A
arktype•2mo ago
L0xus

Narrow type based on another property's value

Hey, I've been using ArkType for a few days now - loving it so far. One thing I'd love to be able to do is narrow a type based on the value of another property in the object. eg. 
const myType = type("'string' | 'Color'")

const myValue = type({
  type: myType,
  value: type("'string' | 'string.hex'")
})
const myType = type("'string' | 'Color'")

const myValue = type({
  type: myType,
  value: type("'string' | 'string.hex'")
})
given this code, I would want the type of 'value' to change based on the 'type' property. if 'type ' is 'string', then I'd want value to be narrowed to 'string', but if 'type' is 'Color', I'd want it to be narrowed to 'string.hex'. Is there currently a way to do this in the ArkType system? Many thanks in advance, Chris
4 Replies
TizzySaurus
TizzySaurus•2mo ago
You may be able to do something with generics, but I'm not sure ArkType's generic definitions support conditionals like this... you could try
const myValue = type("<t extends 'string' | 'Color', v extends t extends 'Color' ? 'string.hex' : 'string'>", {
    type: "t",
    value: "v"
})
const myValue = type("<t extends 'string' | 'Color', v extends t extends 'Color' ? 'string.hex' : 'string'>", {
    type: "t",
    value: "v"
})
Otherwise you can do a top-level narrow. Something like
const stringHex = type("string.hex");

const myValue = type({
    type: [stringHex, "|", "string"],
    value: "string | string.hex"
}).narrow((data) => {
    if (data.type === "Color") {
        if (!stringHex.allows(data.value)) {
            return ctx.mustBe({key: "value", expected: "a valid hex since 'type' was 'Color'"});
        }
     }
     return true
})
const stringHex = type("string.hex");

const myValue = type({
    type: [stringHex, "|", "string"],
    value: "string | string.hex"
}).narrow((data) => {
    if (data.type === "Color") {
        if (!stringHex.allows(data.value)) {
            return ctx.mustBe({key: "value", expected: "a valid hex since 'type' was 'Color'"});
        }
     }
     return true
})
Potentially with an else on the data.type === "Color", if you want to disallow hex when type isn't "Color" Not that the types probably won't be narrowed though Maybe you can fix that with .narrow(...).as<NarrowedType>() if it matters to you
🌺🇫🇷 Shigu :3 🌺
🌺🇫🇷 Shigu :3 🌺•2mo ago
what you are describing sounds more suited for a discriminated union and wouldn't need all this code
L0xus
L0xusOP•2mo ago
thank you so much for the quick response. the generic approach didn't seem to support he conditionals, but the top level narrow did the trick!
Dimava
Dimava•2mo ago
type({
  type: "'string'",
  value: '"string"
}).or({
  type: "'Color'",
  value: "string.hex"
})
type({
  type: "'string'",
  value: '"string"
}).or({
  type: "'Color'",
  value: "string.hex"
})
Generic is also possible you just need the hkt one

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