I agree with you that it might be nice if you could actually rely on it, but OTOH I have never personally encountered an error caused by a non-boolean predicate.
However, it's not only nonzero? or defined? that don't return boolean values. see http://news.ycombinator.com/item?id=5074676 for more examples. If you read through the core libs documentation you'll find more examples. You just cannot rely on predicates returning true/false in all cases, so you either have to learn not to rely on it at all or learn every example where it doesn't. So just don't rely on it.
Comments
I agree with you that it might be nice if you could actually rely on it, but OTOH I have never personally encountered an error caused by a non-boolean predicate.
However, it's not only nonzero? or defined? that don't return boolean values. see http://news.ycombinator.com/item?id=5074676 for more examples. If you read through the core libs documentation you'll find more examples. You just cannot rely on predicates returning true/false in all cases, so you either have to learn not to rely on it at all or learn every example where it doesn't. So just don't rely on it.