The statement in the article is: "We must distinguish :@ (prefixing) from :* (unit multiplication) because Kilo :@ Gram can, for example, be added to something expressed in Gram s, but Meter :* Gram cannot."
I infer this as: Gram can be added to Kilogram (of course by fixing for appropriate unit conversions) but it cannot be added to "Meter :* Gram". I think that's what Bob meant. On Sat, Apr 11, 2015 at 6:10 PM, Raul Miller <[email protected]> wrote: > That one strikes me as odd, also. > > Perhaps it's to prevent people from calculating center of mass? > > But why would anyone want to prevent that type of calculation? > > -- > Raul > > On Sat, Apr 11, 2015 at 5:41 PM, Devon McCormick <[email protected]> wrote: >> Hi Bob - >> >> this sounds interesting but why would Meter*Gram be forbidden? If I'm >> calculating Newtons, I want Kg * M/s^2, don't I? >> >> Regards, >> >> Devon >> >> On Sat, Apr 11, 2015 at 4:37 PM, Robert Bernecky <[email protected]> >> wrote: >> >>> In the recent discussion on typing in J, I cited the Gimley Glider >>> and the Mars Climate Orbiter as events in which confusion over >>> units of measure caused expensive problems and near-disaster >>> (disaster in the case of the MCO). >>> >>> This morning, I was reading the SIGPLAN Haskell Symposium '14 >>> Proceedings, and found this article: >>> >>> http://www.cis.upenn.edu/~eir/papers/2014/units/units.pdf >>> >>> By way of introduction, the paper cites the same two events, and also >>> cites (bottom of page 1) a paper that gives more of the same. >>> The paper is worth reading, IMO, because the authors create >>> DIMENSIONs: mass, length, density, volume, etc., that are >>> independent of units (kilogram, furlong, fortnight). >>> They also introduce two kinds of combinators (adverbs) that >>> can modify the dimensions, e.g,: >>> >>> type Velocity = Length: / Time >>> type Kilogram = Kilo :@ Gram NB. Kilo is prefix, not unit >>> >>> This lets them do things such as multiplying Kilograms times Gram >>> (and getting the right answer) , while forbidding Meter * Gram. >>> >>> I think these folks have some good ideas that could be exploited >>> in J and APL to everyone's benefit. >>> >>> Bob >>> >>> -- >>> Robert Bernecky >>> Snake Island Research Inc >>> 18 Fifth Street >>> Ward's Island >>> Toronto, Ontario M5J 2B9 >>> >>> [email protected] >>> tel: +1 416 203 0854 >>> >>> ---------------------------------------------------------------------- >>> For information about J forums see http://www.jsoftware.com/forums.htm >>> >> >> >> >> -- >> Devon McCormick, CFA >> ---------------------------------------------------------------------- >> For information about J forums see http://www.jsoftware.com/forums.htm > ---------------------------------------------------------------------- > For information about J forums see http://www.jsoftware.com/forums.htm ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
