On 10/20/05, Deane Yang <deane_yang@yahoo.com> wrote:
And if I recall correctly that was the problem with some space mission that went wrong. I just don't think implicit conversion of units would have been the right fix for that problem.
There is nothing logically wrong with the expression at all. If such a mistake was made it wasn't because they mathematically shouldn't add inches and meters, it's because they forgot to convert the units or because they were adding together two unrelated values. If it was the former problem, this is already solved since the conversion is automatic. If it was the latter problem, it, again, has nothing to do with the fact that different units were used, it's because they were adding together the wrong values. Just as I described above, you can have the same problem if both quantities had the same units. The reason why you shouldn't add them together has nothing to do with the units used, it's because there is a higher level abstraction that isn't represented. You can either choose to encapsulate the two quantities with separate types to disallow the conversion, or you can just add quantities together only where you should in your particular project. Forcing explicit conversion does not "fix" any of this. Adding a length with another length always makes sense. It's a higher level of abstraction entirely specific to individual project which might make you believe otherwise. Forcing explicit conversions does not solve this. On 10/20/05, Deane Yang <deane_yang@yahoo.com> wrote:
Is it really sometimes necessary or desirable to write computation code that mixes different units for the same dimension? Is anyone patient and indulgent enough to explain to this naive mathematician why?
I am standing on a stool that is 3 feet high. I am 1.6 meters tall. Adding the heights together describes how high above the ground I reach when standing on the stool. The same applies to mass: my mass plus the mass of a bowling ball, or anything else you can think of. Units are merely implementation details to their dimension and should not directly affect the way expressions combining them are written. Special cases occur, especially in the "empty classification" (radians to steradians should not be convertible), however this is specific to individual unit types, not general to the entire system. Disallowing adding together two different lengths just results in exposing implementation details to high-level code. The point in having them be two different units of the same dimension is to show that they express the same thing just with a different representation. Since they represent the same exact same concept there is absolutely no reason why you shouldn't allow them to be used in the same manner. Again, all of the "problems" you describe have nothing to do with the units used, but rather occur because of an unrepresented abstraction in your specific project. Encapsulate the different types separately if you want that abstraction as it has no place in a units library as I see it. -- -Matt Calabrese