Brandon Allbery via RT wrote: >Erm. Isn't Nil a silent Failure? It may well represent such a thing, but it is also a reified object. Putting objects into sets is an operation that's applicable to any kind of object, and which (for comparison) does in fact work on objects of the Failure class. Expecting an object to be propagated doesn't assert that it doesn't represent failure; rather, it's just embracing the reification. The great benefit of reification is that the object can be processed in all the familiar ways that are used on ordinary value objects.
If you want to impose a restriction that failure objects can't go into some of the places I've tried to put Nil, then in some cases that would be a sensible decision. It would make perfect sense to restrict Range endpoints, for example, but it doesn't make any sense to restrict the domain of Sets. But in any case, wherever such a restriction is desirable it should be enforced by signalling an exception: silently substituting a different value sucks. And if your logic for rejecting Nil is that it represents failure, then presumably the same logic would lead you to reject objects of the Failure class too, and maybe also Exception. -zefram
