Haskell语法

http://www.ibm.com/developerworks/cn/java/j-cb07186.html

1. 构造符号

:

比如：

1:2:3:[]

而常用的

[1,2,3]

是一种语法糖（http://en.wikipedia.org/wiki/Syntactic_sugar）

2. 一切都是函数

函数定义语法：

函数名 :: 参数类型中可能用到的基本类型 基本类型名 => 参数1 -> 参数2 -> 返回值类型

函数名 模式1 = 实现1

函数名 模式2 = 实现2

......

3. 从集合中取出元素

元素 <- 集合

4. 定义为

名称 :: 定义

5. 模式

基本类型

a b

Tuple

a b
(x1, y1)(x2, y2)

List

像x:xs这样的模式可以将list的头部绑定为x，尾部绑定为xs。如果这list只有一个元素，那么xs就是一个空list。

http://fleurer-lee.com/lyah/syntax-on-function.htm

[]
(x:xs)
(x:_)
(x:y:[])

---

类型

When we write a type explicitly, we use the notation

expression :: MyType

to say that  expressionhas the type MyType.

As you can see, we can apply head and tail to lists of different types. Applying head to
a [Char] value returns a Char value, while applying it to a [Bool] value returns a Bool
value. The head function doesn’t care what type of list it deals with.
Because the values in a list can have any type, we call the list type polymorphic.
When we want to write a polymorphic type, we use a type variable, which must begin with a
lowercase letter. A type variable is a placeholder, where we’ll eventually substitute a
real type.

Haskell实际上代表了一种方向，即编程并不需要关注How，而是关注What。如果Haskell能够流行起来，或者更加简化，或者更加强大，那么很多程序员本身的价值就会褪色。

a type name, and hence a type constructor, must start with
a capital letter.

Defining a New Data Type

data BookInfo = Book Int String [String]
deriving (Show)

BookInfo是Type的名称，而Book是Value名称，听上去很不可思议，说白了，BookInfo是系统用的名称，而Book是给用户用的名称。

ghci> :type myInfo
myInfo :: BookInfo

或者说，可以用Book来定义一个数据，但是它的实际类型是BookInfo.

也可以让二者拥有相同的名字

data Book = Book Int String [String]
deriving (Show)

The :info command gets ghcito tell us everything it knows about a name:

下面的定义类似于 typedef

type CustomerID = Int
type ReviewBody = String
data BetterReview = BetterReview BookInfo CustomerID ReviewBody

下面关于Bool的定义也可以让我们豁然开朗

data Bool = False | True

---

尝试写一些常用的函数

Fibonacci number

http://en.wikipedia.org/wiki/Fibonacci_number

fabonacci :: (Integral a) => a -> a
fabonacci 0 = 1
fabonacci 1 = 1
fabonacci n = fabonacci(n - 2) + fabonacci(n - 1)

保存为fabonacci.hs, 在命令行中执行：

H:\haskell>ghci
GHCi, version 7.6.3: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Prelude> :l fabonacci.hs
[1 of 1] Compiling Main             ( fabonacci.hs, interpreted )
Ok, modules loaded: Main.
*Main> map fabonacci [1..20]
[1,2,3,5,8,13,21,34,55,89,144,233,377,610,987,1597,2584,4181,6765,10946]

对数组执行幂操作

pow :: (Integral a) => a -> a -> a

pow 0 y = 1

pow x 0 = 1

pow x 1 = x

pow x y = x * (pow x (y-1))

power :: (Integral a) => a -> [a] -> [a]

power y xs = map (\x -> pow x y) xs 

执行结果

*Main> power 3 [1..10]

[1,8,27,64,125,216,343,512,729,1000]

*Main> power 4 [1..10]

[1,16,81,256,625,1296,2401,4096,6561,10000]

*Main> power 5 [1..10]

[1,32,243,1024,3125,7776,16807,32768,59049,100000]

*Main> power 6 [1..10]

[1,64,729,4096,15625,46656,117649,262144,531441,1000000]

*Main> power 7 [1..10]

[1,128,2187,16384,78125,279936,823543,2097152,4782969,10000000]

*Main> power 8 [1..10]

[1,256,6561,65536,390625,1679616,5764801,16777216,43046721,100000000]

*Main> power 9 [1..10]

[1,512,19683,262144,1953125,10077696,40353607,134217728,387420489,1000000000]

*Main> power 10 [1..10]

[1,1024,59049,1048576,9765625,60466176,282475249,1073741824,3486784401,100000000

00]

当Haskell与数学结合在一起的时候，真是威力无穷！

查找100以内的所有勾股数组合

http://zh.wikipedia.org/wiki/%E7%95%A2%E9%81%94%E5%93%A5%E6%8B%89%E6%96%AF%E4%B8%89%E5%85%83%E6%95%B8

通过List Comprehension的方式，可以很快地完成

Prelude> [(x, y, z)| x <- [1..100], y <- [1..x], z <- [1..y], x*x == y*y + z*z]

[(5,4,3),(10,8,6),(13,12,5),(15,12,9),(17,15,8),(20,16,12),(25,20,15),(25,24,7)

(26,24,10),(29,21,20),(30,24,18),(34,30,16),(35,28,21),(37,35,12),(39,36,15),(4

,32,24),(41,40,9),(45,36,27),(50,40,30),(50,48,14),(51,45,24),(52,48,20),(53,45

28),(55,44,33),(58,42,40),(60,48,36),(61,60,11),(65,52,39),(65,56,33),(65,60,25

,(65,63,16),(68,60,32),(70,56,42),(73,55,48),(74,70,24),(75,60,45),(75,72,21),(

8,72,30),(80,64,48),(82,80,18),(85,68,51),(85,75,40),(85,77,36),(85,84,13),(87,

3,60),(89,80,39),(90,72,54),(91,84,35),(95,76,57),(97,72,65),(100,80,60),(100,9

,28)]

怎么用函数写呢？