逆波兰表达式计算器

今天在learnyouahaskell.com上看见一段逆波兰表达式计算器的代码，挺美妙的。

solveRPN :: (Num a, Read a) => String -> a
solveRPN = head . foldl foldingFunction [] . words
    where foldingFunction (x:y:ys) "*" = (x * y):ys
          foldingFunction (x:y:ys) "+" = (x + y):ys
          foldingFunction (x:y:ys) "-" = (y - x):ys
          foldingFunction xs numberString = read numberString:xs

第一行是函数solveRPN的类型声明，略过。简单的流程解释一下就是：先用words将字符串tokenize，然后用上foldl扫描归纳之，最后将结果存在list的第一个节点，用head取出来。简单明了，一气呵成。比如，给定字符串"10 8 + 2 -"。下面是归纳步骤。

words "10 8 + 2 -" 得到 ["10", "8", "+", "2", "-"]，接下来便是：foldl [] ["10", "8", "+", "2", "-"]。应用传递给foldl的函数foldingFunction，得到运算过程：

1. fold1 [] "10" [""8", "+", "2", "-"]


2. foldl [10] [""8", "+", "2", "-"]


3. foldl [10, 8] ["+", "2", "-"]


4. foldl [18] ["2", "-"]


5. foldl [18, 2] ["-"]


6. foldl [20]

foldl结束后head [20]，便得到20。需要注意的是第六行，它的意思是(read numberString) : xs，而不是read (numberString:xs)。优先级问题哈。