Maybe vs. exception vs. failure thunk in Scheme

It's pretty easy to switch back and forth between representing a function with a "maybe" result type as:

1. returning X or #f;
2. returning X or raising an exception; or
3. returning X and taking a failure thunk

In Scheme, sometimes X might include the value #f. In that case I think representation #3 is the best one, since raising an exception is clunky. You can also combine #2 and #3 and take an optional thunk that by default raises an exception.

This is one case where true unions do cause some problems, so maybe I should have tempered my earlier enthusiasm a little. Still, it's not insurmountable, and this cost needs to be weighed against the price of strictly disjoint unions.