Julia 1.0.0: What does the `=>` operator do?

Question

I see this Stackoverflow code for =>, but when I search Julia 1.0.0 on-line help for "=>", I get zero hits.

replace!(x, 0=>4)  # The last expression is the focus of this question.

In the REPL help I get:

help?> =>
search: =>

  Pair(x, y)
  x => y

Construct a Pair object with type Pair{typeof(x), typeof(y)}. The elements are stored in the fields first and second. They can also be accessed via iteration.

See also: Dict

Examples ≡≡≡≡≡≡≡≡≡≡

  julia> p = "foo" => 7
  "foo" => 7

  julia> typeof(p)
  Pair{String,Int64}

  julia> p.first
  "foo"

  julia> for x in p
             println(x)
         end
  foo
  7

What does => do in replace!(x, 0=>4)? Does it create a pair, a replacement of all zeros by fours, or what? Why do I seem to not find it in the Julia 1.0.0 on-line docs?

EDIT

Code added to help me understand @Bill's helpful answer below:

julia> x = [1, 0, 3, 2, 0]
5-element Array{Int64,1}:
 1
 0
 3
 2
 0

julia> replace!(x, 0=>4)
5-element Array{Int64,1}:
 1
 4
 3
 2
 4

Edit 2

Besides @Bill's accepted answer, I found @Steven's answer helpful as well. Sorry I could not check them both, but Bill's came in first and they both offered useful information.

Answer 1

"What does => do in replace!(x, 0=>4)? Does it create a pair, a replacement of all zeros by fours, or what?"

It creates a Pair. In the function replace, a Pair in the second argument position means the multiple dispatch of replace() chooses a version of the replace function where, given a numeric array or string x, all items within x fitting the first part of the Pair are replaced with an instance of the second part of the Pair.

You can check the REPL docs for replace for details.

Answer 2

This small example should show how "=>" makes a pair

julia> replace("julia", Pair("u", "o"))

"jolia"

julia> replace("julia", "u" => "o")

"jolia"