[Haskell-cafe] reading and writing Data.Ratio.Rational

[Olaf Klinke]

Before asking on this list, I wrote my own version which did just what David suggested: Parse the integer before the decimal point, parse the part behind the decimal point and divide the latter by 10^length, then sum both parts. This function has type

Fractional t => String -> t

I thought that this was maybe very inefficient and therefore looked for prior art. If someone feels it is worth adding to some package, I'm happy to provide the code. 

Olaf


> Am 06.03.2017 um 21:46 schrieb David Feuer <david.feuer at gmail.com>:
> 
> I think your best bet is probably to get your hands dirty and parse it yourself: first grab an integer, then optionally a decimal point, etc.
> 
> On Mar 6, 2017 3:43 PM, "Olaf Klinke" <olf at aatal-apotheke.de> wrote:
> Ah, thanks! That is something to build on.
> 
> Olaf
> > Am 24.02.2017 um 15:35 schrieb Patrick Chilton <chpatrick at gmail.com>:
> >
> > Prelude> import Numeric
> > Prelude Numeric> fst $ head $ readFloat "0.1234" :: Rational
> > 617 % 5000
> >
> > On Fri, Feb 24, 2017 at 1:17 PM, Olaf Klinke <olf at aatal-apotheke.de> wrote:
> > Dear cafe,
> >
> > when processing text files containing numbers of the form "xxxx.yyyy" I used to parse them into Double using that type's Read instance. Obviously, even with arithmetic no more complicated than the field operations the result might have ugly rounding errors like 12.000000000002 due to the fact that numbers like 0.7 are not dyadic rationals. The math in my program is not complicated and the numbers are not large, so I don't care about Rationals having potentially huge memory footprints.
> >
> > So here's my question. A literal like 0.7 has type Fractional a => a, but the Read instance of Rational rejects the string "0.7". Must it be this way? Do I have to go via toRational.(read :: String -> Data.Scientific.Scientific)?
> >
> > Note that the documentation of Data.Scientific explicitly states that using (/) is unsafe, so I'd rather stay with the field Rational.
> >
> > For the output as decimal expansion, there is of course long division as described here [1], but I wonder whether either this exists in some library or there is even a more efficient solution.
> >
> > Any pointers are appreciated.
> > Thanks,
> > Olaf
> >
> > [1] http://stackoverflow.com/questions/30931369/how-to-convert-a-rational-into-a-pretty-string
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