ab1dd87540
When we did the earlier documentation rework for Terraform v0.12 we still had one big "Expressions" page talking about the various operators and constructs, and so we had to be a bit economical with the details about some more complicated constructs in order to avoid the page becoming even more overwhelming. However, we've recently reorganized the language documentation again so that the expressions section is split across several separate pages, and that gives some freedom to go into some more detail about and show longer examples for certain features. My changes here are not intended to be an exhaustive rewrite but I did try to focus on some areas I've commonly seen questions about when helping in the community forum and elsewhere, and also to create a little more connectivity between the different content so readers can hopefully find what they are looking for more easily when they're not yet sure what terminology to look for.
2.3 KiB
layout, page_title
| layout | page_title |
|---|---|
| language | Conditional Expressions - Configuration Language |
Conditional Expressions
Hands-on: Try the Create Dynamic Expressions tutorial on HashiCorp Learn.
A conditional expression uses the value of a bool expression to select one of two values.
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 != "" ? var.a : "default-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.
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"