Concat functions in haskell

Question

I'm new to Haskell so I have lots of questions so I'd like your help.

Make a function "func" that receives a function " f " (String -> String) type and a String "s" that returns the reverse of "s" concat with " f " application in "s"

I managed to make the " f " function like this:

f:: String -> String
f x -> reverse(x) ++ x

console: f "HASKELL" -> "LLEKSAHHASKELL"

But when it comes to the "func" function I don't know what to do or type in the console. This code below worked in GHC, but I don't know why, it doesn't accepts entries.

func:: (String -> String) -> String -> String
func f s = (reverse(s) ++ f "")

console: func "HASKELL" "ROCKS"

<interactive>:49:6: error:
    • Couldn't match expected type ‘String -> String’
                  with actual type ‘[Char]’
    • In the first argument of ‘func’, namely ‘"HASKELL"’
      In the expression: func "HASKELL" "ROCKS"
      In an equation for ‘it’: it = func "HASKELL" "ROCKS"

If there's a better way to do it, please can you tell me ?

Thanks for your help.

Why f ""? The text seems to ask for f s instead. Anyway, you can't call func with two strings, the first argument must be a function. — chi
– chi, Commented Sep 22, 2019 at 16:32

willeM_ Van Onsem · Accepted Answer · 2019-09-22 16:34:59Z

3

Your function is almost correct, except that you did not use s as parameter for the function application with s:

func:: (String -> String) -> String -> String
func f s = reverse s ++ f s

You furthermore use the function the wrong way, since the first parameter is not a string, but a function that maps a string on a string. We can for example use id :: a -> a to return s, or reverse :: [a] -> [a] to reverse the string:

Prelude> func id "haskell"
"lleksahhaskell"
Prelude> func reverse "haskell"
"lleksahlleksah"

answered Sep 22, 2019 at 16:34

willeM_ Van Onsem

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