[Haskell-cafe] Proposal: Rational read

David Feuer david.feuer at gmail.com
Tue Aug 16 16:48:59 UTC 2016

That choice has already been made, with a different conclusion, in the
Haskell numeric syntax. In Haskell syntax, decimal syntax is interpreted
via fromRational *already*. In source, I can write 3.796::Rational and get
a good result, without any conversion from Double anywhere--it just puts
3796/1000 into lowest terms as it should. I see no particular reason to do
otherwise here. In either case, we need to read digits, building an
integer, until we get to a non-digit. Only what happens next depends on the
notation. Either notation should work for any Fractional type, as it does
in Haskell syntax.

On Aug 15, 2016 11:09 AM, "Michael Orlitzky" <michael at orlitzky.com> wrote:

> On 08/15/2016 09:04 AM, Benno Fünfstück wrote:
> >
> > Well, you are showing 1/3 as a Float and reading it as a Double, which
> > of course won't work since the String represents a Float and not a
> > Double. The following does work:
> >
> Sorry, unclear example. We have a choice to make for how to read in
> decimal strings that could also represent Floats or Doubles. I'm worried
> about the resulting confusion from whatever choice we make.
> Read/Show are supposed to be machine-readable, and representing a
> Rational as a decimal (the same way a Float is represented) gives away
> some type-safety. This is the same argument I would make against having
> (read "1.0" :: Integer) return 1.
> Suppose that this is what the rational read instance would do...
>   ghci> let ratread :: String -> Rational;
>         ratread s = realToFrac (read s :: Double) :: Rational
> We're compatible with the Fractional/Show instance for Float/Double:
>   ghci> fromRational $ ratread (show (0.33333334 :: Float)) :: Float
>   0.33333334
>   ghci> fromRational $ ratread (show (0.33333334 :: Double)) :: Double
>   0.33333334
> But obviously things will go wrong for rationals themselves, when they
> don't fit into a Double. The other choice we could make is to take
> "0.33333334", multiply it by ten-to-the-whatever, and then make that the
> denominator (over ten-to-the-whatever). That's probably a better choice,
> but then you have other inconsistencies...
>   ghci> 0.33333334 :: Rational
>   16666667 % 50000000
>   ghci> toRational (0.33333334 :: Float)
>   11184811 % 33554432
> Namely, that that's not how the Float itself is converted into a
> Rational. Basically, beware of confusing the hell out of everyone in
> order to avoid an explicit type conversion.
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