blog/2025-07-31/the-loop-and-more-loops/

[giscus[bot]]

blog/2025-07-31/the-loop-and-more-loops/

Program in YAML — Code is Data

https://yamlscript.org/blog/2025-07-31/the-loop-and-more-loops/

[pwdrysdale]

Hey ingy, I'm very fresh to Yamlscript and clojure.. I have tried for a few hours to write a simple yamlscript isPrime function, and am having no luck (similarly, none of the AI chat bots are having any luck either!).

How would you do it?

I am coming from more of a Javascript / Typescript / Go background.

Trying to assign / reassign variables outside of loops, etc? The whole thing has got me pretty messed up :/

I looked over some of the rosetta stuff too, but still am just missing something

[ingydotnet]

Hi @pwdrysdale !

Apologies for not seeing this sooner!

Here's a quick example of defining and calling a prime? function.

!YS-v0

defn main(*nums):
  each num nums: !:say
    if prime?(num): +
      "$num is prime"
      "$num is not prime"

defn prime?(n):
  (n > 1) &&
  \(zero?(n % _))
    .not-any?(range(2 n))

and run it:

$ ys prime.ys 1 2 3 4 5 6 7 8 9
1 is not prime
2 is prime
3 is prime
4 is not prime
5 is prime
6 is not prime
7 is prime
8 is not prime
9 is not prime

There's a lot to discuss here. How about instead of me trying to tell you the finer points here, let's turn this into a dialog.
Tell me what you understand here and what isn't clear.
Not only would that be a better way to teach/learn, it would be helpful for others in the future.
And maybe others can chime in whilst we discuss it!

Looking forward to your replies...

[ingydotnet]

WRT to functional (FP) versus imperative programming, it's a bit of a mind switch but not a huge one.
YS/Clojure is really pretty simple.
Just remember that variables hold values that are immutable.
And functions take arguments and return a value.

Say you have a variable called m that contains a mapping {"foo" 111} and you want to add a new key/value pair bar and 222 and you want to change the value of foo to 123.
You can't change the mapping because it is immutable. But you can call a function that takes the mapping and the new stuff and returns an entirely new mapping:

$ ys -U -e '
m =:: {foo: 111}
m =: m.assoc("bar" 222)
m =: m.assoc("foo" 123)
say: m
'
{foo 123, bar 222}

You never changed a mapping here. You just created a new one and then re-bound m to the new one.

Another thing to keep in mind is that you can't return early from a function.

Hope this sparks more questions.