Cn/十分钟学会 Haskell

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概要

Haskell 是函数式，静态严格类型，惰性语言（一切通过函数调用来完成；类型检测是编译器的事，声明类型不是必需的；除非必要，否则什么也不做，nothing is done until it needs to be）。其流行近亲可能是 ML 族语言。

最流行(common)的 Haskell 编译器是 GHC，下载地址。GHC 在 GNU/Linux, FreeBSD, MacOS, Windows 以及 Solaris 平台上都有可供使用的二进制包。安装 GHC，即获得 ghc 和 ghci。前者用于将 Haskell 程序库或应用程序编译成二进制码。后者为解释器，可在编写 Haskell 代码后立即得到反馈，lets you write Haskell code and get feedback right away.

表达式

大部份数学表达式都可以输入 ghci 直接解答。Prelude 是 GHCi 默认提示符。 

 Prelude> 3 * 5
 15
 Prelude> 4 ^ 2 - 1
 15
 Prelude> (1 - 5)^(3 * 2 - 4)
 16

字符串需要双引号引用，以 ++ 连接。 

 Prelude> "Hello"
 "Hello"
 Prelude> "Hello" ++ ", Haskell"
 "Hello, Haskell"

调用函数时，参数紧接函数即可，其间无须添加括号。 

 Prelude> succ 5
 6
 Prelude> truncate 6.59
 6
 Prelude> round 6.59
 7
 Prelude> sqrt 2
 1.4142135623730951
 Prelude> not (5 < 3)
 True
 Prelude> gcd 21 14
 7

控制台

调用 I/O actions 进行控制台输入和输出。如： 

 Prelude> putStrLn "Hello, Haskell"
 Hello, Haskell
 Prelude> putStr "No newline"
 No newlinePrelude> print (5 + 4)
 9
 Prelude> print (1 < 2)
 True

putStr 和 putStrLn 输出字符串到终端。print 输出任意类型的值。（如果 print 作用于字符串，输出将用引号引用。)

复杂的 I/O acttions 需要 do 语句块，以分号间隔。 

 Prelude> do { putStr "2 + 2 = " ; print (2 + 2) }
 2 + 2 = 4
 Prelude> do { putStrLn "ABCDE" ; putStrLn "12345" }
 ABCDE
 12345

通过 getLine（返回字符串）或 readLn（返回任意你需要的类型）获得输入。用<- 符号给 I/O action 的结果命名。 

 Prelude> do { n <- readLn ; print (n^2) }
 4
 16

（4 是输入。16 是结果。）

do 语句块的另一种方式，以缩进取代花括号和分号。虽然在 ghci 中未能获得完美支持，但是可以把它们塞进源文件（如 Test.hs）里然后编译： 

main = do putStrLn "What is 2 + 2?"
          x <- readLn
          if x == 4
              then putStrLn "You're right!"
              else putStrLn "You're wrong!"

运行 ghc --make Test.hs，得到 Test（Windows 上是 Test.exe）。顺便接触了 if 语句。

do 之后首个非空白字符，如上例 putStrLn 的 p，诡异。每新起一行，行首与之对齐，即视为同一 do 块之新句；缩进较之多则继续前句；较之少则结束此 do 块。是为页面布局 layout，Haskell 以之回避语句结束标记和花括号。（then 和 else 子句务必缩进，否则将脱离 if 语句，导致错误。）

（注意：切勿使用制表符。从技术上讲，八格制表符可以正常工作，但不是个好主意。也不要使用比例字体。显然，有些人在编程的时候会犯此糊涂！）

类型

到目前为止，我们一直没有提到过类型声明。那是因为 Haskell 暗中推断，不必声明之。如果非要声明类型，可用 :: 符号明确指出，如： 

 Prelude> 5 :: Int
 5
 Prelude> 5 :: Double
 5.0

类型 types（以及类类型 type classes，稍后提及）总是以大写开头。量 variables 总是以小写开头。这是语言规则，而不只是命名习惯。

你也可以让 ghci 告诉你选择的内容的类型，这种方法很有用，因为类型声明并不是必需的。 

 Prelude> :t True
 True :: Bool
 Prelude> :t 'X'
 'X' :: Char
 Prelude> :t "Hello, Haskell"
 "Hello, Haskell" :: [Char]

(在这个例子中，[Char] 是 String 的另外一种表达方式。参见后面的 section on lists 。)

更多有关数字的例子。 

 Prelude> :t 42
 42 :: (Num t) => t
 Prelude> :t 42.0
 42.0 :: (Fractional t) => t
 Prelude> :t gcd 15 20
 gcd 15 20 :: (Integral t) => t

这些类型用到了 "类型类（type classes）" 含义如下：

  • 42 可作为任意数字(numeric)类型。(这就是为什么我既可以把5声明为Int类型，也可以声明为Double类型的原因。)
  • 42.0 可作为任意分数(fractional)类型，但不能是整数(integral)类型。
  • gcd 15 20 (此为函数调用) 可作为任意整数(integral)类型，但不能是分数类型。

在Haskell "Prelude"(你不需要import任何东西就能使用的那部分库)中有五种数字(numeric)类型:

  • Int 是一个至少30位(bit)精度的整数。
  • Integer 是一个无限精度的整数。
  • Float 是一个单精度浮点数。
  • Double 是一个双精度浮点数。
  • Rational 是一个没有舍入误差的分数/小数类型。

上面5个都是Num类型的instances。其中前两个是Integral类型的instances，后面三种是Fractional类型的instances。

总的一块来看一下， 

 Prelude> gcd 42 35 :: Int
 7
 Prelude> gcd 42 35 :: Double
 
 <interactive>:1:0:
     No instance for (Integral Double)

最后值得一提的类型是()，念做"unit"。 它只有一个取值，也写作()并念做"unit"。 

 Prelude> ()
 ()
 Prelude> :t ()
 () :: ()

你可以把它看作类似C语言中的void关键字。在一个I/O动作中，如果你不想返回任何东西，你可以返回()。

有结构的数据

基本数据类型可以很容易的通过两种方式组合在一起：通过 [方括号] 组合的列表，和通过 (圆括号) 组合的元组。

列表可以用来储存多个相同类型的值： 

 Prelude> [1, 2, 3]
 [1,2,3]
 Prelude> [1 .. 5]
 [1,2,3,4,5]
 Prelude> [1, 3 .. 10]
 [1,3,5,7,9]
 Prelude> [True, False, True]
 [True,False,True]

Haskell 中的字符串（String）其实只不过是字符（Character）类型的 List： 

 Prelude> ['H', 'e', 'l', 'l', 'o']
 "Hello"

冒号 : 运算符用来把一个项（item）添加到列表的开始处。(It is Haskell's version of the cons function in the Lisp family of languages.) 

 Prelude> 'C' : ['H', 'e', 'l', 'l', 'o']
 "CHello"

元组则和列表不同，它用来储存固定个数，但类型不同的值。【译者注：列表是类型相同，但个数不固定，甚至还可以是无限个数】 

 Prelude> (1, True)
 (1,True)
 Prelude> zip [1 .. 5] ['a' .. 'e']
 [(1,'a'),(2,'b'),(3,'c'),(4,'d'),(5,'e')]

上面这个例子用到了 zip 函数，它可以把两个列表组合成一个元组的列表。

The types are probably what you'd expect. 

 Prelude> :t ['a' .. 'c']
 ['a' .. 'c'] :: [Char]
 Prelude> :t [('x', True), ('y', False)]
 [('x', True), ('y', False)] :: [(Char, Bool)]

Lists are used a lot in Haskell. There are several functions that do nice things with them. 

 Prelude> [1 .. 5]
 [1,2,3,4,5]
 Prelude> map (+ 2) [1 .. 5]
 [3,4,5,6,7]
 Prelude> filter (> 2) [1 .. 5]
 [3,4,5]

There are two nice functions on ordered pairs (tuples of two elements): 

 Prelude> fst (1, 2)
 1
 Prelude> snd (1, 2)
 2
 Prelude> map fst [(1, 2), (3, 4), (5, 6)]
 [1,3,5]

Also see how to work on lists

Function definitions

We wrote a definition of an IO action earlier, called main: 

main = do putStrLn "What is 2 + 2?"
          x <- readLn
          if x == 4
              then putStrLn "You're right!"
              else putStrLn "You're wrong!"

Now, let's supplement it by actully writing a function definition and call it factorial. I'm also adding a module header, which is good form. 

module Main where

factorial n = if n == 0 then 1 else n * factorial (n - 1)

main = do putStrLn "What is 5! ?"
          x <- readLn
          if x == factorial 5
              then putStrLn "You're right!"
              else putStrLn "You're wrong!"

Build again with ghc --make Test.hs. And, 

 $ ./Test
 What is 5! ?
 120
 You're right!

There's a function. Just like the built-in functions, it can be called as factorial 5 without needing parentheses.

Now ask ghci for the type. 

 $ ghci Test.hs
 << GHCi banner >>
 Ok, modules loaded: Main.
 Prelude Main> :t factorial
 factorial :: (Num a) => a -> a

Function types are written with the argument type, then ->, then the result type. (This also has the type class Num.)

Factorial can be simplified by writing it with case analysis. 

factorial 0 = 1
factorial n = n * factorial (n - 1)

Convenient syntax

A couple extra pieces of syntax are helpful. 

secsToWeeks secs = let perMinute = 60
                       perHour   = 60 * perMinute
                       perDay    = 24 * perHour
                       perWeek   =  7 * perday
                   in  secs * perWeek

The let expression defines temporary names. (This is using layout again. You could use {braces}, and separate the names with semicolons, if you prefer.) 

classify age = case age of 0 -> "newborn"
                           1 -> "infant"
                           2 -> "toddler"
                           _ -> "senior citizen"

The case expression does a multi-way branch. The special label _ means "anything else".

Using libraries

Everything used so far in this tutorial is part of the Prelude, which is the set of Haskell functions that are always there in any program.

The best road from here to becoming a very productive Haskell programmer (aside from practice!) is becoming familiar with other libraries that do the things you need. Documentation on the standard libraries is at http://haskell.org/ghc/docs/latest/html/libraries/. There are modules there with: 

module Main where

import qualified Data.Map as M

errorsPerLine = M.fromList
    [ ("Chris", 472), ("Don", 100), ("Simon", -5) ]

main = do putStrLn "Who are you?"
          name <- getLine
          case M.lookup name errorsPerLine of
              Nothing -> putStrLn "I don't know you"
              Just n  -> do putStr "Errors per line: "
                            print n

The import says to use code from Data.Map and that it will be prefixed by M. (That's necessary because some of the functions have the same names as functions from the prelude. Most libraries don't need the as part.)

If you want something that's not in the standard library, try looking at http://hackage.haskell.org/packages/hackage.html or this wiki's applications and libraries page. This is a collection of many different libraries written by a lot of people for Haskell. Once you've got a library, extract it and switch into that directory and do this: 

 runhaskell Setup configure
 runhaskell Setup build
 runhaskell Setup install

On a UNIX system, you may need to be root for that last part.

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