'use strict';

// Lens

const view = (lens, obj) => lens.get(obj);

const lens = (source, destination = source) => ({

get: (obj) => obj[source],

set: (val, obj) => ({ ...obj, [destination]: val })

});

const id = (x) => x;

const field = (name, map = id) => (obj) => map(view(lens(name), obj));

// Projection

const projection = (meta) => {

const keys = Object.keys(meta);

return (obj) => {

const hash = {};

keys.forEach((key) => {

const field = meta[key];

hash[key] = field(obj);

});

return hash;

};

};

// Dataset

const persons = [

{ name: 'Marcus Aurelius', city: 'Rome', born: 121 },

{ name: 'Victor Glushkov', city: 'Rostov on Don', born: 1923 },

{ name: 'Ibn Arabi', city: 'Murcia', born: 1165 },

{ name: 'Mao Zedong', city: 'Shaoshan', born: 1893 },

{ name: 'Rene Descartes', city: 'La Haye en Touraine', born: 1596 },

];

// Metadata

const age = (year) =>

new Date().getFullYear() -

new Date(year.toString()).getFullYear();

const upper = (s) => s.toUpperCase();

const md = {

name: field('name'),

place: field('city', upper),

age: field('born', age)

};

// Usage

const p1 = projection(md);

const data = persons.map(p1);

console.dir(data);