Question

Hello I am new at Haskell and i’m having problems trying to get this script to work. This script reads in arguements from a command line and find them in a seperate text file.

E.G: cat.txt | ./redact house big cat (in compiler)

It redacts certain words in a text file by replacing them with stars (**)asterisks. The number of stars used for each redacted word should equal the number of characters in the word.

module Main where

import System
import Data.Char
import Data.List

lowercase :: String -> String
lowercase = map toLower

main = do 
arg1 <- getArgs
txt <- getContents
putStr (redact txt arg1)

redact :: String -> String -> String
redact input xWords = unlines [ work line | line <- lines input ]
where work line = unwords [ foo word | word <- words line ]
  foo w | lowercase(w) == lowercase(xWords) = convertWord w 1
    | otherwise                         = w

convertWord Eq a => [a] -> [a]
convertWord = map (const '*') 

However, when i try to compile this, GHCi returns the error:

redact.hs:13:38:
    Couldn't match expected thye 'Char' with actual type '[Char]'
    Expected type: String
     Actual type: [String]
    In the second argument of 'redact', namely 'arg1'
    In the first of 'putStr', namely '<redact txt arg1>'
Failed, module loaded: none.

So the code:

putStr (redact txt arg1)

is causing the problem.

Thank you in advance for any help and if you can improve the code in anyway that would be great.

EDIT:

I want to enter as many args as possible, it doesnt matter how many args you enter, i tried:

(arg1:arg2:arg3:arg4:arg5:_) <- getArgs

but I have to enter EXACT 5 args, It shouldn’t matter how many args I enter.
I was thinking of using some kind of loop but I am not sure?

Again thank you for your help.

Answer 1

To get it to work with multiple arguments, use getArgs as you have it. The problem lies with

foo w | lowercase(w) == lowercase(xWords) = convertWord w 1
      | otherwise                         = w

where you compare the lowercase of one word to lowercase of multiple words. The latter is not defined, you’d like to compare it to the lowercase of each of the xWords. So first you need to bring them all to lowercase, that’s most efficiently done by calling from main redact txt (map lowercase arg1) rather than just redact txt arg1. Then you need to determine if a read word is in the list xWords, that’s what the elem function is there for.

foo w | lowercase w `elem` xWords = convertWord w 1
      | otherwise                 = w

BTW, you should maybe not call this function foo even if it’s only a local one.