In any case,

• it's not an issue ;-)
• it's up to you find a solution that can pass the tests.

I know, my solution does not have a type declaration since that occupies a lot of words.. it returns something of type Integral since I use the sum function... I tried the toInteger and fromIntegral functions but it overflows

No, it's you using the wrong type ;-)

It only happens if your f's type is f :: Int -> Int. Look at the initial code, it says f :: Int -> Integer.

I don't think you read the error properly..

Your return type should not be Int, it's too tiny to hold the result ;-)

I have an issue with this kata. I was able to come up with a solution but there's something wrong

test/HaskellGolfSpec.hs:38:33: error:
    • Couldn't match expected type ‘Int’ with actual type ‘Integer’
    • In the second argument of ‘shouldBe’, namely ‘myf n’
      In the expression: f (n :: Int) `shouldBe` myf n
      In the second argument of ‘($)’, namely
        ‘\ n -> f (n :: Int) `shouldBe` myf n’
   |
38 |         f (n :: Int) `shouldBe` myf n
   |                                 ^^^^^```