Moonscript static fields

I want to make class like that:

class Example
  field: false --some field shared for all instances of the class
  init: (using field) ->
    field = true --want to change value of the static field above

But in lua I got:

<...>
field = false,
init = function()
  local field = true //Different scopes of variable field
end
<...>

In docs I read that writing using helps to deal with it

You can change the value as you described by editing the metatable from the instance:

class Example
  field: false
  init: ->
    getmetatable(@).field = true

I don't recommend doing that, class fields are probably what you want to use:

class Example
  @field: false
  init: ->
    @@field = true

When assigning a class field you can prefix with @ to create a class variable. In the context of a method, @@ must be used to refernece the class, since @ represents the instance. Here's a brief overview of how @ works:

class Example
  -- in this scope @ is equal to the class object, Example
  print @

  init: =>
    -- in this score @ is equal to the instance
    print @

    -- so to access the class object, we can use the shortcut @@ which
    -- stands for @__class
    pirnt @@

Also, your use of using is incorrect. field is not a local variable. It's a field on the classes's instance meta table.