createType.js

const createType = name => {
  const of = x => Object.freeze({
    '@@type': name,
    fold: f => f(x),
    map: f => of(f(x)),
    inspect: () => `${name}(${x})`
  })
  
  return {of}
}

const Left = createType('Left')

const x = {a: 1}

Left.of(x).map(({a}) => a + 1)

Well apparently that's already the case just because they are Symbols 👍

https://www.keithcirkel.co.uk/metaprogramming-in-es6-symbols/

Will add freeze

👍