add char to every tuple second element in list-CodePudding

getNames 'A' [('A', "Smith"), ('J', "Brown")] -> ["A.Smith", "A.Taylor"]

I try to add just char to every string element in list. My code is:

addForEvery :: Char -> [String] -> [String]
addForEvery a (x:xs) = [a]    x : addForEvery a xs
print(addForEvery 'a' ["fa"])

But I get the following error:

jdoodle.hs:6:32: error:
    • Couldn't match type ‘[Char]’ with ‘Char’
      Expected type: [Char]
        Actual type: [String]
    • In the second argument of ‘(  )’, namely ‘x : addForEvery a xs’
      In the expression: [a]    x : addForEvery a xs
      In the expression: [[a]    x : addForEvery a xs]
  |
6 | addForEvery a (x:xs) = [[a]    x : addForEvery a xs]
  |                                ^^^^^^^^^^^^^^^^^^^^

CodePudding user response：

Did it!

getNames :: Char -> [(Char, String)] -> [String]
getNames a [] = []
getNames a ((z, b):xs) = ([a]    ['.']    b) : getNames a xs

CodePudding user response：

The problem is that [a] x : addForEvery a xs is interpreted as [a] (x : addForEvery a xs). This makes not much sense since [a] is a String, it expects x : addForEvery a xs, but that is a list of Strings.

You can use map :: (a -> b) -> [a] -> [b] to apply the same function to all items in the list. We thus can implement this as:

getNames :: [(Char, String)] -> [String]
getNames = map toFullName
    where toFullName fn ln = fn : '.' : ln