Tail type and Semi-path type

tail-semi-path.ts

type Tail<T extends any[]> = ((...args: T) => any) extends ((
    _: infer First,
    ...rest: infer Rest
) => any)
    ? T extends any[] ? Rest : ReadonlyArray<Rest[number]>
    : []


type GrabDeepType<T extends Record<any, any>, PathType> = {
    'deeper': PathType extends [infer HeadType, ...any[]]
    ? HeadType extends undefined
      ? never
      : GrabDeepType<T[PathType[0]], Tail<PathType>>
    : never,
    'end': T
}[
    PathType extends [infer HeadType, ...any[]]
    ? HeadType extends undefined
        ? 'end'
        : 'deeper'
    :'end'
]



const obj = {
    some: {
        deep: {
            value: true as true,
            other: 'hello' as 'hello'
        }
    }
}


// Can't use it for now for ramda's path definition since array type widening TS 3.2
var aa: GrabDeepType<typeof obj, ['some', 'deep', 'value']>;