How to pass a function typesafe in julia

Let's say, I want to pass a function to another function:

function foo()
    return 0;
end
function bar(func)
    return func();
end
print(bar(foo));

But you can make functions typesafe:

function func(t::Int)
    print(t);
end
func(0);                 #produces no error
func("Hello world");     #produces an error

I didn't found out, how I combine both, that means, how can I explicitly define a parameter of bar, like func, to be a function, possibly with certain input / output argument types.

Thanks in advance for any help.

A function is of type Function. You can easily check this:

julia> foo() = 1;

julia> T = typeof(foo)
typeof(foo)

julia> supertype(T)
Function

julia> foo isa Function
true

This will not necessarily cover all callable types, as you can make any type callable:

julia> struct Callable end

julia> (::Callable)(x::Number) = x + one(x)

julia> callable = Callable()
Callable()

julia> callable(5)
6

julia> callable isa Function
false