set(…): a collection of unique values that do not have any secondary identifiers or ordering.

Set collections are unordered and cannot contain duplicate values, so the ordering of the argument elements is lost and any duplicate values are coalesced:

Since Terraform’s concept of a set requires all of the elements to be of the same type, mixed-typed elements will be converted to the most general type:

Example Code

# Slice a set by first casting it to a list and then accessing it as a list.

variable "set" {
  type = set(string)
  default = [
    "foo",
    "bar",
  ]
}

output "set" {
  value = var.set
}

output "set_first_element" {
  value = var.set[0]
}
Code language: PHP (php)

How to convert list to set type?

Function name – toset

Similar kinds of complex types (list/tuple/set and map/object) can usually be used interchangeably within the Terraform language, and most of Terraform’s documentation glosses over the differences between the kinds of complex type.

Pass a list value to toset to convert it to a set, which will remove any duplicate elements and discard the ordering of the elements.

Explicit type conversions are rarely necessary in Terraform because it will convert types automatically where required. Use the explicit type conversion functions only to normalize types returned in module outputs.

Example

> toset(["a", "b", "c"])
[
  "a",
  "b",
  "c",
]

> toset(["a", "b", 3])
[
  "3",
  "a",
  "b",
]

> toset(["c", "b", "b"])
[
  "b",
  "c",
]Code language: JavaScript (javascript)

Sets are almost similar to both tuples and lists:

• When a list or tuple is converted to a set, duplicate values are discarded and the ordering of elements is lost.
• When a set is converted to a list or tuple, the elements will be in an arbitrary order. If the set’s elements were strings, they will be in lexicographical order; sets of other element types do not guarantee any particular order of elements.

Reference

• https://developer.hashicorp.com/terraform/language/expressions/type-constraints
• https://developer.hashicorp.com/terraform/language/functions/toset

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