Conditional Expressions

A conditional expression uses the value of a boolean expression to select one of two values.

Hands-on: Try the Create Dynamic Expressions tutorial.

Syntax

The syntax of a conditional expression is as follows:

condition ? true_val : false_val

If condition is true then the result is true_val. If condition is false then the result is false_val.

A common use of conditional expressions is to define defaults to replace invalid values:

var.a == "" ? "default-a" : var.a

If var.a is an empty string then the result is "default-a", but otherwise it is the actual value of var.a.

Conditions

The condition can be any expression that resolves to a boolean value. This will usually be an expression that uses the equality, comparison, or logical operators.

Custom Condition Checks

You can create conditions that produce custom error messages for several types of objects in a configuration. For example, you can add a condition to an input variable that checks whether incoming image IDs are formatted properly.

Custom conditions can help capture assumptions, helping future maintainers understand the configuration design and intent. They also return useful information about errors earlier and in context, helping consumers more easily diagnose issues in their configurations.

Refer to Custom Condition Checks for details.

Result Types

The two result values may be of any type, but they must both be of the same type so that Terraform can determine what type the whole conditional expression will return without knowing the condition value.

If the two result expressions don't produce the same type then Terraform will attempt to find a type that they can both convert to, and make those conversions automatically if so.

For example, the following expression is valid and will always return a string, because in Terraform all numbers can convert automatically to a string using decimal digits:

var.example ? 12 : "hello"

Relying on this automatic conversion behavior can be confusing for those who are not familiar with Terraform's conversion rules though, so we recommend being explicit using type conversion functions in any situation where there may be some uncertainty about the expected result type.

The following example is contrived because it would be easier to write the constant "12" instead of the type conversion in this case, but shows how to use tostring to explicitly convert a number to a string.

var.example ? tostring(12) : "hello"