github.com/Ilhicas/nomad@v1.0.4-0.20210304152020-e86851182bc3/website/content/docs/job-specification/hcl2/expressions.mdx

---
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 arithmetic, conditional
evaluation, and a number of built-in functions.

Expressions can be used in a number of places in HCL, particularly as attribute
values. Attribute value expressions must adhere to the attribute type. Block
labels must be string literals without any interpolation. Each language
feature's documentation describes any restrictions it places on expressions.

The rest of this page describes all of the features of Nomad'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 to `null`, Nomad 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 Nomad documentation uses each
pair of terms interchangeably (with a historical preference for "list" and
"map").

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

Nomad 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. Nomad 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
  Nomad, 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
  `<KEY> = <VALUE>` 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/job-specification/hcl2/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"`.

### Available Functions

For a full list of available functions, see [the function
reference](/docs/job-specification/hcl2/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 != ""}
```

<!---
## TODO: revamp this section

## 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
service {
  port = "db" # can use expressions here

  check {
    # but the "check" block is always a literal block
  }
}
```

You can dynamically construct repeatable nested blocks like `check` using a
special `dynamic` block type, which is supported anywhere, example:

```hcl
locals {
  check_paths = ["/health", "/"]
}

job "example" {
  # ...
  service {
    port = "lb" # can use expressions here

    dynamic "check" {
      for_each = local.check_paths

      content {
        type     = "http"
        port     = "lb"
        path     = check.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 (`"check"` 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 (`"check"` 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 (`check` 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.

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/job-specification/hcl2/functions/collection/flatten) and
[`setproduct`](/docs/job-specification/hcl2/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
<<EOF
hello
world
EOF
```

The `<<` marker followed by any identifier at the end of a line introduces the
sequence. Nomad 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:

```hcl
block {
  value = <<EOF
hello
world
EOF
}
```

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

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

In this case, Nomad 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:

```text
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, Nomad 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.