Making Data.Map a functor in Haskell

dictionary, functor, haskell

Solution

First thing I noticed is that your `instance` syntax isn't quite right:

instance (Ord k) => MyFunctor (DataMap.Map k) where
    ...

Otherwise it seems fine.

Problem

I'm studying the basics of Haskell from Learn You a Haskell for Great Good! There is an exercise in the book where you need to make Data.Map into a Functor. I'm trying to make my own functor typeclass and make Data.Map into an instance of it. So here's what I tried: ``` import Data.Map as DataMap import Data.List as DataList class MyFunctor f where myfmap :: (a -> b) -> f a -> f b instance MyFunctor (Ord k) => (DataMap.Map k) where myfmap f x = DataMap.fromList $ DataList.map (\(p,q) ->(p,f q)) $ DataMap.toList x ``` When I try to compile this, GHCI gives me this error: ``` `myfmap' is not a (visible) method of class `Map' ``` I tried looking around other SO answers, blogs, mailing list threads etc without much luck. The only thing I found was the description of the error message in the Haskell Wiki which says that GHCI throws this error when one tries to instantiate a class, but did not import the functions one tries to implement. I have imported Data.Map and Data.List so I don't think that's the real cause. What am I doing wrong?

Original source