HCL is the HashiCorp configuration language.
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ptgeorge and apparentlymart Allow strings "true", "false", "1" and "0" to be used where booleans …
…are expected

There is precedent for allowing strings containing digits where numbers are expected,
and so this extends that to also allow for boolean values to be given as strings.

This applies only to callers going through the decoder API. Direct access via the AST
will reflect exactly what was given in the input configuration.
Latest commit 65a6292 Sep 6, 2018

README.md

HCL

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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, 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 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). 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:

    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:

{
  "variable": {
      "ami": {
          "description": "the AMI to use"
        }
    }
}

Thanks

Thanks to:

  • @vstakhov - The original libucl parser and syntax that HCL was based off of.

  • @fatih - The rewritten HCL parser in pure Go (no goyacc) and support for a printer.