Question

I have a String like "1 2 3 4 5". How can I convert it into a list of integers like [1,2,3,4,5] in Haskell? What if the list is "12345"?

Answer 1

You can use

Prelude> map read $ words "1 2 3 4 5" :: [Int]
[1,2,3,4,5]

Here we use words to split "1 2 3 4 5" on whitespace so that we get ["1", "2", "3", "4", "5"]. The read function can now convert the individual strings into integers. It has type Read a => String -> a so it can actually convert to anything in the Read type class, and that includes Int. It is because of the type variable in the return type that we need to specify the type above.

For the string without spaces, we need to convert each Char into a single-element list. This can be done by applying (:"") to it — a String is just a list of Chars. We then apply read again like before:

Prelude> map (read . (:"")) "12345" :: [Int]
[1,2,3,4,5]