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github.com/hashicorp/hcl

Package hcl decodes HCL into usable Go structures. hcl input can come in either pure HCL format or JSON format. It can be parsed into an AST, and then decoded into a structure, or it can be decoded directly from a string into a structure. If you choose to parse HCL into a raw AST, the benefit is that you can write custom visitor implementations to implement custom semantic checks. By default, HCL does not perform any semantic checks.

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# HCL

[![GoDoc](https://godoc.org/github.com/hashicorp/hcl?status.png)](https://godoc.org/github.com/hashicorp/hcl) [![Build Status](https://travis-ci.org/hashicorp/hcl.svg?branch=master)](https://travis-ci.org/hashicorp/hcl)

HCL (HashiCorp Configuration Language) is a configuration language built
by HashiCorp. The goal of HCL is to build a structured configuration language
that is both human and machine friendly for use with command-line tools, but
specifically targeted towards DevOps tools, servers, etc.

HCL is also fully JSON compatible. That is, JSON can be used as completely
valid input to a system expecting HCL. This helps makes systems
interoperable with other systems.

HCL is heavily inspired by
[libucl](https://github.com/vstakhov/libucl),
nginx configuration, and others similar.

## Why?

A common question when viewing HCL is to ask the question: why not
JSON, YAML, etc.?

Prior to HCL, the tools we built at [HashiCorp](http://www.hashicorp.com)
used a variety of configuration languages from full programming languages
such as Ruby to complete data structure languages such as JSON. What we
learned is that some people wanted human-friendly configuration languages
and some people wanted machine-friendly languages.

JSON fits a nice balance in this, but is fairly verbose and most
importantly doesn't support comments. With YAML, we found that beginners
had a really hard time determining what the actual structure was, and
ended up guessing more often than not whether to use a hyphen, colon, etc.
in order to represent some configuration key.

Full programming languages such as Ruby enable complex behavior
a configuration language shouldn't usually allow, and also forces
people to learn some set of Ruby.

Because of this, we decided to create our own configuration language
that is JSON-compatible. Our configuration language (HCL) is designed
to be written and modified by humans. The API for HCL allows JSON
as an input so that it is also machine-friendly (machines can generate
JSON instead of trying to generate HCL).

Our goal with HCL is not to alienate other configuration languages.
It is instead to provide HCL as a specialized language for our tools,
and JSON as the interoperability layer.

## Syntax

For a complete grammar, please see the parser itself. A high-level overview
of the syntax and grammar is listed here.

  * Single line comments start with `#` or `//`

  * Multi-line comments are wrapped in `/*` and `*/`. Nested block comments
    are not allowed. A multi-line comment (also known as a block comment)
    terminates at the first `*/` found.

  * Values are assigned with the syntax `key = value` (whitespace doesn't
    matter). The value can be any primitive: a string, number, boolean,
    object, or list.

  * Strings are double-quoted and can contain any UTF-8 characters.
    Example: `"Hello, World"`

  * Multi-line strings start with `<<EOF` at the end of a line, and end
    with `EOF` on its own line ([here documents](https://en.wikipedia.org/wiki/Here_document)).
    Any text may be used in place of `EOF`. Example:
```
<<FOO
hello
world
FOO
```

  * Numbers are assumed to be base 10. If you prefix a number with 0x,
    it is treated as a hexadecimal. If it is prefixed with 0, it is
    treated as an octal. Numbers can be in scientific notation: "1e10".

  * Boolean values: `true`, `false`

  * Arrays can be made by wrapping it in `[]`. Example:
    `["foo", "bar", 42]`. Arrays can contain primitives,
    other arrays, and objects. As an alternative, lists
    of objects can be created with repeated blocks, using
    this structure:

    ```hcl
    service {
        key = "value"
    }

    service {
        key = "value"
    }
    ```

Objects and nested objects are created using the structure shown below:

```
variable "ami" {
    description = "the AMI to use"
}
```
This would be equivalent to the following json:
``` json
{
  "variable": {
      "ami": {
          "description": "the AMI to use"
        }
    }
}
```

## Thanks

Thanks to:

  * [@vstakhov](https://github.com/vstakhov) - The original libucl parser
    and syntax that HCL was based off of.

  * [@fatih](https://github.com/fatih) - The rewritten HCL parser
    in pure Go (no goyacc) and support for a printer.

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Last updated on 13 Oct 2022

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