This is part of an ongoing series of posts documenting the built-in interpolation functions in Terraform. For more information, check out the beginning post. In this post I am going to cover the coalesce() function. The example file is on GitHub here.
Function name: coalesce(string,string,…)
Returns: Takes two or more strings and returns the first non-empty string in the list. Must include at least two arguments to evaluate. Does not accept a list of strings.
Example:
variable "string" {
default = "some_string"
}
variable "empty_string" {
default = ""
}
# Returns some_string
output "coalesce" {
value = "${coalesce(var.empty_string,var.string)}"
}
##############################################
# Function: coalesce
##############################################
##############################################
# Variables
##############################################
variable "1_string" {
default = false
}
variable "2_string" {
default = 0
}
variable "3_string" {
default = "three"
}
variable "4_string" {
default = "four"
}
##############################################
# Resources
##############################################
##############################################
# Outputs
##############################################
output "coalesce_output" {
value = "${coalesce(var.1_string,var.2_string,var.3_string,var.4_string)}"
}
Run the following from the coalesce folder to get example output for a number of different cases:
#Evaluate to one
terraform apply -var "1_string=one" -var "2_string=two" -var "3_string=three" -var "4_string=four"
#First string is empty, evaluate to second string
terraform apply -var "1_string=" -var "2_string=two" -var "3_string=three" -var "4_string=four"
#What will a default value of bool false do? Evaluate as string 0
terraform apply -var "2_string=two" -var "3_string=three" -var "4_string=four"
#What will a default value of int 0 do? Evaluate as string 0
terraform apply -var "1_string=" -var "3_string=three" -var "4_string=four"
#All empty, evaluate to empty
terraform apply -var "1_string=" -var "2_string=" -var "3_string=" -var "4_string="
I have to admit I was a little stumped on when you would actually use this. It seems like you would need a situation where you would check the output of multiple resources, and then be okay with selecting the first non-empty value as the value you would want to use. I am sure there is a situation for this, but I have no idea what it might be.
The function will not take a list as a value, only multiple strings, which I found really odd. The function will evaluate the boolean value false as an empty string. It evaluates the int value of 0 as a string value of 0. Other than that, it does precisely what it advertises. I do find the naming a little confusing, since coalesce means to combine multiple things into a single entity. But then I found that there is a Coalesce function in T-SQL that also returns the first non-null value in a list. I suppose in the world of programming, coalesce has this other established definition.
Coming up next is the coalescelist() function, you’ll never guess what it does.
Resourcely Guardrails and Blueprints
November 15, 2024
Deploying Azure Landing Zones with Terraform
November 12, 2024
October 18, 2024
What's New in the AzureRM Provider Version 4?
August 27, 2024