Terraform Tutorial · Terraform Language · Lesson 14 of 25

Terraform for Expressions

Transform lists and maps with for expressions: filter, reshape and build new collections.

List and map comprehensions

Lists, maps and filters

locals {
  users = ["ada", "linus", "grace"]
}

output "upper_names" {
  value = [for u in local.users : upper(u)]
}

output "name_lengths" {
  value = { for u in local.users : u => length(u) }
}

output "short_names" {
  value = [for u in local.users : u if length(u) <= 4]
}

Output

name_lengths = {
  "ada" = 3
  "grace" = 5
  "linus" = 5
}
short_names = [
  "ada",
]
upper_names = [
  "ADA",
  "LINUS",
  "GRACE",
]

Run this on your computer — see “Get Started” for setup.

[for x in list : expr] builds a list.
{for x in list : key => value} builds a map.
Add if condition to filter.
... after the value groups duplicate keys into lists.

Reshaping data

Group by a key

locals {
  instances = [
    { name = "web-1", zone = "a" },
    { name = "web-2", zone = "b" },
    { name = "db-1", zone = "a" },
  ]
}

output "by_zone" {
  value = { for i in local.instances : i.zone => i.name... }
}

Output

by_zone = {
  "a" = [
    "web-1",
    "db-1",
  ]
  "b" = [
    "web-2",
  ]
}

Run this on your computer — see “Get Started” for setup.

Test yourself

Which expression builds a map?

[for u in users : u]
{for u in users : u => length(u)}
for_each = users
users[*]

Show answer

Answer: {for u in users : u => length(u)}