Julia handling void return type

Question

What is the best method for handling a Void type when it is returned by a function? The suggestions in http://docs.julialang.org/en/release-0.5/manual/faq/#how-does-null-or-nothingness-work-in-julia don't work.

A MWE (must be run from the REPL so Base.source_dir() returns Void):

julia> isempty(Base.source_dir())
ERROR: MethodError: no method matching start(::Void)
Closest candidates are:
  start(::SimpleVector) at essentials.jl:170
  start(::Base.MethodList) at reflection.jl:258
  start(::IntSet) at intset.jl:184
  ...
 in isempty(::Void) at ./iterator.jl:3
 in isempty(::Void) at /Applications/Julia-0.5.app/Contents/Resources/julia/lib/julia/sys.dylib:?

julia> isdefined(Base.source_dir())
ERROR: TypeError: isdefined: expected Symbol, got Void

julia> typeof(Base.source_dir()) == Void
true

This is on Julia 0.5. The latter option works, but it's a bit ugly.

Answer 1

Void is a singleton -- a type with exactly one instance. That one instance is Void() also called nothing. Be aware that nothing === Void()

You can treat it just like any other value.

It is returned by a bunch of functions, like println.

You can check if something has returned nothing -- ie and instance of type Void.

By

julia> println()===nothing
true

For the sake of type-stability, a method should not return nothing some of the time, and something some of the time. in those case it should instead return a Nullable, generally.