Optional output arguments

[stevengj]

@mahmah

I guess that this cannot be done in such a straightforward way in Julia. Is this correct?

See also previous discussions, which include a link to exactly this topic in the Julia manual: Implement of Matlab function "nargout" in Julia

@GunnarFarneback

If the extra output is expensive and something you don’t want to spend time on unless it’s wanted, my preferred way is to add an extra input with a mutable struct, vector, or Ref that the function can fill in with the desired information.

Two other options:

1. If you just do something that changes the input argument types, you can return an additional result by dispatching to a different method. For example, you could pass WantsFooToo() as an argument, where WantsFooToo is a singleton type, and it could dispatch to a different method that also computes and returns some data “foo”
2. You can return an object that computes the additional data on demand.

For an exampe, Julia’s linear-algebra factorizations often return a “factorization objection” that have overloaded getproperty to compute additional data, such as the explicit Q matrix from a QR factorization (which otherwise is stored implicitly).

Another example is reverse-mode AD systems like Zygote.jl and ChainRules.jl, where the adjoint rule is returned as a pullback function object that computes a vector–Jacobian product only if/when it is actually needed.