Comparing Julia variable to `nothing` using !== or !=

Question

In some Julia code when can see conditional expression such as

if val !== nothing
    dosomething()
end

where val is a variable of type Union{Int,Nothing}

What is the difference between conditons val !== nothing and val != nothing?

Answer 1

First of all, it is generally advisable to use isnothing to compare if something is nothing. This particular function is efficient, as it is soley based on types (@edit isnothing(nothing)):

isnothing(::Any) = false
isnothing(::Nothing) = true

(Note that nothing is the only instance of the type Nothing.)

In regards to your question, the difference between === and == (and equally !== and !=) is that the former checks whether two things are identical whereas the latter checks for equality. To illustrate this difference, consider the following example:

julia> 1 == 1.0 # equal
true

julia> 1 === 1.0 # but not identical
false

Note that the former one is an integer whereas the latter one is a floating point number.

What does it mean for two things to be identical? We can consult the documentation of the comparison operators (?===):

help?> ===
search: === == !==

  ===(x,y) -> Bool
  ≡(x,y) -> Bool

  Determine whether x and y are identical, in the sense that no program could distinguish them. First the types
  of x and y are compared. If those are identical, mutable objects are compared by address in memory and
  immutable objects (such as numbers) are compared by contents at the bit level. This function is sometimes
  called "egal". It always returns a Bool value.

Sometimes, comparing with === is faster than comparing with == because the latter might involve a type conversion.