修改真實世界Haskell程序來計算字數和行數使用MapReduce

Question

我想對真正的世界哈斯克爾第24章中的程序做一些簡單的修改。下面是一些計算文件中行數的代碼：

import qualified Data.ByteString.Lazy.Char8 as LB 
lineCount :: [LB.ByteString] -> Int64 
lineCount = mapReduce rdeepseq (LB.count '\n') 
         rdeepseq sum 

我想寫一些代碼來計算文件中的單詞。以下是我想出了：

import qualified Data.ByteString.Lazy.Char8 as LB 
lineCount :: [LB.ByteString] -> Int64 
lineCount = mapReduce rdeepseq (length LB.words) 
         rdeepseq sum 

不過，我得到：

Couldn't match expected type `LB.ByteString -> b0' 
      with actual type `Int' 
In the return type of a call of `length' 
Probable cause: `length' is applied to too many arguments 
In the second argument of `mapReduce', namely `(length LB.words)' 
In the expression: 
    mapReduce rdeepseq (length LB.words) rdeepseq sum 

可能是什麼問題呢？

Answer 1

你想申請length的結果致電LB.words，而不是LB.words的功能。嘗試(length . LB.words)。