Changing a digit to a integer within string comprehension

Question

multDigits :: String -> Int
multDigits "" = 1
multDigits str = [product x | x <- str, isDigit x, digitToInt x]

I basically need to get all of the digits from a string and find their product, but can't find much information online on how to use isDigit.

"Hello, m7 name is 9m1" should give the output 63. I am not allowed to use any other functions bar digitToInt.

• willeM_ Van Onsem: hackage.haskell.org/package/base-4.14.0.0/docs/…
• amalloy: @WillemVanOnsem How is that link helpful at all? That function is already in the question.
• Robin Zigmond: seems fair to me as a response to "[I] can't find much information online on how to use isDigit."
• Robert Lutino: I meant within string comprehension, sorry about that.

Answer 1

digitToInt x will not work, since that is a filter in the list comprehension, and this thus means that digitToInt x should return a Bool. Even if somehow that would work, it would not do much, since it would convert it to an Int, and then you ignore the result.

You should put the digitToInt in the "yield" part of the list comprehension, so:

[digitToInt x | x <- str, isDigit x]

now we have a list of Ints, but this will not calculate the product, you can not put the product :: (Foldable f, Num a) => f a -> a function in the yield part, since then you calculate the product per item, but you can not calculate the product of a simple Int. You thus calculate the product over the entire list, so:

multDigits :: String -> Int
multDigits str = product [digitToInt x | x <- str, isDigit x]

You can also make use of map :: (a -> b) -> [a] -> [b] and filter :: (a -> Bool) -> [a] -> [a], instead of using list comprehension, so:

multDigits :: String -> Int
multDigits = product . map digitToInt . filter isDigit

• Robert Lutino: That you very much, you gave me a lot of useful key words to look up, and a very good explanation!