Another approach can be to represent numbers representing an amount of a unit as a pair or a struct containing both the number and the unit. However, this would require updates of a number of arithmetic operators. In addition, read should accept numbers annotated with a unit. Writing them is less of a problem. Jos
-----Original Message----- From: users-boun...@racket-lang.org [mailto:users-boun...@racket-lang.org] On Behalf Of Neil Van Dyke Sent: jueves, 17 de noviembre de 2011 23:52 To: Shriram Krishnamurthi Cc: users@racket-lang.org Subject: Re: [racket] Units/measures library Neil Van Dyke wrote at 11/17/2011 05:49 PM: > Shriram Krishnamurthi wrote at 11/17/2011 09:43 AM: >> DSSSL (-: >> >> (Since I believe Bigloo and/or Gambit implemented DSSSL, lurking in >> their implementations is code that does this...) > > Also, the Curl programming language had units of measurement built-in: > http://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.131.6818 > >> 2.4.1.3 Quantities >> >> Presentation-oriented applications frequently deal with >> measurements of space and time. Curl accordingly includes >> "quantity types," which are numeric data types with units, such as >> Distance and Time. These types are distinct from the >> conventional dimensionless numeric types such as float and >> double. There are also quantity literals, such as 2.54cm, >> 72pt, 50ms, 1week, and even 800dpi and 32(ft/(s^2)). >> By using quantities, programmers can let Curl handle conversions >> between units and can write more descriptive (and checkable) >> interfaces to graphical APIs. There is also a pixel-denominated >> unit px, which can be converted to a Distance by dividing by >> the spatial resolution of a display device. Quantity types are not >> found in other languages for creating Web content, but they are >> used pervasively in the Curl GUI APIs and are tremendously >> useful. in application programs. > > It seems easy for someone to add to Racket as a library, so long as > the library user doesn't mind having to explicitly ``cast'' these > objects to normal Racket numbers when they want to interoperate with > Racket libraries that only work with numbers. Maybe use Typed Racket > or contracts info to add in the type coercions manually, or maybe > having to do the explicit casts is actually good programming practice. Typo. I meant not "manually", but "automatically across module boundaries". > > I am doing something like this with an (as yet unreleased) time > library, and have thought of doing it for financial apps as well. > > One thing I haven't decided is whether and how to represent precision > (or resolution) information, and what to do with that information. For > a simple example, what is the answer to the difference "(2 days) - (3 > minutes)"? I'm wondering whether the unit of measure alone can > indicate the precision for 99.9% of practical purposes, or we have to > be able to support more flexible specification of precision (e.g., "(2 > days) - (3 minutes to a precision of 15 seconds)"). > _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
