2016-01-31 08:41:53 +01:00
|
|
|
# HIL
|
|
|
|
|
|
|
|
[![GoDoc](https://godoc.org/github.com/hashicorp/hil?status.png)](https://godoc.org/github.com/hashicorp/hil) [![Build Status](https://travis-ci.org/hashicorp/hil.svg?branch=master)](https://travis-ci.org/hashicorp/hil)
|
|
|
|
|
|
|
|
HIL (HashiCorp Interpolation Language) is a lightweight embedded language used
|
|
|
|
primarily for configuration interpolation. The goal of HIL is to make a simple
|
|
|
|
language for interpolations in the various configurations of HashiCorp tools.
|
|
|
|
|
|
|
|
HIL is built to interpolate any string, but is in use by HashiCorp primarily
|
|
|
|
with [HCL](https://github.com/hashicorp/hcl). HCL is _not required_ in any
|
|
|
|
way for use with HIL.
|
|
|
|
|
|
|
|
HIL isn't meant to be a general purpose language. It was built for basic
|
|
|
|
configuration interpolations. Therefore, you can't currently write functions,
|
|
|
|
have conditionals, set intermediary variables, etc. within HIL itself. It is
|
|
|
|
possible some of these may be added later but the right use case must exist.
|
|
|
|
|
|
|
|
## Why?
|
|
|
|
|
|
|
|
Many of our tools have support for something similar to templates, but
|
|
|
|
within the configuration itself. The most prominent requirement was in
|
|
|
|
[Terraform](https://github.com/hashicorp/terraform) where we wanted the
|
|
|
|
configuration to be able to reference values from elsewhere in the
|
|
|
|
configuration. Example:
|
|
|
|
|
|
|
|
foo = "hi ${var.world}"
|
|
|
|
|
|
|
|
We originally used a full templating language for this, but found it
|
|
|
|
was too heavy weight. Additionally, many full languages required bindings
|
|
|
|
to C (and thus the usage of cgo) which we try to avoid to make cross-compilation
|
|
|
|
easier. We then moved to very basic regular expression based
|
|
|
|
string replacement, but found the need for basic arithmetic and function
|
|
|
|
calls resulting in overly complex regular expressions.
|
|
|
|
|
|
|
|
Ultimately, we wrote our own mini-language within Terraform itself. As
|
|
|
|
we built other projects such as [Nomad](https://nomadproject.io) and
|
|
|
|
[Otto](https://ottoproject.io), the need for basic interpolations arose
|
|
|
|
again.
|
|
|
|
|
|
|
|
Thus HIL was born. It is extracted from Terraform, cleaned up, and
|
|
|
|
better tested for general purpose use.
|
|
|
|
|
|
|
|
## Syntax
|
|
|
|
|
|
|
|
For a complete grammar, please see the parser itself. A high-level overview
|
|
|
|
of the syntax and grammer is listed here.
|
|
|
|
|
|
|
|
Code begins within `${` and `}`. Outside of this, text is treated
|
|
|
|
literally. For example, `foo` is a valid HIL program that is just the
|
|
|
|
string "foo", but `foo ${bar}` is an HIL program that is the string "foo "
|
|
|
|
concatened with the value of `bar`. For the remainder of the syntax
|
|
|
|
docs, we'll assume you're within `${}`.
|
|
|
|
|
|
|
|
* Identifiers are any text in the format of `[a-zA-Z0-9-.]`. Example
|
|
|
|
identifiers: `foo`, `var.foo`, `foo-bar`.
|
|
|
|
|
|
|
|
* Strings are double quoted and can contain any UTF-8 characters.
|
|
|
|
Example: `"Hello, World"`
|
|
|
|
|
|
|
|
* 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".
|
|
|
|
|
|
|
|
* Unary `-` can be used for negative numbers. Example: `-10` or `-0.2`
|
|
|
|
|
|
|
|
* Boolean values: `true`, `false`
|
|
|
|
|
|
|
|
* The following arithmetic operations are allowed: +, -, *, /, %.
|
|
|
|
|
|
|
|
* Function calls are in the form of `name(arg1, arg2, ...)`. Example:
|
|
|
|
`add(1, 5)`. Arguments can be any valid HIL expression, example:
|
|
|
|
`add(1, var.foo)` or even nested function calls:
|
|
|
|
`add(1, get("some value"))`.
|
|
|
|
|
2016-04-19 01:31:19 +02:00
|
|
|
* Within strings, further interpolations can be opened with `${}`.
|
2016-01-31 08:41:53 +01:00
|
|
|
Example: `"Hello ${nested}"`. A full example including the
|
|
|
|
original `${}` (remember this list assumes were inside of one
|
|
|
|
already) could be: `foo ${func("hello ${var.foo}")}`.
|
|
|
|
|
|
|
|
## Language Changes
|
|
|
|
|
|
|
|
We've used this mini-language in Terraform for years. For backwards compatibility
|
|
|
|
reasons, we're unlikely to make an incompatible change to the language but
|
|
|
|
we're not currently making that promise, either.
|
|
|
|
|
|
|
|
The internal API of this project may very well change as we evolve it
|
|
|
|
to work with more of our projects. We recommend using some sort of dependency
|
|
|
|
management solution with this package.
|
|
|
|
|
|
|
|
## Future Changes
|
|
|
|
|
|
|
|
The following changes are already planned to be made at some point:
|
|
|
|
|
|
|
|
* Richer types: lists, maps, etc.
|
|
|
|
|
|
|
|
* Convert to a more standard Go parser structure similar to HCL. This
|
|
|
|
will improve our error messaging as well as allow us to have automatic
|
|
|
|
formatting.
|
|
|
|
|
|
|
|
* Allow interpolations to result in more types than just a string. While
|
|
|
|
within the interpolation basic types are honored, the result is always
|
|
|
|
a string.
|