From: user5857@newsgrouper.org.invalid   
      
   EricP  posted:   
      
   > Terje Mathisen wrote:   
   > > Anton Ertl wrote:   
   > >> Terje Mathisen  writes:   
   > >>   
   > >>> This would have simplified all sorts of array/matrix sw where both   
   > >>> errors (NaN) and missing (None) items are possible.   
   > >>   
   > >> In what ways would None behave differently from SNaN?   
   > >   
   > > It would be transparently ignored in reductions, with zero overhead.   
   >   
   > There is also the behavior with operators - how is it different from xNan?   
   > xNan behaves like an error and poisons any calculation it is in,   
   > which is also how SQL behaves wrt NULL values:   
   >   
   >    value + xNan => xNan   
   >    value * xNan => xNan   
   >   
   > whereas Null is typically thought of as a missing value:   
   >   
   >    value + Null => value?   
   >    value * Null => 0?   
   >   
      
   I think they would want::   
      
        value + xNaN  =>  xNaN   
        value × xNaN  =>  xNaN   
        value / xNaN  =>  xNaN   
        xNaN  / value =>  xNaN   
      
   Where the non-existent operand is treated as turning the calculation   
   into a copy of the xNaN. Some architectures put a payload into the   
   xNaN such as a 3-bit code for why the xNaN was created, others also   
   IP to help identify the instruction the xNaN first occurred.   
      
   > It could also have different operator instruction options that select   
   > different behaviors similar to rounding mode or exception handling bits.   
   > All those option bits would take up a lot of instruction space.   
   >   
   >   
      
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