It just is so convenient. instance (Integral a, Show a) => PrintfArg (Ratio a) where formatArg x f@(FieldFormat w p a s l _ c) = if elem c "gGv" then showsPrec 0 x else if elem c "doxXb" then formatArg (round x :: Integer) (f {fmtPrecision = Nothing, fmtChar = 'd'}) else case p of Nothing -> error "Text.Printf.formatArg: precision not given" Just p' -> if p' <= 0 then formatArg x (f {fmtPrecision = Nothing, fmtChar = 'd'}) else if elem c "fF" then let n = truncate x sig = '.' : goF (x - fromInteger n) p' b = case a of Just ZeroPad -> formatArg n (f {fmtWidth = fmap (subtract (length sig)) w, fmtPrecision = Nothing, fmtChar = 'd'}) "" _ -> show n in formatArg (b ++ sig) (f {fmtPrecision = Nothing, fmtChar = 's'}) else if elem c "eE" then let (q,e) = log10 x sig = c : show e a' = case a of Just ZeroPad -> a _ -> Nothing fp = formatArg q (f {fmtWidth = fmap (subtract (length sig)) w, fmtAdjust = a', fmtChar = 'f'}) "" in formatArg (fp ++ sig) (f {fmtPrecision = Nothing, fmtChar = 's'}) else error "Text.Printf.formatArg: bad format character" where goF _ 0 = "" goF x p = case compare x 0 of LT -> '-' : goF (negate x) p EQ -> "0" GT -> if 1 == p then show (round (10 * x) :: Integer) else let x10 = 10 * x n = truncate x10 in show n ++ goF (x10 - fromIntegral n) (p - 1) log10 x | x < 1 = let (q,e) = log10 (x * 10) in (q, e - 1) | 10 <= x = let (q,e) = log10 (x / 10) in (q, e + 1) | otherwise = (x, 0)