Opaque Types

Some operations (primarily ones imported from FFI) may have “opaque” types which can only be referenced but not directly manipulated. These might be concrete types under the hood, but a well-written policy should not depend on knowledge of their internal structure. A good example of this is the Envelope type, use in seal and open blocks. It can only be accessed via FFI methods.

command Foo {
    ...

    open {
        // `envelope` is auto-defined in `open` but cannot be used directly. Here
        // it is processed through an FFI function to produce a usable object.
        let obj = envelope::do_open(envelope)
        // `deserialize` is a built-in that turns a serialized `bytes` object into a
        // struct.
        return deserialize(obj)
    }

    ...
}