Reading floating point

David Feuer david.feuer at
Tue Oct 11 05:54:08 UTC 2016

It may currently be true for floats, but it's never been true in general,
particularly with regard to records. Read is not actually designed to parse
Haskell; it's for parsing "Haskell-like" things. Because it, unlike a true
Haskell parser, is type-directed, there are somewhat different trade-offs.

On Oct 11, 2016 1:50 AM, "Carter Schonwald" <carter.schonwald at>

> How is that not a bug? We should be able to read back floats
> On Monday, October 10, 2016, David Feuer <david.feuer at> wrote:
>> It doesn't, and it never has.
>> On Oct 10, 2016 6:08 PM, "Carter Schonwald" <carter.schonwald at>
>> wrote:
>>> Read should accept exactly the valid source literals for a type.
>>> On Monday, October 10, 2016, David Feuer <david.feuer at> wrote:
>>>> What does any of that have to do with the Read instances?
>>>> On Oct 10, 2016 1:56 PM, "Carter Schonwald" <carter.schonwald at>
>>>> wrote:
>>>>> 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.
>>>>>> >
>>>>>> >
>>>>>> > _______________________________________________
>>>>>> > ghc-devs mailing list
>>>>>> > ghc-devs at
>>>>>> >
>>>>>> >
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