Stop writing CLI validation. Parse it right the first time

> Think about it. When you get JSON from an API, you don't just parse it as any and then write a bunch of if-statements. You use something like Zod to parse it directly into the shape you want. Invalid data? The parser rejects it. Done.

Isn’t writing code and using zod the same thing? The difference being who wrote the code.

Of course, you hope zod is robust, tested, supported, extensible, and has docs so you can understand how to express your domain in terms it can help you with. And you hope you don’t have to spend too much time migrating as zod’s api changes.

Yeah, the "parse, don't validate" advice seems vacuous to me because of this. Someone is doing that validation. I think the advice would perhaps be phrased better as "try to not reimplement popular libraries when you could just use them".

Sibling says this with code, but to distil the advice: reflect the result of your validation in the type system.

Then instead of validating a loose type & still using the loose type, you're parsing it from a loose type into a strict type.

The key point is you never need to look at a loose type and think "I don't need to check this is valid, because it was checked before"; the type system tracks that for you.

Everyone seems hung up on the type system, but I think the validity of the data is the important part. I'd still want to convert strings to ints, trim whitespace, drop extraneous props and all of that jazz even if I was using plain JS without types.

I still wouldn't need to check the inputs again because I know it's already been processed, even if the type system can't help me.

Pure js without typescript also has "types". Typescript doesn't give you nominal types either. It's only structural. So when you say that you "know it's already been processed", you just have a mental type of "Parsed" vs "Raw". With a type system, it's like you have a partner dedicated to tracking that. But without that, it doesn't mean you aren't doing any parsing or type tracking of your own.