Get default function arguments



If I have a function with optional arguments like


is it possible to find out what the defaults are?


I don’t think so. Default arguments just result in Julia creating extra methods for your function. For example:

julia> f(x=5) = x + 1
f (generic function with 2 methods)

julia> methods(f)
# 2 methods for generic function "f":
f() in Main at REPL[1]:1
f(x) in Main at REPL[1]:1

Defining f(x=5) = ... actually defined two methods:

f(x) = ...
f() = f(5)

so inside the inner method (f(x)), there’s no indication whether the x it received was a default argument or not. Keyword arguments do something similar by creating an inner function without keyword arguments.

What are you actually trying to do? Perhaps there’s another way.


I have a simulation with certain default parameters. Then I want to double and halve these parameters and record the results. I can do this simply by putting all the parameters in a list but I wondered if I can also do it via writing something like


or something.


Ah, no, that’s not possible. After all, you could have arbitrarily many methods named add, so what would defaults(add) possibly return?

Instead, if you put your parameters in a struct and pass that struct to your function, manipulating the parameters as a group becomes much easier. Check out for some macros to make constructing parameter structs easier.