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- Date: Mon, 23 Nov 1992 20:00:16 -0500
Any language construct can be abused. While this consideration has effected
many language designs, I have yet to see one that prohibits hard to read, hard
to maintain code yet! :-) This is a valid consideration, though, although I
might be faster to use it as a reason to leave out multiple-inheritance than
functional condition handling.
|I think that it's extremely unlikely that we would reconsider this decision
|at this time. There are other solutions to the problem, and there are
|plenty of other problems in Dylan which don't yet have existing solutions.
I can understand this. It is difficult to do cleanly, and would require
revision of method-dispatch (IMHO) to do it right. Some would say this was
Since #none (or some equivalent bottom value) is not going to be provided in
Dylan, I have a proposal which might satisfy a large number of those concerned
about the use of #f. Of course, I expect some people will have problems with
parts of this, so I'll try to make it modular in the hopes that it can serve
as a basis for resolving what concerns remain about the current handling of
#f as a bottom value.
1) Provide a <boolean> class, and let people explicitely cast to it
for the 'anything but #f is true' cases.
2) Provide a special-form predicate for testing if a bottom condition
is being returned. I say a special form since it needs to be given
a parameter that may not normally evaluate without causing an error.
Possible names (and semmantics for which cases return #t and #f)
specified?/unspecified? (I like these, personally)
3) Use this special form in the places where #f is currently supplied
as a bottom value. If desired, multiple special forms could be
given (along the vein of slot-initialized?), or all these could be
unified under a single name. In all other cases, an attempt to
access these values should result in an exception.
These places where #f is currently supplied include: bind, optional
parameters, and where else?
What do people think of this? Is it a fair compromise?
I don't want Dylan to contain hygenic problems. If the designers of Dylan have
found #none to be one, I think it should be abandoned (at least in 'Dylan'),
but in order to do so hygenically we need some way to avoid LISPs 'nil [or #f]
is either a bottom value or a real value.' What do _you_ think of this, alms?