Function calls

The Ruby code in the function can do any number of things to produce the final value, including:

• Evaluate templates.
• Do mathematical calculations.
• Look up values from an external source.
• Cause side effects that modify the catalog.
• Evaluate a provided block of Puppet code, possibly using the function's arguments to modify that code or control how it runs.

An entire function call—including the name, arguments, and lambda—constitutes an expression. It resolves to a single value, and can be used anywhere a value of that type is accepted. A function call might also have an effect, such as adding a class to the catalog. You can also use function calls on their own, which causes their effects to occur while their value is ignored.

All functions run during catalog compilation, which means they can access code and data only from the primary Puppet server. To make changes to an agent node, you must use a resource; to collect data from an agent node, you must use a custom fact.

Each function defines how many arguments it takes, what data types it expects those arguments to be, what values it returns, and any effects it has. For details about any functions built into Puppet, see the function reference. For details about a function included in a module, see that module's documentation.