Haskell doubt: how to transform a Matrix represented as: [String] to a Matrix Represented as [[Int]] ?

Question

Im trying to solve Problem 11 of Project Euler in haskell. I almost did it, but right now im stuck, i want to transform a Matrix represented as [String] to a Matrix represented as [[Int]].

I "drawed" the matrices:

What i want:

"08 02 22 97 38 15 00 40                  [ ["08","02","22","97","38","15","00","40"],                [[08,02,22,97,38,15,00,40]
 49 49 99 40 17 81 18 57  map words lines   ["49","49","99","40","17","81","18","57"],      ??a        [49,49,99,40,17,81,18,57]
 81 49 31 73 55 79 14 29  ---------->       ["81","49","31","73","55","79","14","29"],  --------->     [81,49,31,73,55,79,14,29]
 52 70 95 23 04 60 11 42                    ["52","70","95","23","04","60","11","42"],                 [52,70,95,23,04,60,11,42]
 22 31 16 71 51 67 63 89                    ["22","31","16","71","51","67","63","89"],                 [22,31,16,71,51,67,63,89]
 24 47 32 60 99 03 45 02"                   ["24","47","32","60","99","03","45","02"] ]                [24,47,32,60,99,03,45,02]]

Im stuck in doing the last transformation (??a)

---

for curiosity(and learning) i also want to know how to do a matrix of digits:

Input:

"123456789               [ "123456789"                    [ [1,2,3,4,5,6,7,8,9]
 124834924    lines        "124834924"         ??b          [1,2,4,8,3,4,9,2,4]
 328423423  --------->     "328423423"      --------->      [3,2,8,4,2,3,4,2,3]
 334243423                 "334243423"                      [3,3,4,2,4,3,4,2,3]
 932402343"                "932402343" ]                    [9,3,2,4,0,2,3,4,3] ]

What is the best way to make (??a) and (??b) ?

Answer 1

What you want is the read function:

read :: (Read a) => String -> a

This thoughtfully parses a string into whatever you're expecting (as long as it's an instance of the class Read, but fortunately Int is such).

So just map that over the words, like so:

parseMatrix :: (Read a) => String -> [[a]]
parseMatrix s = map (map read . words) $ lines s

Just use that in a context that expects [[Int]] and Haskell's type inference will take it from there.

To get the digits, just remember that String is actually just [Char]. Instead of using words, map a function that turns each Char into a single-element list; everything else is the same.