whens and lets

Do you know how a YS assignment actually works?

It compiles into a Lisp let form.

Let's take a look...

Assignments are let bindings🔗

Here we compile a main function that assigns 2 variables, x and y.

$ ys -ce '
defn main():
  x =: 6
  y =: 7
  say: x * y
'
(defn main [] (let [x 6 y 7] (say (mul+ x y))))
(apply main ARGS)

This makes it fairly easy to understand how variables inside functions are scoped.

Let's try to use let and when together:

$ NAME=Cammie ys -e '
defn main():
  when ENV.NAME:
    warn: "Hello, $(ENV.NAME)!"
'
Hello, Cammie!

This works, but we had to calculate ENV.NAME twice. We might not want to do that.

We can solve it with a let binding.

$ NAME=Sammie ys -e '
defn main():
  name =: ENV.NAME
  when name:
    warn: "Hello, $name!"
'
Hello, Sammie!

This also works, but I don't like needing an assignment just to ask the question.

There's a when-let function that does exactly what we want.

 NAME=Tammie ys -e '
defn main():
  when-let name ENV.NAME:
    warn: "Hello, $name!"
'
Hello, Tammie!

Nice!

There's also an if-let function, but it doesn't set the variable when the condition is not true.

That's all I got for today.

Let's get into some more interesting stuff this weekend!