Field values are, by default, unrestricted text strings. However, it is often useful to impose some restrictions on the values of certain fields. For example, consider the following record:
Id: 111 Name: Jose E. Marchesi Age: 30 MaritalStatus: single Phone: +49 666 666 66
Id is a numeric identifier for a person.
never use several lines.
Age will tipically be in the range
0..120, and there are only a few valid values for
MaritalStatus: single, married and widow. Phones may be
restricted to some standard format as well to be valid. All those
restrictions (and many others) can be enforced by using field
There are two kind of field types: anonymous and named. Those are described in the following subsections.