On 2007-11-14, at 19:22 EST, Graydon Hoare wrote: > (As far as I can tell -- not being a dylan hacker -- dylan doesn't > even > go as far as having a global sentinel type like nil)
The Dylan equivalent of a nullable type is a union of your type with a singleton that acts as the sentinel. Most often a singleton of the boolean false value is used. So there is a macro `false-or(<type>)` that expands to `type-union(<type>, singleton(#f))`. This works for any type other than a nullable boolean. Because any value other than #f coerces to true in a boolean context, #f is very similar to nil. I've never known anyone to need a nullable boolean. (Although I have seen whacky es3 code that uses true/false/null as a sloppy 3-valued enumeration -- with attended bug reports when null is passed expecting it to behave like false.) I must say, coming from Dylan, es3's undefined _and_ null seem like overkill... but we're stuck with them now! _______________________________________________ Es4-discuss mailing list Es4-discuss@mozilla.org https://mail.mozilla.org/listinfo/es4-discuss
