    module Data.Ratio (
          Ratio
        , Rational
        , (%)               -- :: (Integral a) => a -> a -> Ratio a
        , numerator         -- :: (Integral a) => Ratio a -> a
        , denominator       -- :: (Integral a) => Ratio a -> a
        , approxRational    -- :: (RealFrac a) => a -> a -> Rational

        -- * Specification

        -- $code
      ) where
    import "base" Data.Ratio

    {- $code
    > module  Data.Ratio (
    >     Ratio, Rational, (%), numerator, denominator, approxRational ) where
    >
    > infixl 7  %
    >
    > ratPrec = 7 :: Int
    >
    > data  (Integral a)      => Ratio a = !a :% !a  deriving (Eq)
    > type  Rational          =  Ratio Integer
    >
    > (%)                     :: (Integral a) => a -> a -> Ratio a
    > numerator, denominator  :: (Integral a) => Ratio a -> a
    > approxRational          :: (RealFrac a) => a -> a -> Rational
    >
    >
    > -- "reduce" is a subsidiary function used only in this module.
    > -- It normalises a ratio by dividing both numerator
    > -- and denominator by their greatest common divisor.
    > --
    > -- E.g., 12 `reduce` 8    ==  3 :%   2
    > --       12 `reduce` (-8) ==  3 :% (-2)
    >
    > reduce _ 0              =  error "Data.Ratio.% : zero denominator"
    > reduce x y              =  (x `quot` d) :% (y `quot` d)
    >                            where d = gcd x y
    >
    > x % y                   =  reduce (x * signum y) (abs y)
    >
    > numerator (x :% _)      =  x
    >
    > denominator (_ :% y)    =  y
    >
    >
    > instance  (Integral a)  => Ord (Ratio a)  where
    >     (x:%y) <= (x':%y')  =  x * y' <= x' * y
    >     (x:%y) <  (x':%y')  =  x * y' <  x' * y
    >
    > instance  (Integral a)  => Num (Ratio a)  where
    >     (x:%y) + (x':%y')   =  reduce (x*y' + x'*y) (y*y')
    >     (x:%y) * (x':%y')   =  reduce (x * x') (y * y')
    >     negate (x:%y)       =  (-x) :% y
    >     abs (x:%y)          =  abs x :% y
    >     signum (x:%y)       =  signum x :% 1
    >     fromInteger x       =  fromInteger x :% 1
    >
    > instance  (Integral a)  => Real (Ratio a)  where
    >     toRational (x:%y)   =  toInteger x :% toInteger y
    >
    > instance  (Integral a)  => Fractional (Ratio a)  where
    >     (x:%y) / (x':%y')   =  (x*y') % (y*x')
    >     recip (x:%y)        =  y % x
    >     fromRational (x:%y) =  fromInteger x :% fromInteger y
    >
    > instance  (Integral a)  => RealFrac (Ratio a)  where
    >     properFraction (x:%y) = (fromIntegral q, r:%y)
    >                             where (q,r) = quotRem x y
    >
    > instance  (Integral a)  => Enum (Ratio a)  where
    >     succ x           =  x+1
    >     pred x           =  x-1
    >     toEnum           =  fromIntegral
    >     fromEnum         =  fromInteger . truncate        -- May overflow
    >     enumFrom         =  numericEnumFrom               -- These numericEnumXXX functions
    >     enumFromThen     =  numericEnumFromThen   -- are as defined in Prelude.hs
    >     enumFromTo       =  numericEnumFromTo     -- but not exported from it!
    >     enumFromThenTo   =  numericEnumFromThenTo
    >
    > instance  (Read a, Integral a)  => Read (Ratio a)  where
    >     readsPrec p  =  readParen (p > ratPrec)
    >                               (\r -> [(x%y,u) | (x,s)   <- readsPrec (ratPrec+1) r,
    >                                                 ("%",t) <- lex s,
    >                                                 (y,u)   <- readsPrec (ratPrec+1) t ])
    >
    > instance  (Integral a)  => Show (Ratio a)  where
    >     showsPrec p (x:%y)  =  showParen (p > ratPrec)
    >                               showsPrec (ratPrec+1) x .
    >                               showString " % " .
    >                               showsPrec (ratPrec+1) y)
    >
    >
    >
    > approxRational x eps    =  simplest (x-eps) (x+eps)
    >         where simplest x y | y < x      =  simplest y x
    >                            | x == y     =  xr
    >                            | x > 0      =  simplest' n d n' d'
    >                            | y < 0      =  - simplest' (-n') d' (-n) d
    >                            | otherwise  =  0 :% 1
    >                                         where xr@(n:%d) = toRational x
    >                                               (n':%d')  = toRational y
    >
    >               simplest' n d n' d'       -- assumes 0 < n%d < n'%d'
    >                         | r == 0     =  q :% 1
    >                         | q /= q'    =  (q+1) :% 1
    >                         | otherwise  =  (q*n''+d'') :% n''
    >                                      where (q,r)      =  quotRem n d
    >                                            (q',r')    =  quotRem n' d'
    >                                            (n'':%d'') =  simplest' d' r' d r
    -}