Type constraints in variables in Terraform - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of type constraints in Terraform variables?

Type constraints ensure that variables only accept values of a specific type, like string, number, list, or map. This helps prevent errors by validating input before applying changes.

beginner

How do you declare a variable with a string type constraint in Terraform?

You declare it like this:

variable "example" {
  type = string
}

This means the variable only accepts string values.

beginner

What happens if you assign a wrong type to a Terraform variable with a type constraint?

Terraform will show an error during plan or apply, stopping the process. This protects your infrastructure from invalid configurations.

intermediate

How can you specify a list of numbers as a variable type in Terraform?

Use the syntax:

variable "numbers" {
  type = list(number)
}

This means the variable must be a list where each item is a number.

intermediate

Explain the difference between 'map(string)' and 'object' type constraints in Terraform variables.

'map(string)' means a collection of key-value pairs where each value is a string. 'object' defines a fixed structure with named attributes and their types, like a small form with specific fields.

Which Terraform variable type constraint accepts only true or false values?

A string

B list

C bool

D number

The 'bool' type constraint only accepts boolean values: true or false.

How do you declare a variable that must be a map with string values in Terraform?

A type = map(string)

B type = list(string)

C type = object

D type = string

The syntax 'map(string)' means a map where each value is a string.

What will happen if you assign a list of strings to a variable declared as 'list(number)'?

A Terraform will ignore the type and accept the value

B Terraform will throw an error during plan or apply

C Terraform will convert strings to numbers automatically

D Terraform will treat it as a string variable

Terraform enforces type constraints strictly and will error if types don't match.

Which type constraint allows defining a fixed structure with named attributes in Terraform?

A string

B map

C list

D object

'object' type lets you define a fixed set of named attributes with their own types.

What is the default type of a Terraform variable if no type constraint is specified?

A any

B string

C number

D bool

If no type is set, Terraform treats the variable as 'any' type, accepting any value.

Describe how type constraints improve Terraform variable usage and give examples of common types.

Think about how specifying types helps avoid mistakes before applying changes.

Explain how to declare a variable for a list of objects with specific attributes in Terraform.

Consider how to enforce structure inside a list.

Practice

1. What is the main purpose of using type constraints in Terraform variables?

easy

A. To assign default values to variables

B. To make variables optional

C. To encrypt variable values

D. To ensure variables only accept specific kinds of data

Solution

1. Step 1: Understand variable type constraints

   Type constraints limit what kind of data a variable can accept, like strings or lists.

2. Step 2: Identify the purpose

   This helps catch errors early by preventing wrong data types from being used.

3. Final Answer:

   To ensure variables only accept specific kinds of data -> Option D

4. Quick Check:

   Type constraints = restrict data type [OK]

Hint: Type constraints restrict variable data types [OK]

Common Mistakes:

• Confusing type constraints with default values
• Thinking type constraints make variables optional
• Assuming type constraints encrypt data

2. Which of the following is the correct syntax to declare a variable with a list of strings type constraint in Terraform?

easy

A. variable "names" { type = list(string) }

B. variable "names" { type = "list of strings" }

C. variable "names" { type = [string] }

D. variable "names" { type = string[] }

Solution

1. Step 1: Recall Terraform type syntax

   Terraform uses list(string) to specify a list of strings as a type.

2. Step 2: Compare options

   variable "names" { type = list(string) } uses the correct syntax. Others use invalid or unsupported formats.

3. Final Answer:

   variable "names" { type = list(string) } -> Option A

4. Quick Check:

   List of strings = list(string) [OK]

Hint: Use list(string) for list of strings type [OK]

Common Mistakes:

• Using quotes around type names incorrectly
• Using array syntax like string[] which is invalid in Terraform
• Writing type as a plain string description

3. Given this variable declaration:

variable "ports" {
  type = set(number)
  default = [80, 443, 8080]
}

What will be the type and value of var.ports when accessed in Terraform?

medium

A. A list of numbers: [80, 443, 8080]

B. A set of numbers: {80, 443, 8080}

C. A map with keys 80, 443, 8080

D. A string containing "80,443,8080"

Solution

1. Step 1: Understand the declared type

   The variable type is set(number), which means a set of unique numbers.

2. Step 2: Check the default value

   The default is a list, but Terraform converts it to a set because of the type constraint.

3. Final Answer:

   A set of numbers: {80, 443, 8080} -> Option B

4. Quick Check:

   Type set(number) = set of numbers [OK]

Hint: set(number) converts list to unique number set [OK]

Common Mistakes:

• Confusing set with list type
• Expecting a map instead of a set
• Thinking default list stays a list despite type

4. Identify the error in this variable declaration:

variable "config" {
  type = map(string)
  default = ["a", "b", "c"]
}

medium

A. Default values must be numbers

B. Type map(string) is invalid syntax

C. Default value is a list, but type expects a map

D. Variable name cannot be "config"

Solution

1. Step 1: Analyze the type constraint

   The type map(string) expects a map with string values, like { key = "value" }.

2. Step 2: Check the default value

   The default is a list, which does not match the map type.

3. Final Answer:

   Default value is a list, but type expects a map -> Option C

4. Quick Check:

   Type map(string) needs map, not list [OK]

Hint: Match default value type to variable type [OK]

Common Mistakes:

• Using list as default for map type
• Thinking map(string) is invalid syntax
• Believing variable names are restricted

5. You want a variable that accepts either a string or a list of strings. Which type constraint correctly allows this in Terraform?

hard

A. type = string | list(string)

B. type = any

C. type = object({ string_or_list = string })

D. type = string, list(string)

Solution

1. Step 1: Understand union types in Terraform

   Terraform supports union types using the pipe symbol | to allow multiple types.

2. Step 2: Identify correct syntax

   type = string | list(string) uses string | list(string), which means the variable can be either a string or a list of strings.

3. Step 3: Check other options

   type = any allows any type, which is too broad. type = object({ string_or_list = string }) defines an object, not a union. type = string, list(string) is invalid syntax.

4. Final Answer:

   type = string | list(string) -> Option A

5. Quick Check:

   Union type uses | to combine types [OK]

Hint: Use | to combine types for union constraints [OK]

Common Mistakes:

• Using commas instead of | for union types
• Choosing any type instead of specific union
• Confusing object type with union type