Sharpen Your Tools

by Ingy döt Net | | 4 min read

Wanna make some fun toys with YAMLScript? You'll need some sharp tools. You think those elves make all those toys with dull tools?

The CLI tool ys is the main tool you'll use to work with YAMLScript. Today we'll learn about all the things you can do with it.

Welcome to Day 7 of the YAMLScript Advent Calendar

On Tuesday you learned how to install YAMLScript. Reminder, here's the quick way to install the latest version:

$ curl | PREFIX=~/.yamlscript bash
$ export PATH=$HOME/.yamlscript/bin:$PATH
$ ys --version
YAMLScript v0.1.21

The best first command to run is ys --help:

$ ys --help

ys - The YAMLScript (YS) Command Line Tool - v0.1.47

Usage: ys [<option...>] [<file>]


--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

Ready, set, actions!

The first thing to notice is that ys has 3 "actions":

  • --run (default) - Compile and evaluate a YAMLScript file
  • --load - Output the evaluated YAMLScript value as JSON (by default)
  • --compile - Compile YAMLScript code to Clojure code

For each action you'll need some YAMLScript source code. This can come from 3 different places:

  • The --eval (-e) option - specifies a line of YAMLScript code. You can use this option multiple times to specify multiple lines of code.
  • A file path - specify a path to a file containing YAMLScript code.
  • Standard input - specify - as the file path to read YAMLScript code from standard input. If no file or -e options are specified, ys will check to see if there is data on stdin. That means you can leave off the - and pipe data into ys like this: echo $'!yamlscript/v0\nsay: "Hello"' | ys. Of course, it doesn't hurt to specify the - anyway.

Running Clojure with ys

Clojure code is often valid YAMLScript code:

$ ys --compile -e '(println (+ 1 2))'
(println (+ 1 2))

This YAMLScript compiles to the exact same Clojure code.

If you want the code you run to be considered to be Clojure code (thus not compiled by the yamlscript compiler), you can use the --clj (-C) option. This is useful when you want to test out the YAMLScript runtime envronment directly with Clojure code.

Also you can pipe the output of ys --compile to ys --clj to run the compiler's Clojure code ouput:

$ ys -c -e 'say: 123' | ys -C

Modes and Formats

We learned about modes and the --mode option yesterday. You can set the mode for --eval (-e) code with the --mode (-m) option. The accepted values are code (c), data (d) and bare (b).

When you "load" YAMLScript using --load you get the result printed to stdout as JSON. These are 3 formatting options to control how the output is displayed:

  • --json (-J) - Output JSON (default) Note that this JSON is a bit more nicely formatted than the default output.
  • --yaml (-Y) - Output YAML
  • --edn (-E) - Output EDN. EDN is Clojure's native data format. It is also valid Clojure code.

Note that when you specify a formatting option, it implies the --load action.


When you "run" a YAMLScript program it doesn't print anything unless you use a printing command. This isn't surprising; all languages work this way.

Sometimes you want to know what the final value of the run was. To get this you could print it with say. You can also use the special --print (-p) option, which does exactly that (with less typing)..

Finally there a 2 special debugging options:

  • --debug (-X) - Print a full stack trace for errors (more info)
  • --debug-stage (-x) - Display the result of a stage/stages

The --debug-stage option is super useful for understanding exactly how YAMLScript code compiles to Clojure code.

For example, to see the internal AST when compiling some YAMLScript:

$ ys -c -e 'say: "Hello"' -xconstruct
*** construct output ***
{:Lst [{:Sym say} {:Str "Hello"}]}

(say "Hello")

And to see all 7 compilation stages:

$ ys -c -e 'say: "Hello"' -xall
*** parse output ***
({:+ "+MAP", :! "yamlscript/v0"}
{:+ "=VAL", := "say"}
{:+ "=VAL", :$ "Hello"}
{:+ "-MAP"})

*** compose output ***
{:! "yamlscript/v0", :% [{:= "say"} {:$ "Hello"}]}

*** resolve output ***
{:ysm ({:ysx "say"} {:ysi "Hello"})}

*** build output ***
{:ysm ({:Sym say} {:Str "Hello"})}

*** transform output ***
{:ysm ({:Sym say} {:Str "Hello"})}

*** construct output ***
{:Lst [{:Sym say} {:Str "Hello"}]}

*** print output ***
"(say \"Hello\")\n"

(say "Hello")

We'll go over all of these stages in detail in a future post.

In the meantime, try out your new ys tool and see what you can do with it. The more you use it, the sharper it will get.

I'll see you tomorrow for day 8 of YAMLScript Advent 2023!