Recursive functor instance in haskell

i come across a problem with haskell about recursive Functor instance.

data List x = Hol | Hoofd x (List x)
            deriving (Show,Eq,Ord)
instance Functor List where 
        fmap f (Hoofd x (Hoofd x)) = Hoofd (f x) Hoofd(f x)
        fmap f Hol               = Hol     
        fmap f Hoofd x (Hol)     = Hoofd (f x)

I made a datatype List in which i need to inplement the Functor Fmap instance, i tried to use in the datatype deriving (Functor) but i gives me back an error with: can't derive Functor must use DeriveFunctor which i also did not understand...

please help

Solution

There are two ways you can make a class-instance for some type: either

Use a deriving clause. This only works for simple classes where there's one obvious way to define the instance's methods. In Haskell98, these classes were only Eq, Ord, Enum, Bounded, Show, and Read. By now, the language has evolved a lot and GHC can now derive many more classes, but it always requires switching on a language extension. In particular, to derive Functor, you need to put
```
{-# LANGUAGE DerivingFunctor #-}
```
at the top of your source file, or alternatively call the compiler/intepreter with
```
$ ghc -XDeriveFunctor
```
(I wouldn't recommend the latter; your file wouln't be “self-sufficient” if you do that.)

With that extension enabled, you can simply write
```
data List x = Hol | Hoofd x (List x)
        deriving (Show, Eq, Ord, Functor)
```
Write the instance yourself. That's what you've done in your code snippet, which has nothing to do with deriving. Your implementation is faulty, which is something you might well ask a question about, but then you should include the information what's currently going wrong.