» Types and Values

The result of an expression is a value. All values have a type, which dictates where that value can be used and what transformations can be applied to it.

» Types

The Terraform language uses the following types for its values:

  • string: a sequence of Unicode characters representing some text, like "hello".
  • number: a numeric value. The number type can represent both whole numbers like 15 and fractional values like 6.283185.
  • bool: a boolean value, either true or false. bool values can be used in conditional logic.
  • list (or tuple): a sequence of values, like ["us-west-1a", "us-west-1c"]. Elements in a list or tuple are identified by consecutive whole numbers, starting with zero.
  • map (or object): a group of values identified by named labels, like {name = "Mabel", age = 52}.

Strings, numbers, and bools are sometimes called primitive types. Lists/tuples and maps/objects are sometimes called complex types, structural types, or collection types.

Finally, there is one special value that has no type:

  • null: a value that represents absence or omission. If you set an argument of a resource or module to null, Terraform behaves as though you had completely omitted it — it will use the argument's default value if it has one, or raise an error if the argument is mandatory. null is most useful in conditional expressions, so you can dynamically omit an argument if a condition isn't met.

» Literal Expressions

A literal expression is an expression that directly represents a particular constant value. Terraform has a literal expression syntax for each of the value types described above.

» Strings

Strings are usually represented by a double-quoted sequence of Unicode characters, "like this". There is also a "heredoc" syntax for more complex strings.

String literals are the most complex kind of literal expression in Terraform, and have their own page of documentation. See Strings for information about escape sequences, the heredoc syntax, interpolation, and template directives.

» Numbers

Numbers are represented by unquoted sequences of digits with or without a decimal point, like 15 or 6.283185.

» Bools

Bools are represented by the unquoted symbols true and false.

» Null

The null value is represented by the unquoted symbol null.

» Lists/Tuples

Lists/tuples are represented by a pair of square brackets containing a comma-separated sequence of values, like ["a", 15, true].

List literals can be split into multiple lines for readability, but always require a comma between values. A comma after the final value is allowed, but not required. Values in a list can be arbitrary expressions.

» Maps/Objects

Maps/objects are represented by a pair of curly braces containing a series of <KEY> = <VALUE> pairs:

  name = "John"
  age  = 52

Key/value pairs can be separated by either a comma or a line break.

The values in a map can be arbitrary expressions.

The keys in a map must be strings; they can be left unquoted if they are a valid identifier, but must be quoted otherwise. You can use a non-literal string expression as a key by wrapping it in parentheses, like (var.business_unit_tag_name) = "SRE".

» Indices and Attributes

Elements of list/tuple and map/object values can be accessed using the square-bracket index notation, like local.list[3]. The expression within the brackets must be a whole number for list and tuple values or a string for map and object values.

Map/object attributes with names that are valid identifiers can also be accessed using the dot-separated attribute notation, like local.object.attrname. In cases where a map might contain arbitrary user-specified keys, we recommend using only the square-bracket index notation (local.map["keyname"]).

» More About Complex Types

In most situations, lists and tuples behave identically, as do maps and objects. Whenever the distinction isn't relevant, the Terraform documentation uses each pair of terms interchangeably (with a historical preference for "list" and "map").

However, module authors and provider developers should understand the differences between these similar types (and the related set type), since they offer different ways to restrict the allowed values for input variables and resource arguments.

For complete details about these types (and an explanation of why the difference usually doesn't matter), see Type Constraints.

» Type Conversion

Expressions are most often used to set values for the arguments of resources and child modules. In these cases, the argument has an expected type and the given expression must produce a value of that type.

Where possible, Terraform automatically converts values from one type to another in order to produce the expected type. If this isn't possible, Terraform will produce a type mismatch error and you must update the configuration with a more suitable expression.

Terraform automatically converts number and bool values to strings when needed. It also converts strings to numbers or bools, as long as the string contains a valid representation of a number or bool value.

  • true converts to "true", and vice-versa
  • false converts to "false", and vice-versa
  • 15 converts to "15", and vice-versa