Composable Nominal Types for Typescript

composableNominalTypes.ts

export type ChainNominal<BaseType, Tag extends string> = BaseType extends {
  __type: infer Tag0
  __baseType: infer BaseType0
}
  ? Tag0 extends [...infer NestedTags]
    ? BaseType0 & { __type: [Tag, ...NestedTags]; __baseType: BaseType0 }
    : never
  : BaseType & {
      __type: [Tag] // Using an array allow us to use multiple type tags :3
      __baseType: BaseType // Only here to ease type inference
    }

export type FlatNominal<BaseType, Tag extends string> = BaseType extends {
  __type: infer Tag0
  __baseType: infer BaseType0
}
  ? BaseType0 & {
      __type: Tag0 & { [key in Tag]: Tag }
      __baseType: BaseType0
    }
  : BaseType & {
      __type: { [key in Tag]: Tag }
      __baseType: BaseType // Only here to ease type inference
    }

// Similar to FlatNominal, but it's able to deal with "inheritance chains"
export type AddTag<BaseType, Tag extends string> = BaseType extends {
  __baseType: infer BaseType0
  __type: infer Tag0
}
  ? BaseType0 &
      (Tag0 extends string // To account for traditional nominal types
        ? {
            __baseType: BaseType0
            __type: { [key in Tag]: Tag } & { [key in Tag0]: Tag0 }
          }
        : Tag0 extends [infer Tag1, ...infer DeeperTags]
        ? Tag1 extends string
          ? {
              __baseType: BaseType0
              __type: { [key in Tag1]: [Tag1, ...DeeperTags] } &
                { [key in Tag]: Tag }
            }
          : never // should not happen
        : Tag0 extends { [key in string]: unknown }
        ? {
            __baseType: BaseType0
            __type: Tag0 & { [key in Tag]: Tag }
          }
        : never)
  : BaseType & {
      __baseType: BaseType
      __type: { [key in Tag]: Tag }
    }

// Similar to ChainNominal, but able to deal with more edge cases
export type InheritFrom<BaseType, Tag extends string> = BaseType extends {
  __baseType: infer BaseType0
  __type: infer Tag0
}
  ? Tag0 extends [...infer NestedTags]
    ? BaseType0 & {
        __baseType: BaseType0
        __type: [Tag, ...NestedTags]
      }
    : Tag0 extends { [key in string]: unknown }
    ? BaseType0 & {
        __baseType: BaseType0
        __type: [Tag, Tag0]
      }
    : never
  : BaseType & {
      __baseType: BaseType
      __type: [Tag]
    }