Terraform - Specifying multiple possible values for Variables - amazon-web-services

Question

CloudFormation provides AllowedValues for Parameters which tells that the possible value of the parameter can be from this list. How can I achieve this with Terraform variables? The variable type of list does not provide this functionality. So, in case I want my variable to have value out of only two possible values, how can I achieve this with Terraform. CloudFormation script that I want to replicate is:
"ParameterName": {
"Description": "desc",
"Type": "String",
"Default": true,
"AllowedValues": [
"true",
"false"
]
}

Answer 1

I don't know of an official way, but there's an interesting technique described in a Terraform issue:
variable "values_list" {
description = "acceptable values"
type = "list"
default = ["true", "false"]
}
variable "somevar" {
description = "must be true or false"
}
resource "null_resource" "is_variable_value_valid" {
count = "${contains(var.values_list, var.somevar) == true ? 0 : 1}"
"ERROR: The somevar value can only be: true or false" = true
}
Update:
Terraform now offers custom validation rules in Terraform 0.13:
variable "somevar" {
type = string
description = "must be true or false"
validation {
condition = can(regex("^(true|false)$", var.somevar))
error_message = "Must be true or false."
}
}

Answer 2

Custom validation rules is definitely the way to go. If you want to keep things simple and check the provided value against a list of valid ones, you can use the following in your variables.tf config:
variable "environment" {
type = string
description = "Deployment environment"
validation {
condition = contains(["dev", "prod"], var.environment)
error_message = "Valid value is one of the following: dev, prod."
}
}

Answer 3

Variation on the above answer to use an array/list.
variable "appservice_sku" {
type = string
description = "AppService Plan SKU code"
default = "P1v3"
validation {
error_message = "Please use a valid AppService SKU."
condition = can(regex(join("", concat(["^("], [join("|", [
"B1", "B2", "B3", "D1", "F1",
"FREE", "I1", "I1v2", "I2", "I2v2",
"I3", "I3v2", "P1V2", "P1V3", "P2V2",
"P2V3", "P3V2", "P3V3", "PC2",
"PC3", "PC4", "S1", "S2", "S3",
"SHARED", "WS1", "WS2", "WS3"
])], [")$"])), var.appservice_sku))
}
}