Counting dashes in numbers

Question

I have written my very first Haskell module and I feel that the code can be improved based on the simplicity of the problem I am solving. Do you have any suggestions?

Problem statement:

You are given a String code containing a message composed entirely of decimal digits ('0'-'9'). Each digit consists of some number of dashes (see diagram below). A "check function" of a message is defined as the total number of dashes in the message. Return the value of the check function for the message represented in code.
Definition
Class: CheckFunction
Method: newFunction
Parameters: String
Returns: int
Method signature: int newFunction(String code)
(be sure your method is public)
Notes
- 0 consists of 6 dashes, 1 consists of 2 dashes, 2 consists of 5 dashes, 3 consists of 5 dashes, 4 consists of 4 dashes, 5 consists of 5 dashes, 6 consists of 6 dashes, 7 consists of 3 dashes, 8 consists of 7 dashes, 9 consists of 6 dashes.
Constraints
- code will contain between 1 and 50 characters, inclusive.
- Each character in code will be a digit ('0'-'9').

Solution:

module Main where
import System.IO()
import Data.Char
-- Problem statement: https://community.topcoder.com/stat?c=problem_statement&pm=4788
-- Method signature: int newFunction(String code)
newFunction :: String -> Int
newFunction input =
 let arr = [6, 2, 5, 5, 4, 5, 6, 3, 7, 6]
 x = [arr !! digitToInt (input !! x) | x <- [0..length input - 1]]
 in sum x
assert :: String -> Int -> String
assert s i = if newFunction s == i
then "test passed" else "test failed"
main :: IO()
main = do
 putStrLn $ show $ assert "13579" 21
 putStrLn $ show $ assert "868177" 28
 putStrLn $ show $ assert "8571" 17
 putStrLn $ show $ assert "3" 5
 putStrLn $ show $ assert "73254370932875002027963295052175" 157

• Caridorc: Please include problem statement in the question (or your re-wording of it)
• Orestis: I have already included a link of the problem statement. Do you need more information?
• Caridorc: it is a site policy we do not like relying on external website as they may go down and if they do our pages lose meaning Thanks

Answer 1

Take items directly from the thing, without indexing:

x = [arr !! digitToInt i | i <- input]

And prefer where over let: (where is so much more common than let, it is easier to read as you see the general picture before the details.)

newFunction input = sum [arr !! digitToInt i | i <- input]
 where
 arr = [6, 2, 5, 5, 4, 5, 6, 3, 7, 6]

arr is a bad name, I would prefer dashesInDigits

assert should be more general (take f as argument) and written using guards.

• Orestis: Is it better to use " ^? element " from lens package instead of " !! "?
• Caridorc: @Orestis in this case you may consider lookup that also works for non-number keys