The read-decimal-as-inexact parameter allows you to change read's behavior in this way. Is that what you're looking for?
Robby On Fri, Jul 23, 2010 at 1:20 AM, Neil Van Dyke <[email protected]> wrote: > This is just a small idea, not something I'd argue for strongly... > > How about making numbers formatted with decimal points, like "12.34", read > as exact rather than inexact? > > The rationale being: decimal point format is a commonplace and very useful > format for writing exact rational numbers, and if one writes a number like > "5.55", there is nothing except an old Scheme convention to imply that one > intends "5.55" to be any less exact than "555/100". (I'm ignoring any > question of precision here, which Scheme does not address either.) > > For example, with this change, the following expression would evaluate to an > exact 5 rather than what it currently does: an inexact 4.999999999999999. > > (/ (read (open-input-string "5.55")) 1.11) > > The exact 5 result would also be what I imagine is expected by, say, casual > programmers using Racket for financial calculations, which I think would be > a small selling point for Racket. > > Python 2.5.2 (r252:60911, Jan 24 2010, 14:53:14) > [GCC 4.3.2] on linux2 > Type "help", "copyright", "credits" or "license" for more information. >>>> 5.55 / 1.11 > 4.9999999999999991 > > The only good linguistic argument I can think of against this is that, when > we have Racket *write* (as in the "write" procedure, not "display") inexact > rationals in decimal-point form, they'd need to be written with an "#i" > prefix, so that inexactness is preserved on a *read* of that same format by > Racket. The drawbacks I see to the "#i" on write are small: (1) visually, > it's a little bit more cluttered; (2) it will break any legacy code that > assumes it can do a write of numbers for some interoperation or UI purpose > that doesn't work if the "#i" prefix is added. "#i" could even be > considered helpful, as in a heads-up to the programmer that they have a > number that might be corrupted by IEEE floating-point operations they didn't > expect. > > The only implementation efficiency argument I can think of against this is > perhaps wanting numbers to be inexact by default, so that native > floating-point representation and operations can be used more often. If > that is indeed a consideration, I'd prefer to leave adding "#i" prefixes and > "exact->inexact" calls as optimizations that can be added consciously by > programmers, with the default being more exact than efficient. > > -- > http://www.neilvandyke.org/ > > _________________________________________________ > For list-related administrative tasks: > http://lists.racket-lang.org/listinfo/users > _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
