How to add the bang character to functors?

Question

By convention, we add a bang character ! to any function name that mutates its arguments, so for the following code example, should we add a ! to the functor name?

mutable struct Foo
    a::Int
end

(foo::Foo)(val) = foo.a = val

f = Foo(1)  # f.a = 1
f(10)       # f.a = 10

In short, is it possible to call the last line as f!(10)? I am just curious. Thanks.

Answer 1

The call here is just the same as the name of your variable. So if you want it to contain a !, you will have to name your variable f!:

julia> f! = Foo(1)  # f.a = 1
Foo(1)

julia> f!(4)
4

There is nothing magical about the ! character, it's just part of the identifier. So you have to put the ! inside the actual name, exactly like you do with functions.