Extract type from or

Hello! Is it possible, using the following schema:

const userType = type({
    intent: "'profile'",
    name: 'string>0',
    surname: 'string>4',
})
.or({
    intent: "'private'",
    password: 'string>0',
})

const userType = type({
    intent: "'profile'",
    name: 'string>0',
    surname: 'string>4',
})
.or({
    intent: "'private'",
    password: 'string>0',
})

to extract one of the types ("find" it by the intent field)? I'd like to use it for multiple forms on a page. It's possible to split it into multiple types, but having it in one type is more readable to me

17 Replies

ssalbdivad•5w ago

Yeah I'd say ideally you'd just want to define it as two types individually and then .or them together (usually best to start by defining any components you need to reuse rather than extract them out). That said, there is an API just like TS for this:

userType.extract({ intent: "'profile'" })

userType.extract({ intent: "'profile'" })

PIat•5w ago

Omg!!!! Perfect, thanks! I won't be reusing it at all, that's why I like to keep it as a single object

PIat•5w ago

Where could I see an example of building such a helper in a type-safe way? (extracting the type by intent and omitting the intent key) Some test maybe?

const formType = type({
    intent: "'profile'",
    name: 'string>0',
    surname: 'string>4',
}).or({
    intent: "'private'",
    password: 'string>0',
})

type ExtractIntents<T> = T extends { intent: infer I } ? I : never

function getFormType<T extends Type, I extends ExtractIntents<inferAmbient<T>>>(
    type: T,
    intent: I,
) {
    return type.extract({ intent: `'${intent}'` }).omit('intent')
}

const formType = type({
    intent: "'profile'",
    name: 'string>0',
    surname: 'string>4',
}).or({
    intent: "'private'",
    password: 'string>0',
})

type ExtractIntents<T> = T extends { intent: infer I } ? I : never

function getFormType<T extends Type, I extends ExtractIntents<inferAmbient<T>>>(
    type: T,
    intent: I,
) {
    return type.extract({ intent: `'${intent}'` }).omit('intent')
}

My abominations end up with never

type step1 = Type<inferAmbient<typeof formType>>

type step2 = Extract<step1, { intent: "'private'"}>

type step1 = Type<inferAmbient<typeof formType>>

type step2 = Extract<step1, { intent: "'private'"}>

PIat•5w ago

type ExtractIntents<T> = T extends { intent: infer I } ? I : never

function getFormType<
    T extends Type<{ intent: string }>,
    I extends ExtractIntents<inferAmbient<T>> & string,
>(type: T, intent: I) {
    return (
        type
            // @ts-ignore
            .extract({ intent: `'${intent}'` })
            // @ts-ignore
            .omit('intent') as unknown as Omit<
                                            Extract<outputOf<T>, { intent: I }>,
                                            'intent'
                                            >
    )
}

const profileType = getFormType(formType, 'profile')

type ExtractIntents<T> = T extends { intent: infer I } ? I : never

function getFormType<
    T extends Type<{ intent: string }>,
    I extends ExtractIntents<inferAmbient<T>> & string,
>(type: T, intent: I) {
    return (
        type
            // @ts-ignore
            .extract({ intent: `'${intent}'` })
            // @ts-ignore
            .omit('intent') as unknown as Omit<
                                            Extract<outputOf<T>, { intent: I }>,
                                            'intent'
                                            >
    )
}

const profileType = getFormType(formType, 'profile')

This works! Sorry for butchering the library 🙈

ssalbdivad•5w ago

It's not your fault, it's not possible for TS to statically compute that this is safe so you have to give up type safety inside an implementation like that to have it externally Here's how I would write it though:

function getFormType<t extends { intent: string }, intent extends t["intent"]>(
    type: Type<t>,
    intent: intent
): Type<Omit<Extract<t, { intent: intent }>, "intent">>
function getFormType(type: Type, intent: string) {
    return type
        .extract({ intent: `'${intent}'` })
        .as<{ intent: string }>()
        .omit("intent") as never
}

const profileType = getFormType(formType, "profile")

function getFormType<t extends { intent: string }, intent extends t["intent"]>(
    type: Type<t>,
    intent: intent
): Type<Omit<Extract<t, { intent: intent }>, "intent">>
function getFormType(type: Type, intent: string) {
    return type
        .extract({ intent: `'${intent}'` })
        .as<{ intent: string }>()
        .omit("intent") as never
}

const profileType = getFormType(formType, "profile")

PIat•5w ago

So much cleaner Is it called function overloading when you declare the function name twice?

ssalbdivad•5w ago

Well like I said I'm very used to having to cast constantly haha so may as well at least minimize the chaos 😛

ssalbdivad•5w ago

Yeah in TS it works a lot like casting. Any of the declarations that are not on the implementation itself act as the external signature, then internally it very loosely asserts that they're correlated but it's basically a cast

PIat•5w ago

I'd have two more questions 🙏 1. What is the .as function and 2. why when you cast as never it still returns the proper value

ssalbdivad•5w ago

Casting to never is just a way to allow an assignment, in this case from the return value to the statically declared type (Type<Omit<Extract<t, { intent: intent }>, "intent">>)

PIat•5w ago

Mindblown, neber used it before. Will need to get used to this syntax

ssalbdivad•5w ago

.as is just a way to cast a type instance Unfortunately it is not supported for arrow functions which are what I normally like to use But it works well if you're already using function

PIat•5w ago

Why do you use arrow functions? 👀 isn't it annoying to be able to use the function only under the declaration?

ssalbdivad•5w ago

Not really, because that only applies if you're actually writing functions you're invoking at the top-level of the module instead of from other functions which is pretty rare. It's mostly just a stylistic preference since they mirror the way functions are declared as types in TS

PIat•5w ago

Makes a lot of sense! Thank you for all these answers 🙏

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Extract type from or