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On 2-Mar-05, at 2:35 PM, Mark Hamburg wrote:
The difference between math.nan and nil is that you can perform arithmetic 
on math.nan.
That's currently true but it could easily be changed; simply substitute a NaN for nil in arithmetic computations. However, there is a more profound difference: 
  nil == nil ==> true
  NaN == NaN ==> false
Were it not for that, I would advocate making nil and NaN the same thing.
I could live with breaking that particular IEEE-749 oddity but there are actually good reasons for it, or at least reasons some people think are good. Worth thinking about, though.
I could personally also live with nil having the semantics described below, rather than create yet another special case. I don't think it would be awful for nil[x] to be nil rather than throwing an error; however, there are good reasons for throwing the error as well (or at least reasons some people think are good). I do not believe that it would be a good idea to silently return a different special value, though: either throw an error or return nil. 


Actually, it would be sort of cool to have a NaN equivalent for other
operations. Call it NotAValue and perhaps it's actually the same as NaN. For 
example:

    x = nil
    y = x.field

y is now equal to NotAValue.
That would, however, change the semantics with respect to error detection. 

Mark