--- layout: docs page_title: Expressions - Configuration Language sidebar_title: Expressions description: |- HCL allows the use of expressions to access data exported by sources and to transform and combine that data to produce other values. --- # 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 sources, 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 arguments. 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](#string-literals) below for information about escape sequences and the heredoc syntax. - See [String Templates](#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 ` = ` pairs: ```hcl { 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](/docs/from-1.5/syntax#identifiers), 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: - `source..` is an object representing a [source](/docs/from-1.5/sources) of the given type and name. ### Available Functions For a full list of available functions, see [the function reference](/docs/configuration/from-1.5/functions). ## `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` is a list of strings, then the following expression produces a list of strings with all-uppercase letters: ```hcl [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. 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 `{` and `}` are used instead, the result is an object, and two result expressions must be provided separated by the `=>` symbol: ```hcl {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. A `for` expression can also include an optional `if` clause to filter elements from the source collection, which can produce a value with fewer elements than the source: ```text [for s in var.list : upper(s) if s != ""] ``` The source value can also be an object or map value, in which case two temporary variable names can be provided to access the keys and values respectively: ```text [for k, v in var.map : length(k) + length(v)] ``` Finally, if the result type is an object (using `{` and `}` delimiters) then the value result expression can be followed by the `...` symbol to group together results that have a common key: ```text {for s in var.list : substr(s, 0, 1) => s... if s != ""} ``` ## Splat Expressions A _splat expression_ provides a more concise way to express a common operation that could otherwise be performed with a `for` expression. If `var.list` is a list of objects that all have an attribute `id`, then a list of the ids could be produced with the following `for` expression: ```hcl [for o in var.list : o.id] ``` This is equivalent to the following _splat expression:_ ```hcl var.list[*].id ``` The special `[*]` symbol iterates over all of the elements of the list given to its left and accesses from each one the attribute name given on its right. A splat expression can also be used to access attributes and indexes from lists of complex types by extending the sequence of operations to the right of the symbol: ```hcl var.list[*].interfaces[0].name ``` The above expression is equivalent to the following `for` expression: ```hcl [for o in var.list : o.interfaces[0].name] ``` Splat expressions are for lists only (and thus cannot be used [to reference resources created with `for_each`](https://www.terraform.io/docs/configuration/resources.html#referring-to-instances), which are represented as maps). However, if a splat expression is applied to a value that is _not_ a list or tuple then the value is automatically wrapped in a single-element list before processing. For example, `var.single_object[*].id` is equivalent to `[var.single_object][*].id`, or effectively `[var.single_object.id]`. This behavior is not interesting in most cases, but it is particularly useful when referring to resources that may or may not have `count` set, and thus may or may not produce a tuple value: ```hcl aws_instance.example[*].id ``` The above will produce a list of ids whether `aws_instance.example` has `count` set or not, avoiding the need to revise various other expressions in the configuration when a particular resource switches to and from having `count` set. ## `dynamic` blocks Within top-level block constructs like sources, expressions can usually be used only when assigning a value to an argument using the `name = expression` or `key = expression` form. This covers many uses, but some source types include repeatable _nested blocks_ in their arguments, which do not accept expressions: ```hcl source "amazon-ebs" "example" { name = "pkr-test-name" # can use expressions here tag { # but the "tag" block is always a literal block } } ``` You can dynamically construct repeatable nested blocks like `tag` using a special `dynamic` block type, which is supported anywhere, example: ```hcl locals { standard_tags = { Component = "user-service" Environment = "production" } } source "amazon-ebs" "example" { # ... tag { key = "Name" value = "example-asg-name" } dynamic "tag" { for_each = local.standard_tags content { key = tag.key value = tag.value } } } ``` A `dynamic` block acts much like a `for` expression, but produces nested blocks instead of a complex typed value. It iterates over a given complex value, and generates a nested block for each element of that complex value. - The label of the dynamic block (`"tag"` in the example above) specifies what kind of nested block to generate. - The `for_each` argument provides the complex value to iterate over. - The `iterator` argument (optional) sets the name of a temporary variable that represents the current element of the complex value. If omitted, the name of the variable defaults to the label of the `dynamic` block (`"tag"` in the example above). - The `labels` argument (optional) is a list of strings that specifies the block labels, in order, to use for each generated block. You can use the temporary iterator variable in this value. - The nested `content` block defines the body of each generated block. You can use the temporary iterator variable inside this block. Since the `for_each` argument accepts any collection or structural value, you can use a `for` expression or splat expression to transform an existing collection. The iterator object (`tag` in the example above) has two attributes: - `key` is the map key or list element index for the current element. If the `for_each` expression produces a _set_ value then `key` is identical to `value` and should not be used. - `value` is the value of the current element. A `dynamic` block can only generate arguments that belong to the source type, data source or provisioner being configured. The `for_each` value must be a map or set with one element per desired nested block. If you need to declare resource instances based on a nested data structure or combinations of elements from multiple data structures you can use expressions and functions to derive a suitable value. For some common examples of such situations, see the [`flatten`](/docs/configuration/from-1.5/functions/collection/flatten) and [`setproduct`](/docs/configuration/from-1.5/functions/collection/setproduct) functions. ### Best Practices for `dynamic` Blocks Overuse of `dynamic` blocks can make configuration hard to read and maintain, so we recommend using them only when you need to hide details in order to build a clean user interface for a re-usable code. Always write nested blocks out literally where possible. ## 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: ```hcl <