On 06/05/17 14:50, Alex Wede wrote:
Now I need to write a functor instance
instance Functor (FunPB a) where
fmap f b = ?
It will help to see the specific types of the instance instance Functor (FunPB a) where fmap :: (c -> d) -> FunPB a c -> FunPB a d ie we need to change only the second argument of FunPB a c. By you definition, that would be the [b] in data FunPB a b = FunPB { runFunPB :: a -> (a,[b]) } so you can remove the constructor to get the inner function, create a new function that uses the inner one and fmap the second part of the tuple instance Functor (FunPB a) where fmap f (FunPB lambda) = FunPB $ \a1 -> let (a2, bs) = lambda a1 in (a2 , fmap f bs) see that we have a `fmap f bs` in the second part, we use that we already have a Functor instance for [b], making our job easier. -- -- Ruben