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 the Waypoint configuration also allows more complex expressions such as references to attributes exported by previous stages, arithmetic, conditional evaluation, and a number of built-in functions.

The rest of this page describes all of the expressions in Waypoint's configuration language. This page is very long so we recommend using the "Jump to Section" dropdown beneath the header above to more easily navigate.

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. Waypoint configuration 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 a parameter value to null, Waypoint 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.

Type Conversion

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

Waypoint 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. Waypoint 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 Waypoint, 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
  }
  

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  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".

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"]).

Arithmetic and Logical Operators

An operator is a type of expression that transforms or combines one or more other expressions. Operators either combine two values in some way to produce a third result value, or transform a single given value to produce a single result.

Operators that work on two values place an operator symbol between the two values, similar to mathematical notation: 1 + 2. Operators that work on only one value place an operator symbol before that value, like !true.

Waypoint configuration has a set of operators for both arithmetic and logic, which are similar to operators in programming languages such as JavaScript or Ruby.

When multiple operators are used together in an expression, they are evaluated in the following order of operations:

1. !, - (multiplication by -1)
2. *, /, %
3. +, - (subtraction)
4. >, >=, <, <=
5. ==, !=
6. &&
7. ||

Parentheses can be used to override the default order of operations. Without parentheses, higher levels are evaluated first, so 1 + 2 * 3 is interpreted as 1 + (2 * 3) and not as (1 + 2) * 3.

The different operators can be gathered into a few different groups with similar behavior, as described below. Each group of operators expects its given values to be of a particular type. Waypoint will attempt to convert values to the required type automatically, or will produce an error message if this automatic conversion is not possible.

Arithmetic Operators

The arithmetic operators all expect number values and produce number values as results:

• a + b returns the result of adding a and b together.
• a - b returns the result of subtracting b from a.
• a * b returns the result of multiplying a and b.
• a / b returns the result of dividing a by b.
• a % b returns the remainder of dividing a by b. This operator is generally useful only when used with whole numbers.
• -a returns the result of multiplying a by -1.

Equality Operators

The equality operators both take two values of any type and produce boolean values as results.

• a == b returns true if a and b both have the same type and the same value, or false otherwise.
• a != b is the opposite of a == b.

Comparison Operators

The comparison operators all expect number values and produce boolean values as results.

• a < b returns true if a is less than b, or false otherwise.
• a <= b returns true if a is less than or equal to b, or false otherwise.
• a > b returns true if a is greater than b, or false otherwise.
• a >= b returns true if a is greater than or equal to b, or false otherwise.

Logical Operators

The logical operators all expect bool values and produce bool values as results.

• a || b returns true if either a or b is true, or false if both are false.
• a && b returns true if both a and b are true, or false if either one is false.
• !a returns true if a is false, and false if a is true.

Conditional Expressions

A conditional expression uses the value of a bool expression to select one of two values.

The syntax of a conditional expression is as follows:

condition ? true_val : false_val

If condition is true then the result is true_val. If condition is false then the result is false_val.

A common use of conditional expressions is to define defaults to replace invalid values:

var.a != "" ? var.a : "default-a"

If var.a is an empty string then the result is "default-a", but otherwise it is the actual value of var.a.

Any of the equality, comparison, and logical operators can be used to define the condition. The two result values may be of any type, but they must both be of the same type so that Waypoint can determine what type the whole conditional expression will return without knowing the condition value.

Function Calls

Waypoint configuration has a number of built-in functions that can be used within expressions as another way to transform and combine values. These are similar to the operators but all follow a common syntax:

<FUNCTION NAME>(<ARGUMENT 1>, <ARGUMENT 2>)

The function name specifies which function to call. Each defined function expects a specific number of arguments with specific value types, and returns a specific value type as a result.

Some functions take an arbitrary number of arguments. For example, the min function takes any amount of number arguments and returns the one that is numerically smallest:

min(55, 3453, 2)

Expanding Function Arguments

If the arguments to pass to a function are available in a list or tuple value, that value can be expanded into separate arguments. Provide the list value as an argument and follow it with the ... symbol:

min([55, 2453, 2]...)

The expansion symbol is three periods (...), not a Unicode ellipsis character (…). Expansion is a special syntax that is only available in function calls.

Available Functions

For a full list of available functions, see the function reference.

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:

[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:

{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:

[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:

[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:

{for s in var.list : substr(s, 0, 1) => s... if s != ""}

Splat Expressions

A splat expression is a 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:

[for o in var.list : o.id]

This is equivalent to the following splat expression:

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:

var.list[*].interfaces[0].name

The above expression is equivalent to the following for expression:

[for o in var.list : o.interfaces[0].name]

String Literals

Waypoint configuration 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:

The alternative syntax for string literals is the so-called "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:

<<EOT
hello
world
EOT

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

stanza {
  value = <<EOT
hello
world
EOT
}

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

stanza {
  value = <<-EOT
  hello
    world
  EOT
}

In this case, Waypoint 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, Waypoint 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.

Interpolation

A ${ ... } sequence is an interpolation, which evaluates the expression given between the markers, converts the result to a string if necessary, and then inserts it into the final string:

"Hello, ${var.name}!"

In the above example, the named object var.name is accessed and its value inserted into the string, producing a result like "Hello, Juan!".

Directives

A %{ ... } sequence is a directive, which allows for conditional results and iteration over collections, similar to conditional and for expressions.

The following directives are supported:

• The if <BOOL>/else/endif directive chooses between two templates based on the value of a bool expression:

  "Hello, %{ if var.name != "" }${var.name}%{ else }unnamed%{ endif }!"
  

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  The else portion may be omitted, in which case the result is an empty string if the condition expression returns false.

• The for <NAME> in <COLLECTION> / endfor directive iterates over the elements of a given collection or structural value and evaluates a given template once for each element, concatenating the results together:

  <<EOT
  %{ for ip in ips }
  server ${ip}
  %{ endfor }
  EOT
  

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  The name given immediately after the for keyword is used as a temporary variable name which can then be referenced from the nested template.

To allow template directives to be formatted for readability without adding unwanted spaces and newlines to the result, all template sequences can include optional strip markers (~), immediately after the opening characters or immediately before the end. When a strip marker is present, the template sequence consumes all of the literal whitespace (spaces and newlines) either before the sequence (if the marker appears at the beginning) or after (if the marker appears at the end):

<<EOT
%{ for ip in ips ~}
server ${ip}
%{ endfor ~}
EOT

In the above example, the newline after each of the directives is not included in the output, but the newline after the server ${ip} sequence is retained, causing only one line to be generated for each element:

server 10.1.16.154
server 10.1.16.1
server 10.1.16.34

When using template directives, we recommend always using the "heredoc" string literal form and then formatting the template over multiple lines for readability. Quoted string literals should usually include only interpolation sequences.