> ## Documentation Index
> Fetch the complete documentation index at: https://errand.nuvrel.dev/llms.txt
> Use this file to discover all available pages before exploring further.

# Expressions

> Operators, conditionals, index access, and function calls.

Errand's expression syntax is defined by HCL. Expressions appear in `computed` block `expression` attributes, task `condition` and `working_dir` values, and inside `${}` interpolations in command strings.

## Operators

### Arithmetic

| Operator | Operation      |
| -------- | -------------- |
| `+`      | Addition       |
| `-`      | Subtraction    |
| `*`      | Multiplication |
| `/`      | Division       |
| `%`      | Remainder      |

```hcl theme={null}
computed "workers" {
  expression = var.cpu_count * 2
}
```

### Comparison

| Operator | Operation             |
| -------- | --------------------- |
| `==`     | Equal                 |
| `!=`     | Not equal             |
| `<`      | Less than             |
| `<=`     | Less than or equal    |
| `>`      | Greater than          |
| `>=`     | Greater than or equal |

```hcl theme={null}
task "deploy" {
  condition = var.replicas > 0
  commands  = ["kubectl apply -f k8s/"]
}
```

### Logical

| Operator | Operation |
| -------- | --------- |
| `&&`     | And       |
| `\|\|`   | Or        |
| `!`      | Not       |

```hcl theme={null}
task "migrate" {
  condition = var.env == "production" && var.run_migrations
  commands  = ["./bin/migrate up"]
}
```

### Precedence

Operators evaluate in this order, highest to lowest:

| Priority | Operators            |
| -------- | -------------------- |
| Highest  | `!`, `-` (unary)     |
|          | `*`, `/`, `%`        |
|          | `+`, `-`             |
|          | `>`, `>=`, `<`, `<=` |
|          | `==`, `!=`           |
|          | `&&`                 |
| Lowest   | `\|\|`               |

Use parentheses when order might be ambiguous:

```hcl theme={null}
condition = (var.env == "production" || var.env == "staging") && var.ready
```

## Conditional

The ternary operator selects between two values based on a condition:

```
condition ? value_if_true : value_if_false
```

```hcl theme={null}
task "build" {
  commands = [
    "go build ${var.debug ? "-race" : ""} ./...",
  ]
}
```

Both branches must produce a compatible type.

## Index and attribute access

Access list elements by zero-based index:

```hcl theme={null}
expression = var.services[0]
```

Access map and object values by key or dot notation:

```hcl theme={null}
expression = var.ports["api"]
expression = var.db.host
```

Dot notation and bracket notation are equivalent for string keys. Accessing an out-of-bounds index or a missing key is an error at evaluation time.

## Function calls

Call a function by name with arguments in parentheses. Functions can be nested:

```hcl theme={null}
expression = lower(replace(var.branch, "/", "-"))
```

All built-in functions are available in every expression context. See the [functions reference](/functions/index).

## HCL expression language

Errand uses a subset of HCL's expression language. For expressions, splat expressions (`[*]`), and template directives (`%{if}`, `%{for}`) are not supported (yet).