I disagree that this is less OO than what @mauro3 showed (regardless of what the term OO actually means). The closure you show in your code is implemented as a struct storing a. The only real difference between the two examples is that yours is one line shorter but produces intermediate outputs which might be less legible and shouldn’t be dispatched on.
julia> dump(mul)
#13 (function of type var"#13#14"{Int64})
a: Int64 2
edited to make the tone seem less aggressive (did not mean to be aggressive)