121 lines
3.3 KiB
Plaintext
121 lines
3.3 KiB
Plaintext
---
|
|
page_title: one - Functions - Configuration Language
|
|
description: |-
|
|
The 'one' function transforms a list with either zero or one elements into
|
|
either a null value or the value of the first element.
|
|
sidebar_title: one
|
|
---
|
|
|
|
# `one` Function
|
|
|
|
-> **Note:** This function is available only in Terraform v0.15 and later.
|
|
|
|
`one` takes a list, set, or tuple value with either zero or one elements.
|
|
If the collection is empty, `one` returns `null`. Otherwise, `one` returns
|
|
the first element. If there are two or more elements then `one` will return
|
|
an error.
|
|
|
|
This is a specialized function intended for the common situation where a
|
|
conditional item is represented as either a zero- or one-element list, where
|
|
a module author wishes to return a single value that might be null instead.
|
|
|
|
For example:
|
|
|
|
```hcl
|
|
variable "include_ec2_instance" {
|
|
type = bool
|
|
default = true
|
|
}
|
|
|
|
resource "aws_instance" "example" {
|
|
count = var.include_ec2_instance ? 1 : 0
|
|
|
|
# (other resource arguments...)
|
|
}
|
|
|
|
output "instance_ip_address" {
|
|
value = one(aws_instance.example[*].private_ip)
|
|
}
|
|
```
|
|
|
|
Because the `aws_instance` resource above has the `count` argument set to a
|
|
conditional that returns either zero or one, the value of
|
|
`aws_instance.example` is a list of either zero or one elements. The
|
|
`instance_ip_address` output value uses the `one` function as a concise way
|
|
to return either the private IP address of a single instance, or `null` if
|
|
no instances were created.
|
|
|
|
## Relationship to the "Splat" Operator
|
|
|
|
The Terraform language has a built-in operator `[*]`, known as
|
|
[the _splat_ operator](/language/expressions/splat), and one of its functions
|
|
is to translate a primitive value that might be null into a list of either
|
|
zero or one elements:
|
|
|
|
```hcl
|
|
variable "ec2_instance_type" {
|
|
description = "The type of instance to create. If set to null, no instance will be created."
|
|
|
|
type = string
|
|
default = null
|
|
}
|
|
|
|
resource "aws_instance" "example" {
|
|
count = length(var.ec2_instance_type[*])
|
|
|
|
instance_type = var.ec2_instance_type
|
|
# (other resource arguments...)
|
|
}
|
|
|
|
output "instance_ip_address" {
|
|
value = one(aws_instance.example[*].private_ip)
|
|
}
|
|
```
|
|
|
|
In this case we can see that the `one` function is, in a sense, the opposite
|
|
of applying `[*]` to a primitive-typed value. Splat can convert a possibly-null
|
|
value into a zero-or-one list, and `one` can reverse that to return to a
|
|
primitive value that might be null.
|
|
|
|
## Examples
|
|
|
|
```
|
|
> one([])
|
|
null
|
|
> one(["hello"])
|
|
"hello"
|
|
> one(["hello", "goodbye"])
|
|
|
|
Error: Invalid function argument
|
|
|
|
Invalid value for "list" parameter: must be a list, set, or tuple value with
|
|
either zero or one elements.
|
|
```
|
|
|
|
### Using `one` with sets
|
|
|
|
The `one` function can be particularly helpful in situations where you have a
|
|
set that you know has only zero or one elements. Set values don't support
|
|
indexing, so it's not valid to write `var.set[0]` to extract the "first"
|
|
element of a set, but if you know that there's only one item then `one` can
|
|
isolate and return that single item:
|
|
|
|
```
|
|
> one(toset([]))
|
|
null
|
|
> one(toset(["hello"]))
|
|
"hello"
|
|
```
|
|
|
|
Don't use `one` with sets that might have more than one element. This function
|
|
will fail in that case:
|
|
|
|
```
|
|
> one(toset(["hello","goodbye"]))
|
|
|
|
Error: Invalid function argument
|
|
|
|
Invalid value for "list" parameter: must be a list, set, or tuple value with
|
|
either zero or one elements.
|
|
```
|