# `A2UICore.Functions.System`
[🔗](https://github.com/a2ui-elixir/a2ui/blob/v0.1.1/umbrella/apps/a2ui_core/lib/a2ui_core/functions/system.ex#L1)

Functions the runtime supplies to every catalog: the `@` namespace.

v1.0 defines `@index`, which evaluates to the iteration index of the
collection template being rendered (plus an optional `offset`).
Catalogs need not declare these; `A2UICore.Catalog.Basic` registers
them so a catalog can evaluate them.

# `functions`
*since 0.1.0* 

```elixir
@spec functions(String.t()) :: [A2UICore.Catalog.Function.t()]
```

The system functions available to a protocol version (none before
v1.0).

# `index`
*since 0.1.0* 

```elixir
@spec index(map(), A2UICore.Resolution.DataContext.t() | nil) ::
  {:ok, integer()} | {:error, A2UICore.Error.t()}
```

Evaluates `@index`. Outside a collection scope with a context it is a
validation error; without any context it is just the offset.

---

*Consult [api-reference.md](api-reference.md) for complete listing*