Question

The program return all possible combinations of ‘0’ and ‘1’ length N.

addToElement :: String -> String -> String
addToElement element symbol = element ++ symbol

addOneToElement :: String -> String
addOneToElement element = addToElement element "1"                

addZeroToElement :: String -> String
addZeroToElement element = addToElement element "0"                

processListOnce :: [String] -> [String]
processListOnce lst = do
    let s1 = map addOneToElement lst
    let s2 = map addZeroToElement lst 
    s1 ++ s2

processList :: [String] -> Integer -> [String]
processList lst 1 = processListOnce lst
processList lst n = do
             let tmp = processListOnce(lst)
             processList tmp (n - 1)

{-                       
processList2 :: [String] -> Integer -> [String]
processList2 lst n = iterate (map processListOnce) lst !! n
-}

main = do
     let s = processList ["0", "1"] 2
     let ss = show s
     putStrLn ss

It is my first Haskell program so I will be thankful if you help me:

• First of all pls refactore my code Haskell-way. I already know one magic refactring:

   Control.Monad.replicateM n [0,1]
  

  but this solution is not good for studying purposes 🙂

• Why I can not use ProcessList2 instead ProcessList and get error:

  all_possible_combinations.hs:44:51:
  Couldn't match expected type `[Char]' against inferred type `Char'
  Expected type: [String]]
  Inferred type: [String]
  In the second argument of `iterate', namely `lst'
  In the first argument of `(!!)', namely
   `iterate (map processListOnce) lst'
  

  • Is there any way to skip (not use) the ‘tmp’ variable in the processList? I have tried, but got the error:

    processList :: [String] -> Integer -> [String]
    processList lst 1 = processListOnce lst
    processList lst n = processList processListOnce(lst) (n - 1)
    
    
    all_possible_combinations.hs:39:32:
    Couldn't match expected type `[String]'
    against inferred type `[String] -> [String]'
    In the first argument of `processList', namely `processListOnce'
    In the expression: processList processListOnce (lst) (n — 1)
    In the definition of `processList':
    processList lst n = processList processListOnce (lst) (n — 1)
    

Thanks in advance.

Answer 1

Is there any way to skip (not use) the ‘tmp’ variable in the processList? I have tried, but got the error:

This definition has used the wrong precedence. You should write

processList lst n = processList (processListOnce lst) (n - 1)
-- #                            ^                   ^

Why I can not use ProcessList2 instead ProcessList and get error:

processListOnce is already a [String] -> [String] function. If you use map processListOnce it will become a [[String]] -> [[String]] function. Thus, remove the map.

processList2 lst n = iterate processListOnce lst !! n