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Could not match type `[] 'with` IO' - Haskell

I am new to Haskell. In this task, I am performing a split operation, but I am facing a problem due to an incorrect type match. I am reading data from a text file and the data is in table format. Ex. 1|2|Rahul|13.25.

In this format. Here |

is the delimiter, so I want to split the data from the delimiter |

and I want to print data from the 2nd column and 4th column, but I get an error like this

  "Couldn't match type `[]' with `IO'
    Expected type: IO [Char]
      Actual type: [[Char]]
    In the return type of a call of `splitOn'"

Here is my code ..

module Main where

import Data.List.Split

main = do
    list <- readFile("src/table.txt")
    putStrLn list
    splitOn "|" list

Any help on this would be appreciated .. Thanks

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2 answers


The problem is when you are trying to return a list from a function main

that is of type IO ()

.

What you probably want to do is print the result.



main = do
    list <- readFile("src/table.txt")
    putStrLn list
    print $ splitOn "|" list
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Not Haskell, but it looks like a typical awk task.

cat src / table.txt | awk -F '|' '{print $ 2, $ 4}'

Back to Haskell, the best I can find is:



module Main where

import Data.List.Split(splitOn)
import Data.List (intercalate)

project :: [Int] -> [String] -> [String]
project indices l = foldl (\acc i -> acc ++ [l !! i]) [] indices

fromString :: String -> [[String]]
fromString = map (splitOn "|") . lines

toString :: [[String]] -> String
toString = unlines . map (intercalate "|")

main :: IO ()
main = do
  putStrLn =<<
    return . toString . map (project [1, 3]) . fromString =<<
    readFile("table.txt")

If you are not reading a file, but from stdin, the function interact

might be useful.

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