How to check if value is v:null?

Question

What is the proper way to check if a particular variable is null? I use v:null as a special value to indicate something in a custom function's return value.

I have tried:

let l:v = v:null
echom l:v == v:null   " Works fine
let l:v = {'file': "something"}
echom l:v == v:null   " E735: Can only compare Dictionary with Dictionary

Note for searchers: null is sometimes called None, empty or nil.

Answer 1

The == operator will check for value.

You need to check identity. This is done with the is (and isnot) operator. Use is# to match the case, is? to ignore it.

For example:

if l:v is v:null
  echom "Null"
endif
if l:v isnot v:null
  echom "Not null"
endif

There is a list of operators at :h expr4. You can use e.g :h expr-== as well, or just :h ==.

I also recommend reading Effective VimScript from Martin Tournoij.