#4189: Add (<.>) operator (generalizing (.) to Functor)

[wren ng thornton]

Maciej Marcin Piechotka wrote:
> 
> The "a <$> b <$> c <$> d" was done to show the relation between $/. and
> <$>/<.>.

Yes, but (.) _is_ (<$>) for (a->). Therefore, trying to make (.) and ($) 
opposing conceptions and then extending it to (<.>) and (<$>) doesn't 
necessarily make sense.

Personally, I think being explicit about the use of fmap here makes code 
a lot clearer overall. One prime example has already been raised where 
it makes it clear that (fmap f . fmap g . h) should be written (fmap(f . 
g) . h) instead. This isn't a case like (<=<) or (<<<) where we are 
actually generalizing composition in Hask to composition in another 
category. I'm not a big fan of making a composition operator that 
crosses between categories; it just doesn't feel like a clean 
abstraction. The notation of (<$>) for fmap is a clean abstraction in 
the context of Applicative because it vs (<*>) captures the semantic 
differences between the spaces in (f x y). Outside of the applicative 
setting, use of (<$>) instead of fmap tends to obfuscate code rather 
than improve legibility, IMO.

-- 
Live well,
~wren