.conf

my_app.complex_list.buzz.type = person
my_app.complex_list.buzz.age  = 25
my_app.complex_list.fido.type = dog
my_app.complex_list.fido.opts.special_value = biz

schema.exs

[mappings: [
  "my_app.complex_list.*": [
    to: "my_app.complex_list",
    datatype: [:complex],
    default: []
  ]
  "my_app.complex_list.*.opts.special_value": [
  to: "my_app.complex_list",
  datatype: :atom,
  default: :bar
  ],
  "my_app.complex_list.*.type" [
    to: "my_app.complex_list",
    datatype: :atom,
    default: :undefined
  ],
  "my_app.complex_list.*.age" [
    to: "my_app.complex_list",
    datatype: :integer,
    default: 0
  ]]
 translations: [
   "my_app.complex_list.*": fn
   	_, {key, value_map}, acc ->
   	# example values
   	# {buzz, %{type: :person, age: 25, opts: %{special_value: :bar}}}
    # {fido, %{type: :dog, age: 0, opts: %{special_value: :biz}}}

   	acc -> [[name: name,
   	         type: value_map[:type],
   	         age: value_map[:age],
   	         opts: [special_value: value_map[:opts][:special_value]]
   	         ] | acc]
  ]]

sys.config

[{my_app,
    complex_list, [
      [{name, buzz},
       {type, person},
       {age, 25},
       {opts, [{special_value, bar}]},
      ],
      [{name, fido},
       {type, dog},
       {age, 0},
       {opts, [{special_value, biz}]}
      ]
       ]}
]