Numbering the strings in a list of strings in Haskell

In Haskell variables are immutable, so you can not change the value of a variable like in an imperative language like Python.

You can for example use zipWith :: (a -> b -> c) -> [a] -> [b] -> [c] to zip the list of strings together with a list of Ints:

numberLines :: [String] -> [String]
numberLines = zipWith f [1 :: Int .. ]
    where f i s = show i ++ ' ' : ':' : s

Here f thus takes an Int and a String and produces a string "i: s" with i the string representation of the number, and s the string.

Others suggested you try using zipWith or mapAccumL to implement the function. Here are two implementations using those functions:

addNumber n string = show n ++ ": " ++ string

numberLines = snd . mapAccumL (\n string -> (n + 1, addNumber n string)) 1

numberLines1 = zipWith addNumber [1..]