AML Constant

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:

const <constant-name> = <expression>

or you can explicitly declare the constant’s type

<constant-type> <constant-name> = <expression>

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

Example usages

Basic types

const a = 'hello' // a's type is String
const b = 2 // b's type is Number
const signup_threshold = 0.6  // signup_threshold'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
Dataset my_dataset {
  metric count {
    definition: @aql count(*) | where users.signup_threshold < ${signup_threshold} ;;
  }
}

tip

String interpolation is a common way to reuse basic constant types.

Lists and dictionaries

• Declare with const keyword

// 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 list
List<String> 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

const my_filter = FilterBlock { // my_filter's type is FilterBlock
  type: 'field'
  label: 'User role'
  source: FieldFilterSource {
    dataset: 'tenant_user'
    field: ref('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) ;;
  }
}

Use-cases

• Define a set of variables and reuse them in multiple places
• Define & reuse global SQL definitions
• Build a Dashboard with Multiple Similar Charts