Configuration is a programming language whether you like it or not

Start with a list of key-value pairs. Perfectly reasonable. Then one value needs to differ between staging and production, so you add a conditional — maybe as an overlay file, maybe as a templating tag, maybe as a naming convention that a script interprets. Then you have twelve services that are almost identical, so you add a loop. Then the loop body needs a name derived from the index, so you add string interpolation.

You have now written a programming language. You did it accidentally, without a spec, without a type system, and without a debugger.

The tell

The tell is always the same: someone asks "why is this value wrong in production?" and the answer requires reading four files and simulating a text substitution in your head.

That's not a configuration problem. That's a program you cannot step through.

What I actually think

Not "YAML bad." YAML is fine as a serialization format, which is what it is. The mistake is asking a serialization format to carry logic, and then bolting the logic on through the one mechanism the format didn't reserve for it — string templating.

Two honest options:

• Keep configuration dumb. No conditionals, no loops, no interpolation. Generate it from something else, and check the generated artifact into the repo where a human can read it and a reviewer can diff it.
• Admit it's code. Use a real language with types, a compiler and an editor that understands it. CUE, Starlark, Pkl, or plain TypeScript.

What doesn't work is the middle: a format that's neither dumb enough to read nor expressive enough to reason about.

The part people push back on

"But then I need a build step." Yes. You already have one — it's Helm, or Kustomize, or a bash script, and the reason it doesn't feel like a build step is that it has no compiler, no type checker, and no errors until deploy time. Making it explicit isn't adding a step. It's noticing one.