arktype•2w ago

Trying to index an array type with .get(...path)

const schema = type('string[]');
console.log(schema.get(0).expression);

const schema = type('string[]');
console.log(schema.get(0).expression);

The code below prints "string | undefined" which at first kind of makes sense but after thinking about it, I feel like it should just be "string". While regular array indexes can return undefined, types are never actually turned into real values so an index can never actually fail. It also doesn't match TypeScript types where indexing an array type like string[][0] equals string. My use case is that I'm building an automatic form generator where I am using .get(...path) to get a specific part of the schema in a deeply nested object schema. I want to be able to effectively get the array's type but to my knowledge it's impossible to do so because any array index always adds | undefined so I cannot differentiate between string[] and (string | undefined)[] for example.

24 Replies

ssalbdivad•2w ago

Well first of all, it's fun to see other people thinking about these kinds of questions in detail because they're really interesting! There are some cases where ArkType specifically diverges from TypeScript to be safer or more precise and this is one of them TypeScript's handling of index access is quite inconsistent. There is a noUncheckedIndexAccess setting that adds | undefined to your value when you access it in JS, but has no effect on how string[][number] is evaluated. The result of a property access is just the answer to the question "what possible value could I have here"? If I have the type { foo?: number }, it wouldn't make sense to say that o.foo couldn't include undefined. What you're asking for is more like extracting part of the structure of the type itself. Luckily I just released an API allowing these kinds of structural queries on the internal type system from @ark/schema! It may require some digging through the structure of the type system to figure out how to get exactly what you want, but it's the coolest part of the project anyways:

type("string[]").select({
    kind: "sequence",
    method: "assertFind",
    boundary: "shallow"
}).variadic?.expression //? string

type("string[]").select({
    kind: "sequence",
    method: "assertFind",
    boundary: "shallow"
}).variadic?.expression //? string

TizzySaurus•2w ago

Out of interest, what's the output with string[] > 1? Because technically there it is guaranteed to be string. Although I can see it not being worth the cost to implement the logic

ssalbdivad•2w ago

Hahah, yeah I literally tested that exact case as I was writing the example it includes undefined still but you're right it shouldn't there. My downfall was probably making sure I reduced [string, ...string[]] and [...string[], string] to the same type (string[] > 1) but then you lose the tuple structure so I'd have to add some logic to put it back It's actually probably pretty easy if you want to give it a shot there's tons of properties on a sequence node like minRequiredLength

TizzySaurus•2w ago

I actually might take a look this weekend if it's easy enough.

ssalbdivad•2w ago

Definitely way easier than the last stuff you were doing 😅 I will get to that PR tonight btw sorry I've been a bit overwhelmed trying to juggle consulting work

TizzySaurus•2w ago

Sounds good :)

JoshOP•2w ago

Thinking about it some more I guess the behaviour isn't black and white. I thought it was nice that you could pass through path as if it were using optional chaining:

const schema = type({
  people: type({
    'names?': type({
      first: 'string | undefined',
    }).array(),
  }).array(),
});
// Prints string | undefined
console.log(schema.get('people', 0, 'names', 0, 'first').expression);

const schema = type({
  people: type({
    'names?': type({
      first: 'string | undefined',
    }).array(),
  }).array(),
});
// Prints string | undefined
console.log(schema.get('people', 0, 'names', 0, 'first').expression);

But then you don't know where the | undefined comes from. Also TS errors when .get has more than 3 arguments, lol I guess I'm using the wrong tool for the job I'm also using the select function, it seemed perfect for this and its the reason I chose arktype :) Although I didn't know it was powerful enough to traverse the whole structure which is why I was using it in conjunction with .get(). I'll take a look through the source!

ssalbdivad•2w ago

Yeah I specifically only built 3 overloads because there should almost never be a good reason to extract types that deeply. What are you trying to build? Yeah it's a lossy transformation, the only goal is to answer the question "what values could be allowed at this path?"

JoshOP•2w ago

My plan was to make a vue library, kind of similar to vee-validate, for a personal project of mine. API would be something like this:

<template>
  <Form :schema>
    <FormControl :schema :path="['color', 'r']" />
    <FormControl :schema :path="['color', 'g']" />
    <FormControl :schema :path="['color', 'b']" />
  </Form>
</template>

<script setup lang="ts">
const { type } from 'arktype';

const schema = type({
  color: {
    r: '0 <= number.integer <= 255',
    g: '0 <= number.integer <= 255',
    b: '0 <= number.integer <= 255',
  }
})
</script>

<template>
  <Form :schema>
    <FormControl :schema :path="['color', 'r']" />
    <FormControl :schema :path="['color', 'g']" />
    <FormControl :schema :path="['color', 'b']" />
  </Form>
</template>

<script setup lang="ts">
const { type } from 'arktype';

const schema = type({
  color: {
    r: '0 <= number.integer <= 255',
    g: '0 <= number.integer <= 255',
    b: '0 <= number.integer <= 255',
  }
})
</script>

Each <FormControl> would render label, input and description HTML elements that take from info from arktype schema. Since the path leads to a number type, it would automatically render as a slider with the min, max and default values taken from the arktype validation. My project is basically an editor for a decently large and nested JSON object so I thought this would be a nice way to avoid specifying things like min and max values multiple times throughout the project.

ssalbdivad•2w ago

Ahh okay I see. Yeah .get is definitely not the right tool for traversing an object Another potentially less intimidating approach you can use for non-arrays is .props The types will be very helpful and you don't have to learn the internal schema representation (although to do what I mentioned with an array you would )

JoshOP•2w ago

I was thinking .get was nice for it's type safe object path Parameters<typeof schema.get> 🤔 not sure that's a good idea though Hmm props does seem like another good option. I was also thinking about just converting it to a JSON schema and doing it that way but I guess props keeps more arktype specific information?

ssalbdivad•2w ago

Do you have the JSON structure ahead of time? I thougth you meant it was dynamic and you had to generate it In which case type safety won't help ArkType's JSON representation is richer than JSON schema you can look at that if you want using .json but that prop is not typed. You are probably underestimating the complexity of traversing a structure that can include unions, morphs, intersections, primitives, constraints, named props, index signatures, sequences etc. If you know your type will never include e.g. an index signature you can avoid certain cases but to be type safe obviously we have to include all that

JoshOP•2w ago

The schema will always be static and known at compile time but I hope to make the UI components generic over any schema and hopefully as type-safe as possible

ssalbdivad•2w ago

What about myType.internal.flatRefs does that help?

JoshOP•2w ago

Interesting, it might be easier to iterate through these regexes to get schema properties rather than do it myself? There are so many ways I could do this now that I'll have to think about it haha

ssalbdivad•2w ago

I mean if it's a JSON structure can you just assume undefined will never be in it and only allow null haha?

JoshOP•2w ago

I was hoping not to use anything internal if I could help it for future proofing concerns but also I couldn't really figure out just by looking at the docs how to do this most efficiently Yes, only nulls

ssalbdivad•2w ago

Well your original concern was (string | undefined)[] but that couldn't happen so why not just use get and exclude undefined?

JoshOP•2w ago

You make a very good point :) I do control the schema so I could just remove any undefined I thought maybe at some point I would have a reason to have optional properties

ssalbdivad•2w ago

You can have optional properties but know that undefined will never actually be a possible value of a present key Like JSON

JoshOP•2w ago

Oh yes that would be the case so (string | undefined)[] would actually be impossible Well my initial example wasn't actually useful for my exact case but this conversation has given me a lot of ideas so it was worth asking 😂

ssalbdivad•2w ago

You're right there are tons of ways you could do it because the internal type nodes are very introspectable. Maybe the best approach would be to decide the API you want first and exactly how you want the types to behave, then figure out how to get the values to do that

JoshOP•2w ago

Thanks for the help!

ssalbdivad•2w ago

Excited to see what you cook up!

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Trying to index an array type with .get(...path)

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