Proposal: Add Compositor class as superclass of Arrow

apfelmus apfelmus at
Fri Oct 19 05:01:42 EDT 2007

Cale Gibbard wrote:
> fmap (f . g) x = fmap f (fmap g x)
>   becomes:
> (f . g) . x = f . (g . x)
> fmap id x = x
>   becomes
> id . x = x

Nice! Can this be done in Category Theory, too? I mean, it would be nice 
to internalize morphism, functors, natural transformations, ... in one 
and the same category (like Hask), so there's less fuss. I.e. given a 
category  C , construct an category  C\infty  that is basically the same 
as  C  but also contains the functors, natural transformations etc. of 
C and has this handy infix (.) operation.

> I've tried this out for a while, and it is actually rather nice to use
> in many cases. Functor application is common enough that having a
> one-character representation for it is great.

I can't remember using fmap/liftM very often, but if I use `liftM`, then 
often in infix notation, so and infix symbol for  fmap  is indeed a very 
good idea.

However, (.) in that role confuses me because I always think that the 
right argument should be function. In other words, I'm fine with

   print . sum . map read . lines . readFile

( with a hypothetical instance Category (a -> IO b) ) while your 
proposal gives rise to

   show . sum . map read . lines . readFile "foo.txt"

which makes me feel ill. In my opinion, function composition and 
function application should have separate notations. The new (.) blurs 
these lines too much for my taste (i.e. (.) :: (a -> b) -> Id a -> Id b) 
and I prefer <$> (or even plain $) for  fmap .

In addition, I always longed for categories without an embedding (a -> 
b) -> c a b , they keep popping up while I program in Haskell and more 
often than I need infix  fmap . Also, I dislike (>>>) or (<<<) and very 
much prefer (.) for them.

But in the end, we can have both worlds of (.) without name clash! 
Simply annotate functors with the category they operate on :)

   class Category c => Functor c f where
      (.) :: c a b -> f a -> f b

   instance Functor (->) [] where
      (.) = map

   instance Category c => Functor c (c d) where
      (.) = `o`


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