Haskell - case
函数中模板匹配的本质实际上就是case语句的语法糖。case语句在函数式编程中用途往往比命令式要大很多，因为这样可以很流畅的定义出一个没有指令流程的函数，使得函数的定义更加流畅。
举一个简单的例子
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| firstLetterV1 :: [a] -> a
firstLetterV1 [] = error "Null list"
firstLetterV1 [x:_] = x
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完全等价于这段代码
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| firstLetterV2 :: [a] -> a
firstLetterV2 str = case str of [] -> error "Null list"
(x:_) -> x
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从中我们可以看到case表达式语法长这个样子
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| case expression of pattern -> result
pattern -> result
pattern -> result
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于是乎就可以写出这样的代码
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| scoreValue :: Int -> Char
scoreValue value =
let range = div value 10
in case range of 6 -> 'D'
7 -> 'C'
8 -> 'B'
9 -> 'A'
10 -> 'S'
_ -> 'F'
describeList :: [a] -> String
describeList ls = "The list is " ++ str ls
where str [] = "empty"
str [x] = "a singleton list"
str ls = "a longer list"
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