How does fmap work for List
functor, haskell
Solution
`map` is just a normal function with type `(a -> b) -> [a] -> [b]`. Unlike `fmap`, it is not part of the `Functor` typeclass. It works exactly how you think it does.
The idea behind typeclasses is that you use the types to figure out which implementation to use. When we say `instance Functor [] where ...`, we're telling the compiler what the implementation of `fmap` for `[]` (the list type) is.
In this case, the implementation for `fmap` is just `map`, which is a normal function.
Problem
Learn you a haskell gives description about Functor typeclass. I can see that for list, it's implemented as follows: ``` instance Functor [] where fmap = map ``` But how does this work ? In the typeclass Functor, fmap doesn't even have an implementation. All it has is just type declaration like this: ``` class Functor f where fmap :: (a -> b) -> f a -> f b ``` Just by having the type declaration, how does Haskell figure out map operation for Lists correctly ?