haskell,formatting,rational , How to convert a Rational into a “pretty” String?


How to convert a Rational into a “pretty” String?

Question:

Tag: haskell,formatting,rational

I want to display some Rational values in their decimal expansion. That is, instead of displaying 3 % 4, I would rather display 0.75. I'd like this function to be of type Int -> Rational -> String. The first Int is to specify the maximum number of decimal places, since Rational expansions may be non-terminating.

Hoogle and the haddocks for Data.Ratio didn't help me. Where can I find this function?


Answer:

Here's one that I wrote a few weeks ago. You can specify the number of decimals you want (correctly rounded), or just pass Nothing in which case it will print the full precision, including marking the repeated decimals.

module ShowRational where
import Data.List(findIndex, splitAt)

-- | Convert a 'Rational' to a 'String' using the given number of decimals.
-- If the number of decimals is not given the full precision is showed using (DDD) for repeating digits.
-- E.g., 13.7/3 is shown as \"4.5(6)\".
showRational :: Maybe Int -> Rational -> String
showRational (Just n) r =
    let d = round (abs r * 10^n)
        s = show (d :: Integer)
        s' = replicate (n - length s + 1) '0' ++ s
        (h, f) = splitAt (length s' - n) s'
    in  (if r < 0 then "-" else "") ++ h ++ "." ++ f
-- The length of the repeating digits is related to the totient function of the denominator.
-- This means that the complexity of computing them is at least as bad as factoring, i.e., it quickly becomes infeasible.
showRational Nothing r =
    let (i, f) = properFraction (abs r) :: (Integer, Rational)
        si = if r < 0 then "-" ++ show i else show i
        decimals f = loop f [] ""
        loop x fs ds =
            if x == 0 then
                ds
            else
                case findIndex (x ==) fs of
                    Just i  -> let (l, r) = splitAt i ds in l ++ "(" ++ r ++ ")"
                    Nothing -> let (c, f) = properFraction (10 * x) :: (Integer, Rational) in loop f (fs ++ [x]) (ds ++ show c)
    in  if f == 0 then si else si ++ "." ++ decimals f

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