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    I realize this is partly because the examples are in Scala, but none of this gets at what a Functor really is.

    Functor is an algebra.

    Functor is an algebra with one operation, usually called map.

    That one operation has a type something like:

    (a -> b) -> f a -> f b
    

    That one operation should respect identity:

    map id = id
    

    And that one operation should be associative:

    map (p . q) = (map p) . (map q)
    

    That’s it people. That’s it. Functor is a very weak structure. Many things can be functor. Many of those things will not look anything like a “list”, “collection”, or even a “data structure”.

    Understanding free objects, free versions of these algebraic structures, can lend a more faithful intuition for what these things are.

    Glancing at Coyoneda (the free functor) should give one some idea of why you’re not dealing with something that has anything to do with lists.

    Want to know more?

    You know the drill: https://github.com/bitemyapp/learnhaskell

    Edit:

    Since I take great satisfaction in excising misunderstandings, I’m going to include a Functor instance that should help drop the “collections” oriented view of what they are.

    -- (->) or -> is the type constructor for functions
    -- a -> a, the identity function's type is a type of
    -- -> taking two parameters of the same type (a and a)
    -- (->) a a   analogous to   Either a b
    instance Functor ((->) r) where
        map = (.)
    
    -- (.) or .  is function composition
    -- (.) :: (b -> c) -> (a -> b) -> a -> c
    -- more on this Functor instance:  http://stackoverflow.com/questions/10294272/confused-about-function-as-instance-of-functor-in-haskell
    

    Bonus round for upvoting me:

    http://www.haskellforall.com/2012/09/the-functor-design-pattern.html

    http://hackage.haskell.org/package/kan-extensions-3.7/docs/Data-Functor-Coyoneda.html

    http://oleksandrmanzyuk.wordpress.com/2013/01/18/co-yoneda-lemma/

    http://www.reddit.com/r/haskell/comments/17a33g/free_functors_the_reason_free_and_operational_are/c83p8k2

    https://gist.github.com/thoughtpolice/5843762

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      My upvote does not express how strongly I appreciate this kind of comment that expands on the post, brings in new material, and links to more. So I’m saying thanks. :)

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        Thanks to your comment, it was my pleasure. I was cringing and expecting downvotes when I posted it.

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        Understanding free objects, free versions of these algebraic structures, can lend a more faithful intuition for what these things are.

        This is a super great point—it also, meaningfully, applies to other structures like Monads, Applicatives, or Monoids, Categories, Arrows. Really quickly, here’s Yoneda and Coyoneda (the “two” free functors)

        newtype Yoneda f a = Yoenda { runYoneda :: forall b . (a -> b) -> f b }
        data Coyoneda f b where Coyoneda :: f a -> (a -> b) -> Coyoneda f b
        

        In each case we see that functor tends to mean having a parametric structure (the f) and a method of transforming the parameter to something else (the functions a -> b). When we “collapse” this free view of a functor we get to decide if, how, when, and why we combine that structure and its mapping function. For lists we, well, map it. For something like

        data Liar a = Liar -- note that `a` does not appear on the right side
        

        we just throw the mapping function away.

        (Another key point that’s a bit harder to see is that if you map the Yoneda/Coyoneda formulation repeatedly it does not store each and every mapping function but instead composes them all together and retains only that composition. This ensures that functors cannot “see” how many times fmap has been called. That would let you violate the functor laws!)

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          Do you have any reference of functor being an algebra? I’m intrigued

          Since we’re clarifying what a functor is, I guess is worth noting that you’re talking about endofunctors in the (idealized) Hask category. In category theory, a functor is defined by two mappings: one for objects in the category and one for arrows, that must preserve identity and composition (the laws you mention). Since the mapping of objects is already given by the type constructor, here one needs to provide only the mapping of functions but it kind of irks me when ppl. say a functor is only defined by “map” :)

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            I didn’t want to force the thunk for categories, natural transformations, etc. None of the readers will care if they’re still at the stage of thinking Functor has something to do with List.

            Monoid et al. are more properly thought of as an algebra, Functors are more accurately thought of as a mapping.

            There’s a tradeoff to be made with lying minimally to avoid false intuitions and not forcing a bunch of thunks that aren’t pertinent to the topic under discussion.

            I anticipated this comment.

            The point was to emphasize the abstract-ness of Functor.

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              Yes, but why characterizing functor as an algebra? I think a mapping is pretty clear and understood (heck it can even be more intuitive!)

              I agree my point could be pedantic, but on the other hand, the basics of CT are pretty simple and should emphasize the abstractness of functor more than mentioning Coyoneda :)

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                Functor is definitely an algebra. Its rules mean that it has tight relation to certain functors in CT.

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                  Interesting… any refereces I can read? Or you’re talking about F-algebras?

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                    I mean “algebra” as “set of operations and equalities”.

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                      Ok. To be honest, I need to familiarize myself with the definition of algebra, is just that I had never heard this before :)

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                        It’s an incredibly overloaded term, tbh. In the context of abstract algebra you’d probably want to think of a (G, L)-algebra as a set inductively defined by generators G and laws L. For instance, here’s a “free” monoid algebra (note that this isn’t a free monoid, but a “free monoid algebra” or a “free algebra of the monoid type” or a “(monoid, {})-algebra” maybe)

                        data FMonoid where
                          Fmempty :: FMonoid
                          Fmappend :: FMonoid -> FMonoid -> FMonoid
                        
                        class Monoid FMonoid where    -- this is wrong! doesn't follow laws!
                          mempty = Fmempty
                          mappend = Fmappend
                        

                        note that it has all the “generators” of the typeclass Monoid but follows none of the rules (mempty <> mempty != mempty). Typically we also want to add a set of constants to form the smallest free algebra over a set

                        data FMonoid a where
                          Embed :: a -> FMonoid a
                          Fmempty :: FMonoid a
                          Fmappend :: FMonoid a -> FMonoid a -> FMonoid a
                        
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                          Really interesting, thanks a lot! Now I’m trying to see how this ties to the Functor typeclass: G are the instance constructors and the functor laws make L ? I think I’m missing an important piece of the puzzle here :)

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                            You’re not, that’s basically it.

                            data FFunctor f a where
                              EmbedFunctor :: f a -> FFunctor f a
                              Ffmap :: (a -> b) -> FFunctor f a -> FFunctor f b
                            

                            This lets you build the free (Functor, {})-algebra over some initial type f. If we translate it naively then it doesn’t follow the laws

                            class Functor (FFunctor f) where -- wrong!
                              fmap = Ffmap
                            

                            but we can implement it properly if we’re a little more clever

                            class Functor (FFunctor f) where
                              fmap f x = case x of
                                EmbedFunctor fa -> Ffmap f x
                                Ffmap g fa -> Ffmap (f . g) fa
                            

                            We need one more function, though, since we can’t use EmbedFunctor directly without exposing information about whether or not we’ve ever fmaped this functor (which shouldn’t be possible to access, that’s what fmap id = id says)

                            embed :: f a -> FFunctor f a
                            embed fa = Ffmap id (EmbedFunctor fa)
                            

                            And now, if we think about it, we can see that every value of FFunctor constructed using embed and fmap is of the form

                            Ffmap fun (EmbedFunctor fa)
                            

                            And so that EmbedFunctor constructor is totally superfluous. Let’s remove it

                            data FFunctor f a where
                              Ffmap :: (a -> b) -> f a -> FFunctor f b
                            
                            embed :: f a -> FFunctor f a
                            embed fa = Ffmap id fa
                            

                            And—well—this is just CoYoneda again!

                            lower :: Functor f => FFunctor f a -> f a
                            lower (Ffmap f fa) = fmap f fa
                            
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                              Nice Haven’t digested it properly but I see the trick is to capture the functor with a datatype (is the same thing with free monads, right?) Now is easier to see from where CoYoneda comes, thanks! (you did show me an important piece of the puzzle :P )

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                                On a side note, I went back to a book about CT, and defines categories as an algebraic structure, so I guess it can be called an algebra :) (besides, identity and composition form a monoid) Since Functors form categories too, it might not be that much of a stretch to call them algebras either