在 Haskell 中对字符串列表中的字符串进行编号

Question

So I have to number each string in list of strings

Like this:

numberLines ["one", "two", "three"] 
   ==> ["1: one", "2: two", "3: three"]

I tried this:

numberLines = map (\g -> (x + 1) : g) where
 x = 0

but of course it didn't work

Answer 1

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 Int s:

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.