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There are operations which can be applied to an arbitrary number of input values. For example, consider the addition operation. Sure, intuitively you know how it works for two values, three values, or 42 values:

• 1 + 6 = 7
• 1 + 6 + 3 = 10
• 1 + 6 + 3 + …more summands… = someNumber

Other examples of such functions include number multiplication, string concatenation or joining, booleans AND’ing or OR’ing, n-th element choice, and so on.

To natively support such operations, XOD offers a mechanism called variadic nodes. The variadic nodes can scale the number of their input pins to accept more or fewer values.

In XOD IDE the variadic nodes have a little gripper on the right, and the pins which can scale are marked with the ellipsis (⋯) on mouse hover. Drag the right border of the node to add as many groups of variadic pins as you need.

The variadic pins replication count is called arity level. By default, the arity level for a node is 1. When you add a pin to, for example, the `multiply` node, the arity level becomes 2, etc.

## Expansion #

A variadic node developer is responsible only for creating a node implementation for the first arity level. All subsequent arity level implementations are induced automatically by XOD. To achieve it, when transpiling the program each particular variadic node passes an expansion stage in which the system replaces it with a cascade of primitive level-1 nodes. The replacement principle is better illustrated with an example.

Pro Developers with functional programming experience could note the expansion is nothing more but a reduce/fold operation. Indeed! XOD variadics work through input value list folding.

## Bigger arity steps #

In the examples above the nodes have exactly one variadic pin. It is not always the case. Nodes may have two or three variadic pins. An example of such node is `select` which have two. Every time you increment an arity level of the `select` node it gets two additional input pins.

Formally speaking, the number of variadic input pins which replicate together in a group is called arity step.

Expansion process for the nodes with an arity step greater than one looks similar.

## Shared pins #

When a variadic node has more inputs than necessary to expand it, the leftmost extraneous pins are shared between all nodes in the cascade.

For example, consider textual string join operation which concatenates input strings placing a specified delimiter between them (e.g., space or comma). The delimiter will be shared between all expanded nodes, so you’ll get the result you expect.

In the example above there was a single shared pin. However, there can be more if the author decided the node to be so.

As a node user, you’ll rarely meditate on which pins are shared, and which are not. The usage pattern should be pretty apparent from the node designation.

Arranging pins properly is a puzzle for the node creator. Read about all rules and nuances if you want to create a new variadic node by yourself.

Found a typo or mistake? Want to improve the text? Edit this page on GitHub and open a pull request. If you have a complex proposal or you want to discuss the content, feel free to start a new thread on XOD forum.