Pattern matching key in Erlang maps

Question

I have a map of form shown below:

Map = #{#{country=>"India"} => #{rank => 1}}.

I am trying to match it as follows:

1. #{Key := V} = Map.

or

2. #{#{country := Country} := #{rank := Rank}} = Map.

But its not working for me. Any help as to how it can be done?

Answer 1

When matching key-value associations from maps the key expression must be an expression with literals or bound variables, see the documentation of maps (section Maps in Patterns).

The problem with a match expression like:

#{Key := V} = M.

Where Key is an unbound variable is that this matches all the key/value bindings in the map M, not a particular key/value. Same with the other match expression you tried, it can match several keys.

The correct way would be to fully specify the key here, like this

#{#{country => "India"} := V} = Map.