for Expressions

A for expression creates a complex type value by transforming another complex type value. Each element in the input value can correspond to either one or zero values in the result, and an arbitrary expression can be used to transform each input element into an output element.

For example, if var.list were a list of strings, then the following expression would produce a tuple of strings with all-uppercase letters:

[for s in var.list : upper(s)]

This for expression iterates over each element of var.list, and then evaluates the expression upper(s) with s set to each respective element. It then builds a new tuple value with all of the results of executing that expression in the same order.

Input Types

A for expression's input (given after the in keyword) can be a list, a set, a tuple, a map, or an object.

The above example showed a for expression with only a single temporary symbol s, but a for expression can optionally declare a pair of temporary symbols in order to use the key or index of each item too:

[for k, v in var.map : length(k) + length(v)]

For a map or object type, like above, the k symbol refers to the key or attribute name of the current element. You can also use the two-symbol form with lists and tuples, in which case the additional symbol is the index of each element starting from zero, which conventionally has the symbol name i or idx unless it's helpful to choose a more specific name:

[for i, v in var.list : "${i} is ${v}"]

The index or key symbol is always optional. If you specify only a single symbol after the for keyword then that symbol will always represent the value of each element of the input collection.

Result Types

The type of brackets around the for expression decide what type of result it produces.

The above example uses [ and ], which produces a tuple. If you use { and } instead, the result is an object and you must provide two result expressions that are separated by the => symbol:

{for s in var.list : s => upper(s)}

This expression produces an object whose attributes are the original elements from var.list and their corresponding values are the uppercase versions. For example, the resulting value might be as follows:

{
  foo = "FOO"
  bar = "BAR"
  baz = "BAZ"
}

A for expression alone can only produce either an object value or a tuple value, but Terraform's automatic type conversion rules mean that you can typically use the results in locations where lists, maps, and sets are expected.

Filtering Elements

A for expression can also include an optional if clause to filter elements from the source collection, producing a value with fewer elements than the source value:

[for s in var.list : upper(s) if s != ""]

One common reason for filtering collections in for expressions is to split a single source collection into two separate collections based on some criteria. For example, if the input var.users is a map of objects where the objects each have an attribute is_admin then you may wish to produce separate maps with admin vs non-admin objects:

variable "users" {
  type = map(object({
    is_admin = bool
  }))
}

locals {
  admin_users = {
    for name, user in var.users : name => user
    if user.is_admin
  }
  regular_users = {
    for name, user in var.users : name => user
    if !user.is_admin
  }
}

Element Ordering

Because for expressions can convert from unordered types (maps, objects, sets) to ordered types (lists, tuples), Terraform must choose an implied ordering for the elements of an unordered collection.

For maps and objects, Terraform sorts the elements by key or attribute name, using lexical sorting.

For sets of strings, Terraform sorts the elements by their value, using lexical sorting.

For sets of other types, Terraform uses an arbitrary ordering that may change in future versions of Terraform. For that reason, we recommend converting the result of such an expression to itself be a set so that it's clear elsewhere in the configuration that the result is unordered. You can use the toset function to concisely convert a for expression result to be of a set type.

toset([for e in var.set : e.example])

Grouping Results

If the result type is an object (using { and } delimiters) then normally the given key expression must be unique across all elements in the result, or Terraform will return an error.

Sometimes the resulting keys are not unique, and so to support that situation Terraform supports a special grouping mode which changes the result to support multiple elements per key.

To activate grouping mode, add the symbol ... after the value expression. For example:

variable "users" {
  type = map(object({
    role = string
  }))
}

locals {
  users_by_role = {
    for name, user in var.users : user.role => name...
  }
}

The above represents a situation where a module expects a map describing various users who each have a single "role", where the map keys are usernames. The usernames are guaranteed unique because they are map keys in the input, but many users may all share a single role name.

The local.users_by_role expression inverts the input map so that the keys are the role names and the values are usernames, but the expression is in grouping mode (due to the ... after name) and so the result will be a map of lists of strings, such as the following:

{
  "admin": [
    "ps",
  ],
  "maintainer": [
    "am",
    "jb",
    "kl",
    "ma",
  ],
  "viewer": [
    "st",
    "zq",
  ],
}

Due to the element ordering rules, Terraform will sort the users lexically by username as part of evaluating the for expression, and so the usernames associated with each role will be lexically sorted after grouping.

Repeated Configuration Blocks

The for expressions mechanism is for constructing collection values from other collection values within expressions, which you can then assign to individual resource arguments that expect complex values.

Some resource types also define nested block types, which typically represent separate objects that belong to the containing resource in some way. You can't dynamically generate nested blocks using for expressions, but you can generate nested blocks for a resource dynamically using dynamic blocks.