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91 lines
3.4 KiB
Markdown
91 lines
3.4 KiB
Markdown
---
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title: Variadic Nodes
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---
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# Variadic Nodes
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There are operations which can be applied to an arbitrary number of input
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values. For example, consider the addition operation. Sure, intuitively you
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know how it works for two values, three values, or 42 values:
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* 1 + 6 = 7
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* 1 + 6 + 3 = 10
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* 1 + 6 + 3 + …more summands… = someNumber
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Other examples of such functions include number multiplication, string
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concatenation or joining, booleans AND’ing or OR’ing, n-th element choice, and
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so on.
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To natively support such operations, XOD offers a mechanism called *variadic*
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nodes. The variadic nodes can scale the number of their input pins to accept
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more or fewer values.
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In XOD IDE the variadic nodes have a little gripper on the right, and the pins
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which can scale are marked with the ellipsis (⋯) on mouse hover. Drag the right
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border of the node to add as many groups of variadic pins as you need.
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The variadic pins replication count is called *arity level*. By default, the
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arity level for a node is 1. When you add a pin to, for example, the `multiply`
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node, the arity level becomes 2, etc.
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## Expansion
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A variadic node developer is responsible only for creating a node
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implementation for the first arity level. All subsequent arity level
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implementations are induced automatically by XOD. To achieve it, when
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transpiling the program each particular variadic node passes an *expansion*
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stage in which the system replaces it with a cascade of primitive level-1
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nodes. The replacement principle is better illustrated with an example.
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<div class="ui segment note">
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<span class="ui ribbon label">Pro</span>
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Developers with functional programming experience could note the expansion is
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nothing more but a reduce/fold operation. Indeed! XOD variadics work through
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input value list folding.
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</div>
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## Bigger arity steps
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In the examples above the nodes have exactly one variadic pin. It is not always
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the case. Nodes may have two or three variadic pins. An example of such node is
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[`select`](/libs/xod/core/select/) which have two. Every time you increment an
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arity level of the `select` node it gets two additional input pins.
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Formally speaking, the number of variadic input pins which replicate together
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in a group is called *arity step*.
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Expansion process for the nodes with an arity step greater than one looks
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similar.
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## Shared pins
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When a variadic node has more inputs than necessary to expand it, the leftmost
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extraneous pins are shared between all nodes in the cascade.
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For example, consider textual string join operation which concatenates input
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strings placing a specified delimiter between them (e.g., space or comma). The
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delimiter will be shared between all expanded nodes, so you’ll get the result
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you expect.
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In the example above there was a single shared pin. However, there can be more
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if the author decided the node to be so.
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---
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As a node user, you’ll rarely meditate on which pins are shared, and which are
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not. The usage pattern should be pretty apparent from the node designation.
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Arranging pins properly is a puzzle for the node creator. Read about all rules
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and nuances if you want to [create a new variadic
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node](../creating-variadics/) by yourself.
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