Haskell中的递归仿函数实例

Question

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

Answer 1

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.

Answer 2

EDIT

found the problem my datatype is recursive but mine Functor instance was not.

data List x = Hol | Hoofd x (List x)
            deriving (Show,Eq,Ord)


instance Functor List where
            fmap f Hol = Hol
            fmap f (Hoofd x rest) = Hoofd (f x) (fmap f rest)

Now it works, if there is another element which needs to be processed it will be send back to the instance until there is noting to process