Haskell - Trying to apply a function to lines of multiple numbers

Question

I am new to Haskell and I am trying to apply a function (gcd) to input on standard in, which is line separated and each line contains no less or more than two numbers. Here is an example of my input:

3
10 4
1 100
288 240

I am currently breaking up each line into a tuple of both numbers, but I am having trouble figuring out how to separate these tuples and apply a function to them. Here is what I have so far:

import Data.List

main :: IO ()
main = do
  n <- readLn :: IO Int
  content <- getContents
  let  
    points = map (\[x, y] -> (x, y)). map (map (read::String->Int)). map words. lines $ content
    ans = gcd (fst points :: Int) (snd points :: Int)
  print ans

Any information as two a good place to start looking for this answer would be much appreciated. I have read through the Learning Haskell tutorial and have not found any information of this particular problem.

Answer 1

You are pretty close. There is no reason to convert to a tuple or list of tuples before calling gcd.

main = do 
  contents <- getContents 
  print $ map ((\[x,y] -> gcd (read x) (read y)) . words) . lines $ contents 

All the interesting stuff is between print and contents. lines will split the contents into lines. map (...) applies the function to each line. words splits the line into words. \[x,y] -> gcd (read x) (read y) will match on a list of two strings (and throw an error otherwise - not good practice in general but fine for a simple program like this), read those strings as Integers and compute their GCD.

If you want to make use of lazy IO, in order to print each result after you enter each line, you can change it as follows.

main = do 
  contents <- getContents 
  mapM_ (print . (\[x,y] -> gcd (read x) (read y)) . words) . lines $ contents