What does it means Instance of Num (Maybe Int) required for definition of mayor-CodePudding

mayor ::[Integer] -> Integer -> Maybe Int
mayor [] _ = 0
mayor (x:xs) y = if x > y then findIndex (==x) (x:xs) else mayor xs y

Why do I get the error "Instance of Num (Maybe Int) required for definition of mayor"

CodePudding user response：

On the second line you use the literal 0, you probably intend this to have type Int. If you add that as a type signature, you will get a more informative error:

mayor ::[Integer] -> Integer -> Maybe Int
mayor [] _ = 0 :: Int
mayor (x:xs) y = if x > y then findIndex (==x) (x:xs) else mayor xs y

B.hs:3:14: error:
    • Couldn't match expected type ‘Maybe Int’ with actual type ‘Int’
    • In the expression: 0 :: Int
      In an equation for ‘mayor’: mayor [] _ = 0 :: Int
  |
3 | mayor [] _ = 0 :: Int
  |              ^^^^^^^^

Now you can see that the type signature of your function specifies that it will return a value of type Maybe Int, but in your code you return a value of type Int.

You can fix this by wrapping 0 in the Just wrapper:

mayor ::[Integer] -> Integer -> Maybe Int
mayor [] _ = Just 0
mayor (x:xs) y = if x > y then findIndex (==x) (x:xs) else mayor xs y

This will fix the errors, but I don't think this does what you expect (hint: findIndex (== x) (x:xs) will always return Just 0).