孩子要上3年级了，里面涉及分数的部分，先准备一下。
haskell中涉及分数的模块是Ratio。
Ratio
Documentation
Rational numbers, with numerator and denominator of some Integral type.
Instances
| Typeable1 Ratio | |
| Integral a => Enum (Ratio a) | |
| Eq a => Eq (Ratio a) | |
| Integral a => Fractional (Ratio a) | |
| Integral a => Num (Ratio a) | |
| Integral a => Ord (Ratio a) | |
| (Integral a, Read a) => Read (Ratio a) | |
| Integral a => Real (Ratio a) | |
| Integral a => RealFrac (Ratio a) | |
| (Integral a, Show a) => Show (Ratio a) |
type Rational = Ratio Integer
Arbitrary-precision rational numbers, represented as a ratio of two Integer values. A rational number may be constructed using the % operator.
(%) :: Integral a => a -> a -> Ratio a
Forms the ratio of two integral numbers.
numerator :: Integral a => Ratio a -> a
Extract the numerator of the ratio in reduced form: the numerator and denominator have no common factor and the denominator is positive.
denominator :: Integral a => Ratio a -> a
Extract the denominator of the ratio in reduced form: the numerator and denominator have no common factor and the denominator is positive.
approxRational :: RealFrac a => a -> a -> Rational
approxRational, applied to two real fractional numbers x and epsilon, returns the simplest rational number within epsilon of x. A rational number y is said to be simpler than another y' if
abs(numeratory) <=abs(numeratory')denominatory <=denominatory'
Any real interval contains a unique simplest rational; in particular, note that 0/1 is the simplest rational of all.
比如：
Prelude Ratio> (2%3) * (3%4)
1 % 2
Prelude Ratio> (2%3) + (3%4)
17 % 12
Prelude Ratio> (2%3) / (3%4)
8 % 9
Prelude Ratio> (2%3) - (3%4)
(-1) % 12
Prelude Ratio> 3 % 2 --分数二分之三
3 % 2
Prelude Ratio> numerator (3 % 2) --分子
3
Prelude Ratio> denominator (3 % 2) --分母
2
Prelude Ratio> approxRational 1.5123 0.2 --在(1.5123 - 0.2, 1.5123 + 0.2)之间的最简单的分数
3 % 2
numerator：分子
denominator：分母
approxRational ：approximate rational 近似分数
