It needs to be exposed publicly for the File provider to set it. The same instance variable can be mutated by the DSL-style `#checksum(arg)` method, except that it's impossible to set the value back to nil. Since providers use resources internally as value objects (`@current_resource`), they need a public API for mutating. I'd like a cleaner separation between "real" objects and the DSL, but this is what we've got. Cf. https://github.com/opscode/omnibus-ruby/pull/38#issuecomment-15804909