Need help in understanding the Erlang code and what exactly it is doing

Question

I am finding it difficult to decipher this code. I could understand that there is an instance of a data structure; is it a record or map?

The query is regarding the last two line of the code does it update the record with Response or does it check message_code?

Response =
  #{header =>
      #{message_code => 'CONN_ESTABLISH_REQUEST',
        protocol_instance => ?MGMT_PROTOCOL_IDENTIFIER,
        transaction_identifier => 1},
    content => #{revision_list => [0]}
  },

#{header := #{message_code := 'CONN_ESTABLISH_CONFIRM'},
      content := Confirmation} = Response

Answer 1

It's a map, not a record. (If it were a record, the record name would be between the # and the {.)

The last two lines perform a pattern match on the variable Response. The code asserts that Response is a map containing at least two keys, header and content. The value for header must be a map containing at least one key, message_code, whose value is 'CONN_ESTABLISH_CONFIRM'. The value for content will be stored in the variable Confirmation. If the value of Response doesn't conform to all those requirements, this code will signal a badmatch error.

Note that the behaviour is different depending on whether the right hand side of := contains:

• a constant
• an unbound variable
• a bound variable (a variable that already has a value)

If it is an unbound variable, the value for that key is simply stored in that variable. If it is a bound variable, the value for that key must match the value of that variable, otherwise you get a badmatch error just like for a non-matching constant.

---

As you see, there are two different delimiters used, => and :=. When constructing a new map (such as the first expression in your example), you can only use =>, and when pattern matching, you can only use :=. The idea is that they do different things: => stores a key-value pair in the map, while := extracts an existing key-value pair.

There is another case: updating an existing map. In that case you can use both. => can be used to add a new key to the map, while := can only be used to update an existing key, otherwise it signals a badarg error. For example, if you wanted to add a "footer" to Response, you have to use =>:

NewResponse = Response#{footer => [some,data]},
%% this signals a badarg error:
NewResponse = Response#{footer := [some,data]},

while if you want to change the content, you can use either:

NewResponse = Response#{content := 42},
NewResponse = Response#{content => 42},