MongoDB commands expect the first field to be named with the command name, and it can be either a scalar, an array or an object. The original IDL supported two variations for this command parameter. IDL should be extended to take a custom type in declarative form.

For example, imagine a command that prints a string, it could be represented in IDL as:

    PrintCommand:
        description: Command that prints its argument
        namespace: type
        type: string

Instead of namespace being limited to "ignore" and "concatentate_with_db", it will support an additional value called "type". When type is specified, the command must contain an additional field called "type" which refers to an existing type, struct, or an array of type/struct. It should support all the types that a field in a struct can support.