When would I use boxes?

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You may be surprised to learn that set! is implemented using boxes. That is, code such as:

(let ([a 1])
  (set! a 2)
  (+ a 1))

Is translated into:

(let ([a (box 1)])
   (set-box! a 2)
   (+ (unbox a) 1))

You will often hear that mutable code is slower in Racket, and this is the reason why: every time you use set! the compiler has to box the variable and unbox it everywhere it is used -- this severely reduces the opportunities for optimizations.

As for box-cas! , a lot of synchronization mechanisms can be implemented using a compare-and-swap operation -- most processors provide a single machine instruction for this.

Boxes are another "under-the-hood" feature, less useful to the application programmer, but useful to language developers.

For a related discution, see What are examples of use-cases for continuations?

Alex.

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