Understanding the match operator

I’d start from some underlying primitives here and build up understanding from there:

• In elixir everything is an expression, which can be evaluated to a value – there are no statements or void return values.
  E.g. x = if 1 < 2, do: :a, else: :b will bind x with :a
• The value returned by a match expression is the value of the right hand side value in the match expression.
  So for ^y = (pattern = y) the pattern = y will evaluate to the value of y
• In the above the binding of pattern is kind of a side effect as it doesn’t affect the return value of the match expression.
  Therefore for the outer match expression ^y = (pattern = y) can be simplified to ^y = y
• Being right associative match expressions can be chained without affecting the above properties even if you leave out explicit parenthesis. Each chained match expression will evaluate to the right most “input”, with all the individual match expressions having the chance to fail matching as well as resulting in variables being bound as side effects of their matching.
  a = %{b: 1} = %{d: d} = %{b: 1, d: 4} and the following function essentially the same.

%{d: d} = %{b: 1, d: 4}
%{b: 1} = %{b: 1, d: 4}
a = %{b: 1, d: 4}

These properties make it essentially irrelevant in which order patterns are placed in a chained set of match expressions, though only the patterns, the right most value needs to stay on the right. Yes technically there is an order – you could move least permissive match patterns more right to match earlier than less permissive ones – but in practise that’s really irrelevant.

Now finally moving to function parameters. Usually these cause confusion because the actually imporant “right most input” doesn’t actually show up when writing a function head.

Having a function shows just a chained set of match patterns.

def my_function(a = %{b: 1} = %{d: d}), do: […]

When calling my_function(%{b: 1, d: 4}) internally you’ll essentially get these patterns evaluated as: a = %{b: 1} = %{d: d} = %{b: 1, d: 4}, adding the provided parameters to the right most side of the chain.

Hence in function heads you can reorder all parts of a chained match expression given the input isn’t present explicitly and it wouldn’t functionally change the function (beyond microoptimizations).

Match is right associative.
So,

%{a: x} = %{b: y} = %{a: "what", b: "happened"}

is really:

%{a: x} = (%{b: y} = %{a: "what", b: "happened"})

Also, Map matched on partial keys, so, %{b: y} = %{a: "what", b: "happened"} is a match, resulting the full map.

I think the article has pretty clear wording, for example if you have:

def hello(%{name: person_name} = person) do
    IO.puts "Hello, " <> person_name
    IO.inspect person
  end

The value person will have the entire map passed as the argument binded to it and person_name will bind the value of the key name. This kind of match also enforces the argument to be a map and contain the name key.