Typescript nominal types with classes

Nominal.ts

import * as D from 'io-ts/Decoder'
import * as fp from 'fp-ts/function'

// Nominal types without much ceremony: "class" types
// AKA I want to convert typescript types into fsharp types while using
// typescript

// Not much syntax needed, so they are easy to create. That's what Fable uses to
// construct their own records and DUs.
class UserId {
    constructor(public value: string, private _?: string) {}
}

class SellerId {
    constructor(public value: string, private _?: string) {}
}

const u1 = new UserId('1234')
const s1: UserId = new SellerId('1234') // Error, not assignable because of the private '_' property

namespace A {
    export class UserId {
        constructor(public value: string, private _?: string) {}
    }
}

namespace B {
    export class UserId {
        constructor(public value: string, private _?: string) {}
    }
}

const u2 = new A.UserId('1234')
const u3: A.UserId = new B.UserId('1234') // Two different UserIds in two domains are distinct and not assignable


// Let's say you want to use these types with some validation library.
// Io ts let's you do just that

function createDecoder<NominalClassType extends { val: unknown }>(
    baseType: D.Decoder<unknown, NominalClassType['val']>,
    classType: new (value: NominalClassType['val']) => NominalClassType
) {
    return fp.pipe(
        baseType,
        D.parse((id) => D.success(new classType(id)))
    )
}

const sellerIdDecoder = createDecoder(D.string, SellerId)

const SellerIdDecoder = D.struct({
    seller_id: sellerIdDecoder,
})

const unparsedString1 = `
    { "seller_id": "1234623" }
`

const unparsedString2 = `
    { "seller_d": "1234623" }
`

const unparsedString3 = `
    { "seller_ "1234623" }
`

const decoded = SellerIdDecoder.decode(JSON.parse(unparsedString1))
const decoded2 = SellerIdDecoder.decode(unparsedString2)
const decoded3 = SellerIdDecoder.decode(unparsedString3)

// Then, pattern match your decoded things.
console.log(decoded)
console.log(decoded2)
console.log(decoded3)