Terraform - Specifying multiple possible values for Variables

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))
    }
}