Haskell完整的文本文件索引器

Question

在早些时候看到关于文本文件索引的帖子后,我开始好奇,因为我正在学习使用Haskell(这不太顺利),如果它可以在Haskell中轻松实现相同的目标: 我如何创建Java来自文件内容的字符串？

该课程的目标是读取文件,按字母顺序排列文件中的单词,并显示这些单词出现的行号.

Haskell中的函数是否能够以类似的方式构建？

Answer 1

这是一个可以完成您需要的繁重工作的功能:

import Data.List

sortWords :: String -> [(Int,String)]
sortWords contents = sortBy (\(_,w1) (_,w2) -> compare w1 w2) ys
  where xs = zip [1..] (lines contents)
        ys = concatMap (\(n,line) -> zip (repeat n) (words line)) xs

剩下要做的唯一事情就是编写一些简单的IO代码来读取文件并很好地打印结果.如果您在以下输入上运行此功能:

Here are some test
lines to see if this works...
What: about? punctuation!

你会得到这个:

(1,"Here")
(3,"What:")
(3,"about?")
(1,"are")
(2,"if")
(2,"lines")
(3,"punctuation!")
(2,"see")
(1,"some")
(1,"test")
(2,"this")
(2,"to")
(2,"works...")

Answer 2

这是一个Haskell解决方案

import Data.List (sortBy)
import Data.Function (on)

numberWordsSorted :: String -> [(String,Int)]
numberWordsSorted  = sortBy (compare `on` fst)
                   . concat 
                   . zipWith (\n xs -> [(x,n)|x<- xs]) [1..]   -- add line numbers
                   . map words
                   . lines

main = fmap numberWordsSorted (readFile "example.txt") >>= mapM_ print

如果你example.txt用内容运行它

the quick brown fox
jumps over the lazy dog

你得到

("brown",1)
("dog",2)
("fox",1)
("jumps",2)
("lazy",2)
("over",2)
("quick",1)
("the",1)
("the",2)

Data.Map如果你更愿意看到the [1,2]而不是两行输出,你应该使用而不是成对.