Making Data.Map a functor in Haskell

Question

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?

Tom Savage · Accepted Answer · 2014-01-07 11:32:10Z

16

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.

edited Jan 7, 2014 at 11:32

answered Jan 7, 2014 at 11:25

Tom Savage

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4 Comments

Anup Cowkur Over a year ago

Stupid me. That was it. Thanks!

The Internet Over a year ago

myfmap f = M.fromList . map (second f) . M.toList hlint can help too

Anup Cowkur Over a year ago

@TheInternet yup. Simpler than my current version. Thanks.

The Internet Over a year ago

@AnupCowkur, this is a little more verbose, but more readable IMO. lpaste.net/98091

DeadDork · Accepted Answer · 2014-05-02 08:05:28Z

7

Alternatively:

import qualified Data.Map as M          

class Functor' f where                  
    fmap' :: (a -> b) -> f a -> f b   

instance (Ord k) => Functor' (M.Map k) where
    fmap' = M.map

answered May 2, 2014 at 8:05

DeadDork

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1 Comment

DeadDork Over a year ago

I honestly believe that Tom Savage's answer is more pedagogical, but I'm pretty sure that Miran was expecting something more along the lines of what I wrote (based on his code for a list as an instance of the Functor class).

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