Reenigne blog

A handy thing to have around (and something I want to put into the UnityALFE language) is a data type for representing physical quantities. This keeps track of a real number plus powers of metres, seconds, kilograms, amps, Kelvins (and maybe others, including user-defined ones). Two values of type Concrete can always be multiplied or divided but can only be added or subtracted if their dimensions match, and can only be converted to other types if they are dimensionless.

Common units of measurement and physical constants can be given as constants. Because the units are tracked automatically you can do things like:

(2*metre+5*inch)/metre

and get the right answer.

Usually the compiler should be able to check the dimensions at compile time and elide them like other type information, or give a compile error for expressions like:

(2*metre+5)/metre

Along similar lines, the log() function could be considered to return a value of type Concrete with some special non-zero dimension. Then you can (and indeed must) specify to which base the logarithm should be by dividing by another logarithm (e.g. log(x)/log(e), log(x)/log(10) or log(x)/log(2)). This syntax is rather more verbose than the usual method of having separate functions for common bases (log(), lg(), ln() etc.) but I find that this is more than made up for by the fact that one doesn't have to remember which function corresponds to which base - it's self-describing.

Another useful type for scientific/engineering work would be a value with confidence interval (e.g. 5±1 meaning "distributed normally with a mean of 5 and a standard deviation of 1"). There are well-defined rules for doing arithmetic with these. A generalization of this to other distribution functions might also be useful.

This entry was posted on Friday, July 25th, 2008 at 4:00 pm and is filed under language. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.