Re: [Haskell-cafe] Why 'round' does not just round numbers ?

Ketil Malde wrote:

Janis Voigtlaender writes:

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Since just about every floating point operation involves some sort of loss of precision, repeated rounding is a fact of life.

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Of course. But that was not the point of the discussion...

Well, allow me to contribute to keeping the discussion on topic by stating that when I was in school, I was taught to round up. Now if you will excuse a momentary digression:

The point *I* wanted to make is that we have two qualitatively different rounding modes: round up on 0.5, or round to even (or randomly, or alternating), and they make sense in different contexts.

Doing computations with fixed precision, you keep losing precision, and rounding bias accumulates - thus the need to use some non-biased rounding.

Doing (small scale) calculations on paper, you can avoid repeated rounding, and only round the result. In which case rounding up is okay, you don't introduce the amount of bias as with repeated rounding. And if your input happens to be truncated, rounding up becomes the right thing to do.

Of course, Haskell should discard the rather tasteless IEEE754 crud, and do its calculations on infinite streams of digits. Then, rounding upwards after 'take'ing a sufficient amount of decimals will be the right thing to do.

Yes, that all makes sense. And I did not intend to cut you short. -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de