bug#13757: Problems with Guile's number printer

[Andy Wingo]

Hi,

I'm redirecting the number printing conversation to bug-guile, so that
we can track it.  Thanks for the debugging!

On Tue 19 Feb 2013 00:12, Mark H Weaver <address@hidden> writes:

> Andy Wingo <address@hidden> writes:
>
>> On Wed 23 Jan 2013 13:20, Daniel Llorens <address@hidden> writes:
>>
>>> scheme@(guile-user)> (f64vector-ref #s64(1 2 3) 0)
>>> $1 = #.#
>>
>> Here you are interpreting an int64 as a double, which should work, but
>> this printed result is really bizarre and looks like a bug in our number
>> printer.  Mark? :)
>
> Yes, our number printer is seriously flawed and needs a rewrite.  It
> prints subnormal[1] floats as "#.#", and even in typical cases often
> fails to print enough digits to get the same number back when you read
> it back in.
>
> Note that this also affects compiled code involving numbers, because the
> compiler serializes numbers using 'number->string'.  For example,
> (* 1e-155 1e-155) returns #f at the REPL, because peval turns this into
> a constant which happens to be a subnormal.  During assembly it serializes
> this to "#.#", and then 'string->number' returns #f.
>
> Also, 3.14159265358979323846264338327950288419716939937510582097494, if
> compiled, fails to produce the float closest to pi.  (acos -1) works
> properly, but only because this expression is not currently folded to a
> constant by the compiler.
>
> I've already started work on this (based on "Printing Floating-Point
> Numbers Quickly and Accurately" by Burger and Dybvig) but got
> distracted.
>
>      Mark
>
> [1] http://en.wikipedia.org/wiki/Subnormal_number

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