The nu-source crate contains types and traits used for keeping track of metadata about values being processed.
Nu uses Tags to keep track of where a value came from, an AnchorLocation,
as well as positional information about the value, a Span.
An AchorLocation can be a Url, File, or Source text that a value was parsed from.
The source Text is special in that it is a type similar to a String that comes with the ability to be cheaply cloned.
A Span keeps track of a value's start and end positions.
These types make up the metadata for a value and are wrapped up together in a Tagged struct,
which holds everything needed to track and locate a value.
Nu's metadata system can be seen when reporting errors. In the following example Nu is able to report to the user where the typo of a column originated from.
1 | ls | get typ
| ^^^ did you mean 'type'?
In addition to metadata tracking, nu-source also contains types and traits related to debugging, tracing, and formatting the metadata and values it processes.