guide: The "Configuration Language Design" section
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Martin Atkins
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@ -75,6 +75,8 @@ complex structures:
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source_file = "${path.module}/foo.txt"
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.. _go-expression-funcs:
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Defining Functions
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------------------
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@ -8,6 +8,8 @@ some more complex situations that can benefit from some additional techniques.
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This section lists a few of these situations and ways to use the HCL API to
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accommodate them.
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.. _go-interdep-blocks:
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Interdependent Blocks
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---------------------
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@ -1,3 +1,285 @@
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Configuration Language Design
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=============================
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In this section we will cover some conventions for HCL-based configuration
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languages that can help make them feel consistent with other HCL-based
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languages, and make the best use of HCL's building blocks.
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HCL's native and JSON syntaxes both define a mapping from input bytes to a
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higher-level information model. In designing a configuration language based on
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HCL, your building blocks are the components in that information model:
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blocks, arguments, and expressions.
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Each calling application of HCL, then, effectively defines its own language.
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Just as Atom and RSS are higher-level languages built on XML, HashiCorp
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Terraform has a higher-level language built on HCL, while HashiCorp Nomad has
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its own distinct language that is *also* built on HCL.
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From an end-user perspective, these are distinct languages but have a common
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underlying texture. Users of both are therefore likely to bring some
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expectations from one to the other, and so this section is an attempt to
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codify some of these shared expectations to reduce user surprise.
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These are subjective guidelines however, and so applications may choose to
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ignore them entirely or ignore them in certain specialized cases. An
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application providing a configuration language for a pre-existing system, for
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example, may choose to eschew the identifier naming conventions in this section
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in order to exactly match the existing names in that underlying system.
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Language Keywords and Identifiers
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---------------------------------
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Much of the work in defining an HCL-based language is in selecting good names
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for arguments, block types, variables, and functions.
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The standard for naming in HCL is to use all-lowercase identifiers with
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underscores separating words, like ``service`` or ``io_mode``. HCL identifiers
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do allow uppercase letters and dashes, but this primarily for natural
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interfacing with external systems that may have other identifier conventions,
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and so these should generally be avoided for the identifiers native to your
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own language.
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The distinction between "keywords" and other identifiers is really just a
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convention. In your own language documentation, you may use the word "keyword"
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to refer to names that are presented as an intrinsic part of your language,
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such as important top-level block type names.
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Block type names are usually singular, since each block defines a single
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object. Use a plural block name only if the block is serving only as a
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namespacing container for a number of other objects. A block with a plural
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type name will generally contain only nested blocks, and no arguments of its
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own.
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Argument names are also singular unless they expect a collection value, in
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which case they should be plural. For example, ``name = "foo"`` but
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``subnet_ids = ["abc", "123"]``.
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Function names will generally *not* use underscores and will instead just run
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words together, as is common in the C standard library. This is a result of
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the fact that several of the standard library functions offered in ``cty``
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(covered in a later section) have names that follow C library function names
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like ``substr``. This is not a strong rule, and applications that use longer
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names may choose to use underscores for them to improve readability.
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Blocks vs. Object Values
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------------------------
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HCL blocks and argument values of object type have quite a similar appearance
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in the native syntax, and are identical in JSON syntax:
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.. code-block:: hcl
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block {
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foo = bar
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}
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# argument with object constructor expression
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argument = {
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foo = bar
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}
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In spite of this superficial similarity, there are some important differences
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between these two forms.
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The most significant difference is that a child block can contain nested blocks
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of its own, while an object constructor expression can define only attributes
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of the object it is creating.
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The user-facing model for blocks is that they generally form the more "rigid"
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structure of the language itself, while argument values can be more free-form.
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An application will generally define in its schema and documentation all of
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the arguments that are valid for a particular block type, while arguments
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accepting object constructors are more appropriate for situations where the
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arguments themselves are freely selected by the user, such as when the
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expression will be converted by the application to a map type.
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As a less contrived example, consider the ``resource`` block type in Terraform
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and its use with a particular resource type ``aws_instance``:
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.. code-block:: hcl
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resource "aws_instance" "example" {
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ami = "ami-abc123"
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instance_type = "t2.micro"
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tags = {
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Name = "example instance"
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}
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ebs_block_device {
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device_name = "hda1"
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volume_size = 8
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volume_type = "standard"
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}
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}
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The top-level block type ``resource`` is fundamental to Terraform itself and
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so an obvious candidate for block syntax: it maps directly onto an object in
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Terraform's own domain model.
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Within this block we see a mixture of arguments and nested blocks, all defined
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as part of the schema of the ``aws_instance`` resource type. The ``tags``
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map here is specified as an argument because its keys are free-form, chosen
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by the user and mapped directly onto a map in the underlying system.
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``ebs_block_device`` is specified as a nested block, because it is a separate
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domain object within the remote system and has a rigid schema of its own.
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As a special case, block syntax may sometimes be used with free-form keys if
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those keys each serve as a separate declaration of some first-class object
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in the language. For example, Terraform has a top-level block type ``locals``
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which behaves in this way:
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.. code-block:: hcl
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locals {
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instance_type = "t2.micro"
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instance_id = aws_instance.example.id
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}
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Although the argument names in this block are arbitrarily selected by the
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user, each one defines a distinct top-level object. In other words, this
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approach is used to create a more ergonomic syntax for defining these simple
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single-expression objects, as a pragmatic alternative to more verbose and
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redundant declarations using blocks:
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.. code-block:: hcl
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local "instance_type" {
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value = "t2.micro"
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}
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local "instance_id" {
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value = aws_instance.example.id
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}
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The distinction between domain objects, language constructs and user data will
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always be subjective, so the final decision is up to you as the language
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designer.
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Standard Functions
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------------------
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HCL itself does not define a common set of functions available in all HCL-based
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languages; the built-in language operators give a baseline of functionality
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that is always available, but applications are free to define functions as they
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see fit.
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With that said, there's a number of generally-useful functions that don't
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belong to the domain of any one application: string manipulation, sequence
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manipulation, date formatting, JSON serialization and parsing, etc.
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Given the general need such functions serve, it's helpful if a similar set of
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functions is available with compatible behavior across multiple HCL-based
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languages, assuming the language is for an application where function calls
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make sense at all.
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The Go implementation of HCL is built on an underlying type and function system
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:go:pkg:`cty`, whose usage was introduced in :ref:`go-expression-funcs`. That
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library also has a package of "standard library" functions which we encourage
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applications to offer with consistent names and compatible behavior, either by
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using the standard implementations directly or offering compatible
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implementations under the same name.
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The "standard" functions that new configuration formats should consider
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offering are:
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* ``abs(number)`` - returns the absolute (positive) value of the given number.
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* ``coalesce(vals...)`` - returns the value of the first argument that isn't null. Useful only in formats where null values may appear.
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* ``compact(vals...)`` - returns a new tuple with the non-null values given as arguments, preserving order.
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* ``concat(seqs...)`` - builds a tuple value by concatenating together all of the given sequence (list or tuple) arguments.
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* ``format(fmt, args...)`` - performs simple string formatting similar to the C library function ``printf``.
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* ``hasindex(coll, idx)`` - returns true if the given collection has the given index. ``coll`` may be of list, tuple, map, or object type.
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* ``int(number)`` - returns the integer component of the given number, rounding towards zero.
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* ``jsondecode(str)`` - interprets the given string as JSON format and return the corresponding decoded value.
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* ``jsonencode(val)`` - encodes the given value as a JSON string.
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* ``length(coll)`` - returns the length of the given collection.
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* ``lower(str)`` - converts the letters in the given string to lowercase, using Unicode case folding rules.
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* ``max(numbers...)`` - returns the highest of the given number values.
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* ``min(numbers...)`` - returns the lowest of the given number values.
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* ``sethas(set, val)`` - returns true only if the given set has the given value as an element.
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* ``setintersection(sets...)`` - returns the intersection of the given sets
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* ``setsubtract(set1, set2)`` - returns a set with the elements from ``set1`` that are not also in ``set2``.
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* ``setsymdiff(sets...)`` - returns the symmetric difference of the given sets.
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* ``setunion(sets...)`` - returns the union of the given sets.
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* ``strlen(str)`` - returns the length of the given string in Unicode grapheme clusters.
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* ``substr(str, offset, length)`` - returns a substring from the given string by splitting it between Unicode grapheme clusters.
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* ``timeadd(time, duration)`` - takes a timestamp in RFC3339 format and a possibly-negative duration given as a string like ``"1h"`` (for "one hour") and returns a new RFC3339 timestamp after adding the duration to the given timestamp.
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* ``upper(str)`` - converts the letters in the given string to uppercase, using Unicode case folding rules.
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Not all of these functions will make sense in all applications. For example, an
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application that doesn't use set types at all would have no reason to provide
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the set-manipulation functions here.
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Some languages will not provide functions at all, since they are primarily for
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assigning values to arguments and thus do not need nor want any custom
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computations of those values.
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Block Results as Expression Variables
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-------------------------------------
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In some applications, top-level blocks serve also as declarations of variables
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(or of attributes of object variables) available during expression evaluation,
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as discussed in :ref:`go-interdep-blocks`.
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In this case, it's most intuitive for the variables map in the evaluation
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context to contain an value named after each valid top-level block
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type and for these values to be object-typed or map-typed and reflect the
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structure implied by block type labels.
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For example, an application may have a top-level ``service`` block type
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used like this:
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.. code-block:: hcl
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service "http" "web_proxy" {
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listen_addr = "127.0.0.1:8080"
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process "main" {
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command = ["/usr/local/bin/awesome-app", "server"]
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}
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process "mgmt" {
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command = ["/usr/local/bin/awesome-app", "mgmt"]
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}
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}
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If the result of decoding this block were available for use in expressions
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elsewhere in configuration, the above convention would call for it to be
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available to expressions as an object at ``service.http.web_proxy``.
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If it the contents of the block itself that are offered to evaluation -- or
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a superset object *derived* from the block contents -- then the block arguments
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can map directly to object attributes, but it is up to the application to
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decide which value type is most appropriate for each block type, since this
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depends on how multiple blocks of the same type relate to one another, or if
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multiple blocks of that type are even allowed.
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In the above example, an application would probably expose the ``listen_addr``
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argument value as ``service.http.web_proxy.listen_addr``, and may choose to
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expose the ``process`` blocks as a map of objects using the labels as keys,
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which would allow an expression like
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``service.http.web_proxy.service["main"].command``.
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If multiple blocks of a given type do not have a significant order relative to
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one another, as seems to be the case with these ``process`` blocks,
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representation as a map is often the most intuitive. If the ordering of the
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blocks *is* significant then a list may be more appropriate, allowing the use
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of HCL's "splat operators" for convenient access to child arguments. However,
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there is no one-size-fits-all solution here and language designers must
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instead consider the likely usage patterns of each value and select the
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value representation that best accommodates those patterns.
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Some applications may choose to offer variables with slightly different names
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than the top-level blocks in order to allow for more concise references, such
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as abbreviating ``service`` to ``svc`` in the above examples. This should be
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done with care since it may make the relationship between the two less obvious,
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but this may be a good tradeoff for names that are accessed frequently that
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might otherwise hurt the readability of expressions they are embedded in.
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Familiarity permits brevity.
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Many applications will not make blocks results available for use in other
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expressions at all, in which case they are free to select whichever variable
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names make sense for what is being exposed. For example, a format may make
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environment variable values available for use in expressions, and may do so
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either as top-level variables (if no other variables are needed) or as an
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object named ``env``, which can be used as in ``env.HOME``.
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