# Standard library

> Find the modules and built-in functions that every Arc automation can use.

The Arc standard library has modules and built-in functions. To use a module, import it at the top of the file, then call its functions by their qualified name. A qualified name without an `import` is an error.

```arc
import time

time.interval(period=1s) -> heartbeat
```

## Modules

| Module                                                                                  | Functions                         | Use                                         |
| --------------------------------------------------------------------------------------- | --------------------------------- | ------------------------------------------- |
| [`control`](https://docs.synnaxlabs.com/reference/control/arc/standard-library/control) | `set_authority`                   | Change control authority while it runs.     |
| [`math`](https://docs.synnaxlabs.com/reference/control/arc/standard-library/math)       | `avg`, `min`, `max`, `derivative` | Get running statistics and rates of change. |
| [`ranges`](https://docs.synnaxlabs.com/reference/control/arc/standard-library/ranges)   | `create`, `end`                   | Mark a period of time as a range.           |
| [`stable`](https://docs.synnaxlabs.com/reference/control/arc/standard-library/stable)   | `for`                             | Pass a value only after it holds steady.    |
| [`status`](https://docs.synnaxlabs.com/reference/control/arc/standard-library/status)   | `set`                             | Set an alarm, a warning, or a state.        |
| [`time`](https://docs.synnaxlabs.com/reference/control/arc/standard-library/time)       | `interval`, `wait`, `now`         | Fire timers and read the time.              |

## Built-in functions

Built-in functions need no `import`.

| Function                                                                              | Signature                           | Use                                                                         |
| ------------------------------------------------------------------------------------- | ----------------------------------- | --------------------------------------------------------------------------- |
| [`select`](https://docs.synnaxlabs.com/reference/control/arc/standard-library/select) | `bool -> select() -> (true, false)` | Send each value to a `true` or a `false` routing entry.                     |
| `on`                                                                                  | `on(channel) -> T`                  | Read a channel. Same as a channel name at the start of a flow.              |
| `write`                                                                               | `T -> write(channel) -> u8`         | Write a channel. Same as a channel name at the end of a flow.               |
| `ge`, `gt`, `le`, `lt`, `eq`, `ne`                                                    | `T -> gt(b) -> bool`                | Compare the trigger `a` with `b`. Same as `>=`, `>`, `<=`, `<`, `==`, `!=`. |
| [`len`](https://docs.synnaxlabs.com/reference/control/arc/reference/functions#len)    | `len(x) -> i64`                     | Get the length of a series or a string. Only in a function body.            |

A channel name does the same work as `on` and `write`:

```arc
on(channel=tank_pt) -> tank_pt_copy
tank_pt -> write(channel=tank_pt_copy)
```

`on` and `write` cannot be in the same flow. The compiler cannot find a type for them and stops with `on_0_T: unresolved type variable: constraints`.

The comparison functions take two numeric values, `a` and `b`, of the same type. A flow value fills the trigger `a`, and `()` sets `b`:

```arc
tank_pt -> gt(b=300.0) -> tank_high
```

`and`, `or`, and `not` are keywords, so text code cannot call them as functions. Use the operators.

## Deprecated names

The names `avg`, `min`, `max`, `derivative`, `interval`, `wait`, `now`, `set_authority`, and `stable_for` without a module still compile, with warning [ARC4003](https://docs.synnaxlabs.com/reference/control/arc/reference/errors#diagnostic-codes). Use the qualified names, such as `math.avg` and `stable.for`.