. The other, cleaner, option is to create a mutable structure just as a placeholder: a single-field mutable structure whose only purpose is to say "the queue's content is that value over there, I'm just the shareable reference to it". Well, you could define that structure, or use the one that's already provided for you: the box. (It's just like you said: when you have a small number of values to return, you don't bother creating a new structure, you just use `values'.) Shriram _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
