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.

Fortran Usage

The load function compiles and evaluates a YAMLScript (or plain YAML) string and returns the raw JSON response string from libys: {"data": ...} on success or {"error": {"cause": ...}} on failure. Fortran has no standard JSON library, so parsing the response is left to the caller (for example with the json-fortran package).

File main.f90:

program main
  use yamlscript
  implicit none

  type(yamlscript_t) :: ys
  character(len=:), allocatable :: json

  call ys%init()
  json = ys%load('!ys-0:' // new_line('a') // 'key:: inc(41)')
  print *, json
  call ys%destroy()
end program main

Installation

Add yamlscript to your fpm.toml and install the libys.so shared library:

[dependencies.yamlscript]
git = "https://github.com/yaml/yamlscript-fortran"
tag = "v0.2.25"

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

Build with the libys library path:

fpm build --flag "-L$HOME/.local/lib"

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

Requirements

• GFortran and the Fortran Package Manager (fpm)
• Linux or macOS

See Also

• YAMLScript Web Site
• Learn YAMLScript
• YAMLScript Blog
• YAMLScript Source Code
• YAMLScript Programs
• YAML
• Clojure

Authors

• Ingy döt Net

License & Copyright

Copyright 2022-2026 Ingy döt Net ingy@ingy.net

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