Reading floating point

David Feuer david.feuer at
Mon Oct 10 20:11:17 UTC 2016

What does any of that have to do with the Read instances?

On Oct 10, 2016 1:56 PM, "Carter Schonwald" <carter.schonwald at>

> The right solution is to fix things so we have scientific notation literal
> rep available.  Any other contortions run into challenges in repsentavility
> of things.  That's of course ignoring denormalized floats, infinities,
> negative zero and perhaps nans.
> At the very least we need to efficiently and safely support everything but
> nan. And I have some ideas for that I hope to share soon.
> On Monday, October 10, 2016, David Feuer <david.feuer at> wrote:
>> I fully expect this to be somewhat tricky, yes. But some aspects of the
>> current implementation strike me as pretty clearly non-optimal. What I
>> meant about going through Rational is that given "625e-5", say, it
>> calculates 625%100000, producing a fraction in lowest terms, before calling
>> fromRational, which itself invokes fromRat'', a division function optimized
>> for a special case that doesn't seem too relevant in this context. I could
>> be mistaken, but I imagine even reducing to lowest terms is useless here.
>> The separate treatment of the digits preceding and following the decimal
>> point doesn't do anything obviously useful either. If we (effectively)
>> normalize in decimal to an integral mantissa, for example, then we can
>> convert the whole mantissa to an Integer at once; this will balance the
>> merge tree better than converting the two pieces separately and combining.
>> On Oct 10, 2016 6:00 AM, "Yitzchak Gale" <gale at> wrote:
>> The way I understood it, it's because the type of "floating point"
>> literals is
>> Fractional a => a
>> so the literal parser has no choice but to go via Rational. Once you
>> have that, you use the same parser for those Read instances to ensure
>> that the result is identical to what you would get if you parse it as
>> a literal in every case.
>> You could replace the Read parsers for Float and Double with much more
>> efficient ones. But you would need to provide some other guarantee of
>> consistency with literals. That would be more difficult to achieve
>> than one might think - floating point is deceivingly tricky. There are
>> already several good parsers in the libraries, but I believe all of
>> them can provide different results than literals in some cases.
>> YItz
>> On Sat, Oct 8, 2016 at 10:27 PM, David Feuer <david.feuer at>
>> wrote:
>> > The current Read instances for Float and Double look pretty iffy from an
>> > efficiency standpoint. Going through Rational is exceedingly weird: we
>> have
>> > absolutely nothing to gain by dividing out the GCD, as far as I can
>> tell.
>> > Then, in doing so, we read the digits of the integral part to form an
>> > Integer. This looks like a detour, and particularly bad when it has many
>> > digits. Wouldn't it be better to normalize the decimal representation
>> first
>> > in some fashion (e.g., to 0.xxxxxxexxx) and go from there? Probably less
>> > importantly, is there some way to avoid converting the mantissa to an
>> > Integer at all? The low digits may not end up making any difference
>> > whatsoever.
>> >
>> >
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>> >
>> >
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