Bypassing Terraform error: “The true and false result expressions must have consistent types”

Terraform wants uniformity, we want freedom (Image Credit: Mulyadi)

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Have you ever came across this Terraform error — when you intentionally want your ternary to output different types?

The true and false result expressions must have consistent types

To bypass this Terraform limitation, check the tip below. I'll follow it with two examples to clarify the usage:

attribute = [
    <desired output if true>, 
    <desired output if false>
    ][<condition> ? 0 : 1]

Simple example

locals {
    dynamic_value = [
      {"region": "${var.region}"}, 
      "unavailable"
      ][var.region == "eu-west-1" ? 0 : 1 ]
}

Here, local.dynamic_value will return an object if the AWS region is Ireland, or the string "unavailable" for any other region.

Wait, what's just happened?

Instead of using the ternary in the traditional way, we defined a tuple (aka, a list with mixed types), and used the ternary to return the index for the output we really want. Thanks mariux for the trick.

Less simple example