Two of the most common things you'll want to do in YAMLScript code is to define variables and functions.

It's very simple. Here's an example:

!yamlscript/v0

name =: 'world'

defn main():
  greeting =: 'Hello'
  say: "$greeting, $name!"

To define a variable with a value we specify a symbol name, followed by one or more spaces, followed by =: for the YAMLScript key (the LHS). The variable will be set to the result of the evaluation of the mapping pair's value (the RHS).

To define a function we use defn, followed by the function name followed by the parenthesized arguments for the LHS. The RHS is the function body.

Let's see how this compiles to Clojure internally using ys -c file.ys:

(def name "world")
(defn main [] (let [greeting "Hello"] (say (str greeting ", " name "!"))))
(apply main ARGS)

In our YAMLScript code we defined 2 variables: name and greeting. But in the Clojure code one became a def expression and the other used let. Using =: outside a function uses def and it's a file scope variable. Using =: inside a function uses let and the scope is the remainder of the function.

This is idiomatic Clojure.