typesafe_set_v1.ts

console.log('hi')

type Focus<T, T2> = (obj: T) => T2

const setIn = <T, T2>(expr: Focus<T, T2>) => (obj: T) => {
    // convert path to array of strings
    const propAccess = /\.([A-Za-z](\.[A-Za-z])*)/
    const path: string[] = expr.toString()
        .match(propAccess)[1]
        .split('.')

    // set
    let targetObj = {} as any
    // focused depth in object
    let focused = targetObj
    // create skeleton
    path.forEach((prop, depth, path) => {
        if (depth < path.length - 1) {
            focused[prop] = {}
            focused = focused[prop]
            console.log(prop, 'focused', JSON.stringify(focused, null, 2))
        } else {
            expr(targetObj)
        }
    })

    // merge source and target
    // return Object.assign(targetObj, obj)
    return Object.assign({}, obj, targetObj)
}

const A = { a: 'b' }

console.log("{ a: 'yo' }", setIn<{ a: string }, string>(obj => obj.a = 'yo')(A))
console.log('A', A)
console.log("{ a: 'x', b: { c: 'yo' }}", setIn<{ a: string, b: { c: string } }, string>(obj => obj.b.c = 'yo')({ a: 'x', b: { c: 'a' } }))

console.log("{ a: 'c'}", setIn<{ a: string }, string>(obj => obj.a = 'c')({ a: 'b' }))