difference between None, Nothing and nothing

[Michele Zaffalon]

I could not see anything in the docs and I could only understand the "nothing" keyword:

julia> function f()

       return

       end

f (generic function with 1 method)

julia> f()==nothing

true

julia> nothing==Nothing

false

julia> nothing==None

false

julia> Nothing==None

false

julia> Nothing==Nothing

true

What is the meaning of Nothing and None?

Thank you,

michele

[Jameson Nash]

Nothing is the type of nothing. An empty return statement implicitly
returns nothing.

julia> isa(nothing, Nothing)
true

julia> nothing === Nothing()
true

None is the type union representing the absence of Any type. You
cannot have an instance of type None.

julia> Nothing <: None
false

julia> None <: Nothing
true

julia> Nothing <: Any
true

julia> Any <: Nothing
false

[Michele Zaffalon]

Now it is clear, thank you.

Wouldn't it make sense to add it to the docs?

[Jameson Nash]

Yes. Can you suggest a good place for it or submit a pull request (you
can edit the manual directly in a browser to create a pull request
https://github.com/JuliaLang/julia/tree/master/doc/manual)?

On Mon, May 5, 2014 at 2:29 AM, Michele Zaffalon

[Ivar Nesje]

It is currently documented in the FAQ.

http://docs.julialang.org/en/latest/manual/faq/#nothingness-and-missing-values

[Michele Zaffalon]

Ah, I could not find it. Searching the docs for "none nothing" "none || nothing" "none or nothing" does not seem to find it. Only null returns the FAQ but null is not a julia keyword, so it is less likely to be searched than none.

[Ivar Nesje]

I get FAQ as the 3. and 6. result on release-0.2 and latest documentation when i search for "nothing".

[Michele Zaffalon]

I don't, please see attachment. And there is nothing (ah!) relevant in the list.

[Ivar Nesje]

The last (partially visible) search result says Frequently Asked Questions (commonly abbreviated FAQ). I count 5 results above it, thus it must be the 6th result.

The search should probably index the FAQ entries individually, but it is still fairly simple to find.

[Michele Zaffalon]

6th entry refers to the top of the FAQ. But you are right, if I click the link and scroll down, nothing is highlighted. On the other hand, searching for nothingness points me to the exact place.

[Ivar Nesje]

Our search functionality (as provided by readthedocs) does not seem to scroll the result page for any query. ctrl-f / cmd-f is usually the best way to find something when the search engine landed you on the correct page.

Ivar