I admit, that my question may stem from a lack of knowledge and be rather vague. But I try to understand, have some doubts and can't resolve them.
So GHC.Base have such definition, and what is the sense in it:
instance Functor ((->) r) where
fmap = (.)
From the viewpoint of programming language: We have really base construction (->), I think more base than anything, but maybe terms, and you describe it as a part of very derivative construction (instance Functor). What is the sense? (->) is (->). Functor have any sense as far as (->) described under Haskell hood meaningfully. But not vice versa: (->) have sense while Functor described in Haskell libraries correctly.
From the viewpoint of lambda calculus: 2.1 If from "common sense" definition "(->) r" is a container around r (let's call it "Any_f"), then how function fmap shoul work? fmap should change value into container, but do not change container-structure, try to write it.
fmap f (Any_f x) <=> Any_f (f x)
(yes, this is non-typed lambda calculus)
2.2. But let's look how Functor ((->) r) defined in Haskell:
instance Functor ((->) r) where
fmap = (.)
-- Or in other words (quotation marks intentionaly):
-- fmap f (Any_f x) = f (Any_f x)
-- fmap :: forall a, b, c => (b -> c) -> (a -> b) -> (a -> c)
So:
"common sense" (not change container structure) tells us to write:
fmap f (Any_f_as_container x) = Any_f_as_container (f x)
types requirements tell us to write:
fmap f (any_f_as_container x) = f (Any_f_as_container x)
Doesn't this means that "instance Functor ((->) r)" is meaningless? And if not - what sense does it has when it changes outermost function (container itself, not container value)?
I will try to convince you that fmap = (.) really is a thing that leaves a container's shape the same, but applies a function to all the elements in the container. But before we do that for (->), let's do it for some simpler types. Specifically, let's do it for types that are containers with a specific number of elements -- i.e., a container with exactly two elements will be TwoF, while one with no elements will be ZeroF. Like this:
data ZeroF a = ZeroF
data OneF a = OneF a
data TwoF a = TwoF a a
data ThreeF a = ThreeF a a a
What should the Functor instances for these look like? Well, the one for OneF looks exactly like in your question:
instance Functor OneF where
fmap f (OneF x) = OneF (f x)
The other ones look pretty similar -- just applying f more (or fewer) times to account for the fact that there are more (or fewer) elements. Here they all are, with some creative whitespace to highlight the similarities/pattern:
instance Functor ZeroF where fmap f (ZeroF ) = ZeroF
instance Functor OneF where fmap f (OneF x0 ) = OneF (f x0)
instance Functor TwoF where fmap f (TwoF x0 x1 ) = TwoF (f x0) (f x1)
instance Functor ThreeF where fmap f (ThreeF x0 x1 x2) = ThreeF (f x0) (f x1) (f x2)
Hopefully for now you agree that this definitely has the flavor of Functor instance that you described in your question: keep the shape of the container the same, and apply the given function f to each element contained within.
So those are containers with a given number of elements. Now, let's write accessors for these containers -- i.e. we want the equivalent of (!!) for lists, where given a number, we pull out that field from the container. Since there's zero elements in a ZeroF, we'll need an indexing type with zero values; while for ThreeF we need an indexing type with three values.
data Zero
data One = One0
data Two = Two0 | Two1
data Three = Three0 | Three1 | Three2
The indexing functions have types that look like this:
indexZero :: ZeroF a -> Zero -> a
indexOne :: OneF a -> One -> a
indexTwo :: TwoF a -> Two -> a
indexThree :: ThreeF a -> Three -> a
I won't implement them all -- they're pretty straightforward -- but here's one to give you the idea in case it's not immediately obvious.
indexTwo (TwoF x0 x1) Two0 = x0
indexTwo (TwoF x0 x1) Two1 = x1
It turns out that the indexing functions have an inverse -- if you give me a function which, when given an index, produces a value, then I can give you a container with those values in it. The types look like this:
tabulateZero :: (Zero -> a) -> ZeroF a
tabulateOne :: (One -> a) -> OneF a
tabulateTwo :: (Two -> a) -> TwoF a
tabulateThree :: (Three -> a) -> ThreeF a
(Do you see why this is the right type for an inverse? Note that, say, TwoF a -> Two -> a is the same type as TwoF a -> (Two -> a)!) Just to give you an idea of how these are implemented, in case it's not immediately obvious, we simply apply the indexing function to each index:
tabulateZero ix = ZeroF
tabulateOne ix = OneF (ix One0 )
tabulateTwo ix = TwoF (ix Two0 ) (ix Two1 )
tabulateThree ix = ThreeF (ix Three0) (ix Three1) (ix Three2)
It's not too hard to prove that tabulateX . indexX = id and indexX . tabulateX = id for each X, i.e. that tabulation really is the inverse of indexing.
Okay, but hold up now and take a look at what we've just done: we have turned a function (like Two -> a) into a container (like TwoF a), and vice versa. The types Two -> a and TwoF a are, morally speaking, exactly the same thing. So it seems reasonable to think we could implement fmap for Two -> a -- for example, just by converting to TwoF a and back as appropriate!
twomap :: (a -> b) -> (Two -> a) -> (Two -> b)
twomap f = indexTwo . fmap f . tabulateTwo
Let's visualize what that's doing. We'll start with an arbitrary indexing function:
\case Two0 -> x0; Two1 -> x1
Now we go through the process:
\case Two0 -> x0; Two1 -> x1
tabulateTwo
TwoF x0 x1
fmap f
TwoF (f x0) (f x1)
indexTwo
\case Two0 -> f x0; Two1 -> f x1
Since f gets applied in both branches, we could pull that out of the case:
f . (\case Two0 -> x0; Two1 -> x1)
That second term is exactly the indexing function we started out with. In other words, we have just determined another, simpler implementation for twomap:
twomap f ix = f . ix
If you work through similar reasoning for zeromap, onemap, and threemap, you'll discover they actually all have that same implementation! We can do this uniformly for all the various sizes of container just by going polymorphic; instead of having onemap for changing One -> a's, etc., let's have an xmap for changing x -> a's:
xmap :: (a -> b) -> (x -> a) -> (x -> b)
xmap f ix = f . ix
Of course, we don't have to name f and ix:
xmap = (.)
...and this is the Functor instance for (x -> _).