# AML Constant

> This document describes how to use AML Constant to declare and reuse values across an AML project

## Introduction

An AML constant allows you to specify a value that can be reused throughout a project.

- Once a constant is declared, user can not change its value through reassignment (it is immutable)
- Cannot declare same name constant in the same scope (i.e. AML modules and functions)

## Syntax

There are two equivalent ways to declare a new constant. You can declare the constant using the `const` keyword without explicit [type](/reference/aml/types):

```aml
const <constant_name> = <expression>
const a = 'hello'
```

or you can explicitly declare the constant’s type

```aml
<constant_type> <constant_name> = <expression>
Int e = 3
```

In the first case, the type of const variable is **automatically inferred** from its value.

## Example usages

### Basic types

```aml
const a = 'hello' // a's type is String
const b = 2 // b's type is Number

// Explicit type declaration
String c = 'world'
Number d = 2.5 + b // 4.5
Int e = 3

// Use the constant with string interpolation
const signup_threshold = 0.6  // signup_threshold's type is Number

Dataset my_dataset {
  metric count {
    definition: @aql count(*)
		// highlight-next-line
      | where users.signup_threshold < ${signup_threshold}
    ;;
  }
}
```

:::tip
[String interpolation](/reference/aml/string-interpolation) is a common way to reuse basic constant types.
:::

### Lists and dictionaries

- Declare with `const` keyword

```aml
// AML list
const models = ['users', 'countries', 'orders']
// access to list's elements
models(0) // 'users'
models(1) // 'countries'
models(2) // 'orders'

// AML dictionary
const modelDict = {
	users: 'users'
	countries: 'countries'
	orders: 'orders'
}
// access to dictionary's elements
modelDict('users') // 'users'
modelDict('countries') // 'countries'
modelDict('orders') // 'countries'
```

- Explicit type

```aml
// AML list
Type ListString = List[String]
ListString models = ['users', 'countries', 'orders']
// access to list's elements
models(0) // 'users'
models(1) // 'countries'
models(2) // 'orders'

// AML dictionary
Type StringDict = Dict[String, String]
StringDict modelDict = {
	users: 'users'
	countries: 'countries'
	orders: 'orders'
}
```

### Object types

```aml
const my_filter = FilterBlock { // my_filter's type is FilterBlock
  type: 'field'
  label: 'User role'
  source: FieldFilterSource {
    dataset: 'tenant_user'
    field: r(public_users.role)
  }
}

const my_dashboard_1 = Dashboard {
	...
	block f1: my_filter // reuse my_filter in multiple dashboards
	...
}

Dashboard my_dashboard_2 {
	...
	block f2: my_filter // reuse my_filter in multiple dashboards
	...
}

Model extended_users = users.extend({
	label: 'Extended users'
	measure count {
    label: 'count'
    type: 'number'
    definition: @aql sum(extended_users.id) ;;
  }
}

// equivalent to
const extended_users2 = users.extend({
	label: 'Extended users'
	measure count {
    label: 'count'
    type: 'number'
    definition: @aql sum(extended_users.id) ;;
  }
}
```

## See also

- [Define & reuse global SQL definitions](/as-code/aml/use-cases/const-reuse-sql-definitions)
- [Build a Dashboard with Multiple Similar Charts](/docs/canvas-dashboard/build-similar-dashboards)