Re: Min and Max
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- Subject: Re: Min and Max
- From: Rici Lake <lua@...>
- Date: 2005年8月27日 12:39:35 -0500
On 27-Aug-05, at 11:15 AM, Adrian Sietsma wrote:
i agree, but i still like Roberto's option of comparison operators
returning the mathing object, instead of "true" - that matches and/or
behaviour.
I think it has its charm; I was playing with that a while ago when I
was first thinking about max and min operators.
The fact that you can't define __lt on booleans is a minor flaw, but it
leads to the issue that you cannot define == to work the same way. That
is, a == b cannot return a if the objects are equal, because that would
then mean that
nil == nil
and
false == false
were nil and false, respectively.
So == cannot be consistent with < and <=, which is a slightly more
serious blemish.
The most serious issue, which also plagues C programs and is the reason
that gcc chose to implement min and max operators (<? and >? if I
recall correctly) is that you cannot open code min and max without
evaluating the arguments twice. So there is nothing wrong with
a < b and a or b
But something desperately wrong with
f(a) < f(b) and f(a) or f(b)
This would not be a problem with
f(a) min f(b)
and is probably the single best argument for having the operators.
Finally, I prefer min and max because there are types where min and max
cannot be defined in terms of < or <=. In those cases, I would prefer
to have an operator backed by a metamethod; failing that, I'm happy to
use a function backed by a metamethod, which is what I actually do in
practice, regardless of speed issues, because I like the
self-documentation.
[Note]
Depending on what you think numeric min and max should return if one of
the arguments is a NaN, numeric min and max can also be difficult to
properly implement in terms of <. However, I was thinking more of
partial order relationships like subset. My preference is that min of
two sets is their intersection and max is their union; that cannot be
defined in terms of a subset predicate. If you prefer the definition
that min/max of two sets returns nil if the sets are not comparable
then you can, but it looks different and arguably suffers from an
efficiency problem:
a <= b and a or b <= a and b
I'm going to shut up about this issue now. :)