README.md



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,
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.