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.