问题
If we have two functions, f and g, then in Haskell h = f . g is equivalent to h x = f(g x). I.e. the functions are applied from right to left to the input. Is there any fundamental reason why it goes from right to left, and not from left to right? I.e. why didn't they make h = f . g equivalent to h x = g(f x) instead?
EDIT: as others pointed out my equivalent functions where the wrong way around, so I fixed those.
回答1:
First of all, there's a mistake in your [original, unedited] question:
h = f . g is equivalent to h x = g(f x)
— that's not true: h = f . g is equivalent to h x = f (g x).
However, as to why it's that way and not the other way around, it's most likely because that's how it works and has worked in math; see http://en.wikipedia.org/wiki/Function_composition:
[...] composite function is denoted g ∘ f : X → Z, defined by (g ∘ f )(x) = g(f(x)) for all x in X.
It's also intuitive because of the equality (f . g) x == f (g x) — as you can see, the order of f and g is the same on both sides.
Moreover, it's trivial to create your own "reverse composition" operator if you desire one for reasons of e.g. readability:
(.>) = flip (.)
so that
Prelude> ((+1) .> (*2)) 3
8
Prelude> ((+1) . (*2)) 3
7
In fact, you can just use Control.Arrow.(>>>) which does the same for functions but is more general and works for other things as well:
Prelude Control.Arrow> ((+1) >>> (*2)) 3
8
来源：https://stackoverflow.com/questions/29878545/why-does-the-dot-compose-from-right-to-left-in-haskell