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"
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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