YAMLScript — Program in YAML

YAMLScript is a new YAML Loader that can add "Super Powers" to your plain old YAML config files.

YAMLScript intends to provide a loader library for every programming language that uses YAML. Currently we have working libraries for Clojure, Go, Java, Julia, NodeJS, Perl, Python, Raku, Ruby and Rust.

By using YAMLScript as your YAML loader, You can dynamically include data from other data files (YAML, JSON, XML, CSV, etc), pull data in from the web or even from a database. You can filter, map, merge, reduce, generate, concatenate, interpolate and manipulate your data to your heart's content.

If you've ever wanted more from your YAML files, YAMLScript has you covered. You can easily mix logic into your data files at any point.

On the other hand, if you just want a rock solid YAML 1.2 loader (without any code evaluation magic) that works the same in any programming language, you should also give YAMLScript a try.

All valid YAML 1.2 Core Schema files are also valid YAMLScript files! That's pretty much any YAML config file you already have.

Without the special !yamlscript/v0 tag at the top, your YAMLScript loader will load any existing YAML (or JSON) just as one would expect a normal YAML loader to do.

Later you can add the special tag and take your YAML capabilities to a whole new level!

Here's an example of using YAMLScript in a YAML file called file.yaml:

--- !yamlscript/v0/ &pets

cats:: load("cats.yaml")
dogs:: curl("https://yamlscript.org/dogs.yaml")

--- !yamlscript/v0/

about: A YAMLScript Example about Pets
title:: "$(ENV.USER.str/capitalize())'s Pets"
birds: !sort:
- Parrot
- Canary
- Owl
cats:: .*pets.cats
dogs:: .*pets.dogs.shuffle().take(2 _)

And these other files:

$ cat cats.yaml
- Siamese
- Persian
- Maine Coon

$ curl -s https://yamlscript.org/dogs.yaml
- Chihuahua
- Pomeranian
- Maltese

- Mastiff
- Great Dane
- Saint Bernard
- Otterhound

From the command line, run:

$ ys --load file.yaml
{"about":"A YAMLScript Example about Pets",
"title":"Ingy's Pets",
"cats":["Siamese","Persian","Maine Coon"],
"dogs":["Otterhound","Saint Bernard"]}

By default YAMLScript outputs JSON, but it can also output YAML by running:

$ ys -Y file.yaml
about: A YAMLScript Example about Pets
title: Ingy's Pets
- Canary
- Owl
- Parrot
- Siamese
- Persian
- Maine Coon
- Great Dane
- Mastiff

You can get the same result from a programming language like Python by using its YAMLScript loader library. Here's a CLI one liner to do the same thing in Python:

$ python -c '
import yamlscript,yaml
ys = yamlscript.YAMLScript()
input = open("file.yaml").read()
data = ys.load(input)

about: A YAMLScript Example about Pets
- Canary
- Owl
- Parrot
- Siamese
- Persian
- Maine Coon
- Otterhound
- Mastiff
title: Ingy's Pets

YAMLScript is also a new, complete, full featured, general purpose, functional and dynamic programming language whose syntax is encoded in YAML. YAMLScript can be used for writing new software applications and libraries.

Here's an example of a YAMLScript program called 99-bottles.ys:

#!/usr/bin/env ys-0

# Print the verses to "99 Bottles of Beer"
# usage:
# ys 99-bottles.ys [<count>]

defn main(number=99):
each [n (number .. 1)]:
say: paragraph(n)

defn paragraph(num): |
$bottles(num) of beer on the wall,
$bottles(num) of beer.
Take one down, pass it around.
$bottles(num - 1) of beer on the wall.

defn bottles(n):
n == 0 : 'No more bottles'
n == 1 : '1 bottle'
=> : "$n bottles"

You can run this program from the command line:

$ ys 99-bottles.ys 3
3 bottles of beer on the wall,
3 bottles of beer.
Take one down, pass it around.
2 bottles of beer on the wall.

2 bottles of beer on the wall,
2 bottles of beer.
Take one down, pass it around.
1 bottle of beer on the wall.

1 bottle of beer on the wall,
1 bottle of beer.
Take one down, pass it around.
No more bottles of beer on the wall.

YAMLScript can compile programs to native binary executables. It's as simple as this:

$ ys -b 99-bottles.ys
* Compiling YAMLScript '99-bottles.ys' to '99-bottles' executable

$ time ./99-bottles 1
1 bottle of beer on the wall,
1 bottle of beer.
Take one down, pass it around.
No more bottles of beer on the wall.

real 0m0.010s
user 0m0.006s
sys 0m0.005s

That's pretty fast!

The YAMLScript language has all the things you expect from a modern programming language including:

  • Using builtin and third party libraries
  • Defining your own namespaces and functions
  • All the standard data types and structures
  • Standard libraries with hundreds of battle tested functions
  • Reasonable performance on par with common dynamic languages

Installing ys - The YAMLScript Command Line Tool

The ys command line tool is the easiest way to get started with YAMLScript. It's currently available on Linux and macOS for both Intel and ARM.

You can try ys out temporarily (for the duration of your shell session) by running this command in your terminal:

$ . <(curl https://yamlscript.org/try-ys)

This will install ys in a temporary directory and add it to the PATH environment variable of your current shell session.

Or you can install the latest release with:

$ curl https://yamlscript.org/install | bash

Make sure that ~/.local/bin is in your PATH environment variable.

To install elsewhere or install a specific version, set the PREFIX and/or VERSION environment variables to the desired values:

$ curl https://yamlscript.org/install | PREFIX=/some/dir VERSION=0.1.xx bash

NOTE: The default PREFIX is ~/.local (or /usr/local if you run the command as root).

You can also install ys from source:

$ git clone https://github.com/yaml/yamlscript
$ cd yamlscript
$ make build
$ make install
$ export PATH=~/.local/bin:$PATH

NOTE: The pre-built binaries currently fail on some older kernels. If you have trouble with the pre-built binaries, try building from source.

The install process has the very minimal dependencies of git, make, curl, and bash. (The libz-dev package is also required on Linux.)

Test your new ys installation by running:

$ ys --help

ys - The YAMLScript (YS) Command Line Tool - v0.1.66

Usage: ys [] []


--run Run a YAMLScript program file (default)
-l, --load Output (compact) JSON of YAMLScript evaluation
-e, --eval YSEXPR Evaluate a YAMLScript expression
multiple -e values joined by newline

-c, --compile Compile YAMLScript to Clojure
-b, --binary Compile to a native binary executable

-p, --print Print the result of --run in code mode
-o, --output FILE Output file for --load, --compile or --binary

-T, --to FORMAT Output format for --load:
json, yaml, edn
-J, --json Output (pretty) JSON for --load
-Y, --yaml Output YAML for --load
-E, --edn Output EDN for --load

-m, --mode MODE Add a mode tag: code, data, or bare (for -e)
-C, --clojure Treat input as Clojure code

-d Debug all compilation stages
-D, --debug-stage STAGE Debug a specific compilation stage:
parse, compose, resolve, build,
transform, construct, print
can be used multiple times
-S, --stack-trace Print full stack trace for errors
-x, --xtrace Print each expression before evaluation

--install Install the libyamlscript shared library
--upgrade Upgrade both ys and libyamlscript

--version Print version and exit
-h, --help Print this help and exit


$ ys --version
YAMLScript 0.1.66

Installing a YAMLScript Library

YAMLScript can be installed as a YAML loader library (module) in several programming languages.

So far there are libraries in these languages: Clojure, Go, Java, NodeJS, Perl, Python, Raku, Ruby and Rust.

Several more are in the works, and the goal is to get it to every language where YAML is used.

Currently to install a YAMLScript library you need to install both the language library and the matching version of libyamlscript.so.

For Python you would do:

$ pip install yamlscript
Successfully installed yamlscript-0.1.66
$ curl https://yamlscript.org/install | VERSION=0.1.66 install
Installed ~/.local/lib/libyamlscript.so - version 0.1.66

For some other language, use that language's library installer. Just make sure the versions match for the library and libyamlscript.

YAMLScript Language Design

YAMLScript code compiles to Clojure code and then is evaluated by a Clojure runtime native binary engine. This means that YAMLScript is a very complete language from the get-go.

NOTE: To see the generated Clojure code for any YAMLScript code just use the -c (--compile) flag for ys:

$ ys -c -e 'say: "Hello"'
(say "Hello")

Clojure is a Lisp dialect that runs on the JVM, however YAMLScript is not run on the JVM. No Java or JVM installation is used to run (or build) YAMLScript programs.

The YAMLScript compiler and runtime interpreter is written in Clojure and then compiled to a native machine code binary using GraalVM's native-image compiler. It is standalone and quite fast.

It is also compiled into a native shared library that can be embedded into almost any programming language. YAMLScript intends to ship language bindings for (at least) 42 popular programming languages.

YAMLScript syntax uses a combination of YAML structure and Clojure Lisp code syntaxes combined together. The code parts have syntax variants that make it feel more like Python or Ruby than a Lisp.

How a YAMLScript program is syntactically styled is very much up to the programmer. She can go Full Lisp or Full YAML, but most likely using a combination of the two will end up reading the best.


YAMLScript is already a working programming language but it does not yet have a stable v0 API release version. In other words, you can use it now but some things might change.

A stable release of YAMLScript v0 is expected in Q2 of 2024.

Once v0 is announced stable, its API will remain backwards compatible for its lifetime. That is to say, any files containing !yamlscript/v0 will always continue to work the same.

YAMLScript Resources

Recent Posts

  • | 5 min read

    Dr. StrangeYAML or How I Learned to Stop Worrying and Love the LLM

    Well now, what happened is, uh, one of our data scientists, uh, well, he went a little funny in the head. You know. Just a little funny. And uh, he went and did a silly thing. Well, I'll tell you...

    read article
  • | 6 min read

    YAMLScript Spring Update

    It's been a while since I let you know what's been happening with YAMLScript. I've been busy working on it every day this year and I have a lot to tell you about! YAMLScript Activity in 2024 Let me...

    read article
  • | 3 min read

    YAMLScript Firsts

    Remember Your First Time? Do you remember the first time you wrote a program in a new language? For YAMLScript, mine was yesterday! This is my first post of 2024. I've been working on YAMLScript...

    read article
  • | 1 min read
    #blog #advent-2023

    2023 Advent Index

    Welcome to the 2023 YAMLScript Advent Blog!   Dec 24 - Wrapped and Ready Dec 23 - Perl to Rust Dec 22 - Flip Flops Dec 21 - YAML, Python and the Holy Graal Dec 20 - Godspeed Dec 19 - Reindeer All...

    read article
  • | 3 min read
    #blog #advent-2023

    Wrapped and Ready

    The Elves have everything wrapped up. Literally! Tonight's the big night. It's Time to Deliver! Welcome to Day 24 of the YAMLScript Advent Calendar It's also time to wrap up this year's YAMLScript...

    read article
  • | 4 min read
    #blog #advent-2023

    Perl to Rust

    When Santa is doing his job in the Luxembourg area, I've always wondered how he gets from Perl to Rust. Maybe he takes this route! Welcome to Day 23 of the YAMLScript Advent Blog! A couple of days ago...

    read article
  • | 3 min read
    #blog #advent-2023

    Flip Flops

    Can you imagine Santa walking around in flip flops? I've never been up to the North Pole, but I'm pretty sure there's no beaches. I always pictured Santa wearing moon boots around the...

    read article
  • | 2 min read
    #blog #advent-2023

    YAML, Python and the Holy Graal

    Which has a greater airspeed velocity... an unladen swallow or Santa's sleigh? Well, that depends... are we talking about an African or European swallow?   Huh?   Welcome to Day 21 of the...

    read article
  • | 4 min read
    #blog #advent-2023


    I wonder if Santa has a Hemi? Supercharged, Turbocharged? Maybe a Nitro Burning Funny Sleigh? Dude's got to get around the world in one night. Godspeed, my festive friend! Welcome to Day 20 of the...

    read article
  • | 3 min read
    #blog #advent-2023

    Reindeer All The Way Down

    Santa is in charge of Christmas. He's the one who makes sure that all the children get presents. But who is in charge of getting Santa his presents? That's where the reindeer come in. They are the...

    read article
View All Posts