Ratio

For each Integral type t, there is a type Ratio t of rational pairs with components of type t. The type name Rational is a synonym for Ratio Integer.

Ratio is an instance of classes Eq, Ord, Num, Real, Fractional, RealFrac, Enum, Read, and Show. In each case, the instance for Ratio t simply "lifts" the corresponding operations over t. If t is a bounded type, the results may be unpredictable; for example Ratio Int may give rise to integer overflow even for rational numbers of small absolute size.

The operator (%) forms the ratio of two integral numbers, reducing the fraction to terms with no common factor and such that the denominator is positive. The functions numerator and denominator extract the components of a ratio; these are in reduced form with a positive denominator. Ratio is an abstract type. For example, 12 % 8 is reduced to 3/2 and 12 % (-8) is reduced to (-3)/2.

The approxRational function, applied to two real fractional numbers x and epsilon, returns the simplest rational number within the open interval (x-epsilon, x+epsilon). A rational number n/d in reduced form is said to be simpler than another n'/d' if |n| <=|n'| and d <=d'. Note that it can be proved that any real interval contains a unique simplest rational.

-- Standard functions on rational numbers

module  Ratio (
    Ratio, Rational, (%), numerator, denominator, approxRational ) where

infixl 7  %

prec = 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 "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)        =  if x < 0 then (-y) :% (-x) else 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
    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 > prec)
                              (\r -> [(x%y,u) | (x,s)   <- reads r,
                                                ("%",t) <- lex s,
                                                (y,u)   <- reads t ])

instance  (Integral a)  => Show (Ratio a)  where
    showsPrec p (x:%y)  =  showParen (p > prec)
                               (shows x . showString " % " . shows 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

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