B. Understanding Filter Interfaces
B.1. Overview
General Syntax
The general syntax for using each filter is shown first. By convention, the filter is shown using the block and endblock macros, although filters can also be used with other macros, including include and execute.
Parameters
If a filter has one or more optional parameters, these are listed in a table containing the following columns:
- Name - the parameter name
- Type - the parameter type
- Rule - the pattern, if any, used to validate the value.
Parameter Types and Rules
Parameter types and rules follow the same conventions as macro argument types and rules. Refer to Understanding Macro Interfaces for details.
B.2. Data Tables
Table Fields
Providing data to a filter via a table is a common idiom in SDF as using tables makes it easy to add additional columns later on.
If a filter expects a table of data in TBL format, the fields in the table of data are listed in a table containing the following columns:
- Field - the field name
- Category - key, mandatory or optional
- Rule - the pattern, if any, used to validate the value.
Table Field Categories
The supported table field categories are explained below.
| Category | Description |
| key | a unique value must be provided for each row |
| mandatory | a value must be provided for each row |
| optional | a value is optional |
Validating Table Fields
As validating table fields can reduce performance quite a bit, the sdf program doesn't validate them by default. To enable table field validation, use the -v option like this:
sdf -v99 ...
Typically, this is only needed when a filter isn't working as you'd expect.