Haskell : Transform String -> [List]

Question

I am trying to read n on the first line then n lines of input and print the sum of the first 2 elements from each line such as :

Input:

2
1 2
3 4

Output:

3
7

so far my code looks like:

import Control.Monad

fromDigits = foldl addDigit 0
 where addDigit num d = 10*num + d

first (x:xs) =   fromDigits x 
second (x:xs) =  fromDigits xs

main = interact processInput
processInput input = unlines [perLine line | line <- lines input]

perLine line =  first line + second line

but I get the following error

Couldn't match type '[Char]' with 'Char'

Couldn't match type 'Char' with '[String]'

I am new to Haskell so I am unsure how to solve it.

Answer 1

Some hints, in order:

• At some point, you need to convert your digits from Char to Int or the like.
  • Haskell won't do that for you unless you ask. Use ord .
• In the x:xs pattern, xs is the rest of the list, not the next element.
  • This is likely where your [Char] vs. Char problem comes from.
• It looks like you want to treat each line as a sequence of words.
  • Try using the words function.
• Finally, you need to convert your numbers into printable form for output.
  • Haskell won't do that for you, either. Use show .

In general, I recommend starting up ghci and playing with it, just to gain some basic familiarity with Haskell. Pull up Hoogle or some other Haskell reference in another window...