Haskell - Reduce list - MapReduce

Question

我试图减少元组列表，其中重复键的值加在一起如下：

[(the, 1), (the, 1)] => [(the, 2)]

我试过了：

reduce :: [(String, Integer)] -> [(String, Integer)]
reduce [] = []
reduce [(k, v) : xs] = (+) [(k, v)] : reduce xs 

我收到此错误：

 Couldn't match expected type `(String, Integer)'
             with actual type `[(String, Integer)] -> [(String, Integer)]'

我做错了什么？

修改

这是完整的程序

toTuple :: [String] -> [(String, Integer)]
toTuple [] = []
toTuple (k:xs) = (k, 1) : toTuple xs

reduce :: [(String, Integer)] -> [(String, Integer)]
reduce [] = []
reduce [(k, v) : xs] = (+) [(k, v)] : reduce xs     

main_ = do list <- getWords "test.txt"
       print $ reduce $ toTuple list

-- Loads words from a text file into a list.
getWords :: FilePath -> IO [String]
getWords path = do contents <- readFile path
               return ([Prelude.map toLower x | x <- words contents]) 

Answer 1

您正在进行错误的模式匹配。模式匹配应该是这样的：

  ((k,v):xs)

(k,v)表示列表的头部，xs表示列表的尾部。同样，这也存在问题：

(+) [(k, v)] : reduce xs 

+的类型是：

λ> :t (+)
(+) :: Num a => a -> a -> a

你不能简单地(+) [(k, v)] : reduce xs做任何不合理的事情。您必须检查字符串的内容，然后添加元组的第二部分。