Injecting a 'distinct symbol' into a function

Question

Injecting a 'distinct symbol' into a function

I have a couple of inquiries regarding the code provided below:

Can something other than an enum be passed as a Param?
Is there a way to achieve more specific typing during execution of e that specifies the param key?

Ideally, both of these issues can be addressed using a unique symbol type such as:

new (class extends Symbol)('alpha')

enum Params {
    alpha = 'alpha'
}

const defaultParam = <T> (param: Params) => {
    return (a: { [param]: T }): T => { 
        throw new Error('need param')
    }
}

const e = defaultParam<number>(Params.alpha)
e({ beta: 2 })

Below is the error message:

Argument of type '{ beta: number; }' is not assignable to parameter of type '{ [param]: number; }'.
  Object literal may only specify known properties, and 'beta' does not exist in type '{ [param]: number; }'.

And with param: string:

A computed property name in a type literal must refer to an expression whose type is a literal type or a 'unique symbol' type.

typescript

Answer 1

Answer №1

No need for any symbols here!

const initialParameter = <G extends string, T>() => {
    return (input: Record<G, T>): T => {
        throw new Error('parameter required')
    }
}

const y = initialParameter<'beta', number>()

Alternatively, if a parameter is needed as a variable:

const initialParameter = <T>() => {
    return <G extends string>(variableParam: G) => {
        return (input: Record<G, T>): T => {
            throw new Error('Need ${variableParam}')
        }
    }
}

const z = initialParameter<number>()('beta')

Answer 2

No need for any symbols here!

const initialParameter = <G extends string, T>() => {
    return (input: Record<G, T>): T => {
        throw new Error('parameter required')
    }
}

const y = initialParameter<'beta', number>()

Alternatively, if a parameter is needed as a variable:

const initialParameter = <T>() => {
    return <G extends string>(variableParam: G) => {
        return (input: Record<G, T>): T => {
            throw new Error('Need ${variableParam}')
        }
    }
}

const z = initialParameter<number>()('beta')

Injecting a 'distinct symbol' into a function

Answer №1

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