Comment by rhdunn

There are three return states that a parser (or partial parser) can return:

1. an item (`T`) for a successful parse;

2. an error (`ParseError`) for an unrecoverable parse error;

3. a partial item (`T with ParseError`) for a recoverable parse error or warning (known statically).

You may also want to differentiate between:

1. leaf nodes (numbers, identifiers, string literals, etc.);

2. composite/nested nodes (addition, multiplication, variable assignment, etc.).