YAMLScript

Add Logic to Your YAML Files

Quick Start

This library lets you load YAML files that may or may not contain YAMLScript functional programming logic. You can use it as a drop-in replacement for your current YAML loader.

Here's an example config.yaml that makes use of YAMLScript functions.

# config.yaml with YAMLScript:
!ys-0:

# Define variables
db-host =: ENV.DB_HOST || 'localhost'
db-port =: ENV.DB_PORT || 5432
deploy =: ENV.DEPLOYMENT || 'dev'
:when deploy !~ /^(dev|stage|prod)/:
  die: |
    Invalid deployment value '$deploy'.
    Must be one of: dev | stage | prod

# Normal YAML data
description: Dynamic application configuration

# Dynamic data values
database:
  host:: db-host
  port:: db-port:num
  name:: "app_$deploy"

# Import external data
features:: load('common.yaml').features

# Use logic and conditions
cache:
  # Variable scoped to this mapping
  enabled =: deploy == 'production'

  directory: .cache
  enabled:: enabled
  limit: 100
  # Conditional key/value pairs
  :when enabled::
    limit:: 1000
    ttl:: 60 * 60  # 3600

What is YAMLScript?

YAMLScript is a functional programming language that can be embedded in YAML. Its syntax is 100% YAML so files that embed it are still valid YAML files.

The YAMLScript project provides YAML loader libraries for many programming languages. They can be used to load any YAML config files properly, whether or not they contain functional programming logic.

It's perfect for:

Configuration files that need logic, variables, and dynamic values
Data transformation with built-in functions for JSON, YAML, and text processing
Templating with powerful string interpolation and data manipulation
Scripting as a complete functional programming language

Key Features

Drop-in YAML replacement – Works with your existing YAML files
Variables & functions – Define and reuse values throughout your files
External data loading – Import JSON, YAML, or data from URLs
Conditional logic – Use if/then/else and pattern matching
Data transformation – Built-ins for transforming & manipulating data
String interpolation – Embed expressions/variables directly in strings
No JVM required – Runs as a native library despite compiling to Clojure

How It Works

YAMLScript extends YAML with a simple, elegant syntax:

# file.yaml
!ys-0:               # Enable YAMLScript

name =: 'World'       # Variable assignment
nums =:: [1, 2, 3]    # Any YAML value

# Literal YAML with ':'
a key: a value

# Evaluated expressions with '::'
message:: "Hello, $name!"
sum:: nums.reduce(+)
timestamp:: now():str

You can load this file from a program as described below, or you can use the ys YAMLScript binary to load the file from the command line:

$ ys -Y file.yaml
a key: a value
message: Hello, World!
sum: 6
timestamp: '2025-09-14T22:35:42.832470203Z'

Under the hood, YAMLScript compiles YAML to Clojure and evaluates it, giving you access to a rich functional programming environment.

Elixir Usage

Use yamlscript as a drop-in replacement for your current YAML loader:

File main.exs:

{:ok, data} = YAMLScript.load(File.read!("config.yaml"))
IO.inspect(data)

# Or the raising variant:
data = YAMLScript.load!(File.read!("config.yaml"))

Installation

Add yamlscript to your mix.exs deps and install the libys.so shared library:

def deps do
  [
    {:yamlscript, "~> 0.2.29"}
  ]
end

curl -sSL https://yamlscript.org/install | LIB=1 bash
export LD_LIBRARY_PATH="$HOME/.local/lib:$LD_LIBRARY_PATH"

See https://yamlscript.org/doc/install/ for more info.

Requirements

Elixir 1.18 or higher (Erlang/OTP 27+)
A C compiler (the package builds a small NIF shim)
Linux or macOS

Authors

Ingy döt Net

License & Copyright

This project is licensed under the terms of the MIT license. See LICENSE for more details.