» Expressions

Expressions are used to refer to or compute values within a configuration. The simplest expressions are just literal values, like "hello" or 5, but HCL also allows more complex expressions such as references to data exported by resources, arithmetic, conditional evaluation, and a number of built-in functions.

Expressions can be used in a number of places in HCL, but some contexts limit which expression constructs are allowed, such as requiring a literal value of a particular type or forbidding. Each language feature's documentation describes any restrictions it places on expressions.

The rest of this page describes all of the features of Packer's expression syntax.

» 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.

HCL 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: 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 source or module to null, Packer 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.

» Advanced Type Details

In most situations, lists and tuples behave identically, as do maps and objects. Whenever the distinction isn't relevant, the Packer 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 source arguments.

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

Packer 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

» Literal Expressions

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

  • 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 Packer, and have additional documentation on this page:
    • See String Literals below for information about escape sequences and the heredoc syntax.
    • See String Templates below for information about interpolation and template directives.
  • Numbers are represented by unquoted sequences of digits with or without a decimal point, like 15 or 6.283185.
  • Bools are represented by the unquoted symbols true and false.
  • The null value is represented by the unquoted symbol null.
  • 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 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. Values can be arbitrary expressions. Keys are strings; they can be left unquoted if they are a valid identifier, but must be quoted otherwise. You can use a non-literal expression as a key by wrapping it in parentheses, like (var.business_unit_tag_name) = "SRE".

» References to Named Values

Packer makes one named values available.

The following named values are available:

» String Literals

HCL has two different syntaxes for string literals. The most common is to delimit the string with quote characters ("), like "hello". In quoted strings, the backslash character serves as an escape sequence, with the following characters selecting the escape behavior:

Sequence Replacement
\n Newline
\r Carriage Return
\t Tab
\" Literal quote (without terminating the string)
\\ Literal backslash
\uNNNN Unicode character from the basic multilingual plane (NNNN is four hex digits)
\UNNNNNNNN Unicode character from supplementary planes (NNNNNNNN is eight hex digits)

The alternative syntax for string literals is the so-called Here Documents or "heredoc" style, inspired by Unix shell languages. This style allows multi-line strings to be expressed more clearly by using a custom delimiter word on a line of its own to close the string:

<<EOF
hello
world
EOF

The << marker followed by any identifier at the end of a line introduces the sequence. Packer then processes the following lines until it finds one that consists entirely of the identifier given in the introducer. In the above example, EOF is the identifier selected. Any identifier is allowed, but conventionally this identifier is in all-uppercase and begins with EO, meaning "end of". EOF in this case stands for "end of text".

The "heredoc" form shown above requires that the lines following be flush with the left margin, which can be awkward when an expression is inside an indented block:

block {
  value = <<EOF
hello
world
EOF
}

To improve on this, Packer also accepts an indented heredoc string variant that is introduced by the <<- sequence:

block {
  value = <<-EOF
  hello
    world
  EOF
}

In this case, Packer analyses the lines in the sequence to find the one with the smallest number of leading spaces, and then trims that many spaces from the beginning of all of the lines, leading to the following result:

hello
  world

Backslash sequences are not interpreted in a heredoc string expression. Instead, the backslash character is interpreted literally.

In both quoted and heredoc string expressions, Packer supports template sequences that begin with ${ and %{. These are described in more detail in the following section. To include these sequences literally without beginning a template sequence, double the leading character: $${ or %%{.

» String Templates

Within quoted and heredoc string expressions, the sequences ${ and %{ begin template sequences. Templates let you directly embed expressions into a string literal, to dynamically construct strings from other values.