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Re: CLTL compatible type contagion?
Raymond Toy writes:
> I agree, it is nice, but I wish it could remind me a bit more
> loudly. :-) Silently making all (following) results a single-float
> makes it very difficult to find out where I made the original
> mistake. :-)
Not so difficult: you just have to look at the number of decimal
places of your intermediate results. It would be more difficult to
find out (or even notice) if you got double-floats with the 8 trailing
decimal places filled with nearly random digits -- which is what ANSI CL
> However, I have discovered the "errant" part of my program. I lose
> precision because I had something like
> (sqrt 22/7)
> and clisp returned a float value. I guess the result would depend on
> whether 22/7 has infinite precision/accuracy in which case the answer
> should be a long-float (?) or whether 22/7 has accuracy of +/- 1/14 in
> which case the answer should be short-float or less. The assumption
> that 22/7 is a single-float is a bit hazy.
Agreed. This is a second flaw in CLtL, this time in the description of
the FLOAT function. 22/7, as a rational number, has infinite precision,
so the result should be a long-float of the longest available float
format. Since CLISP can deal with long-floats of more than 100000 decimal
places, this is not very practical.
For this purpose, CLISP provides a variable LISP:*DEFAULT-FLOAT-FORMAT*.
Set it to DOUBLE-FLOAT, and (sqrt 22/7) will return a float of this type.
> But what about this example?
> 1.000001 +/- 1e-8
> - 1.000000 +/- 1e-8
> 0.000001 +/- 2e-8
> What should the type of the result be?
These phenomena (extinction and the growth of the interval length) are
considered to be inevitable with the usual IEEE floating point arithmetic.
> The current scheme seems like a half-attempt at interval arithmetic.
> Either go all the way or don't go at all.
One should go all the way. Follow-up to the comp.arch.arithmetic newsgroup.
Bruno Haible email: <email@example.com>
Software Engineer phone: +33-1-49083585