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)

How 'bout:

const createType = name => {
  const of = x => ({
    [Symbol.for('@@type')]: Symbol.for(name),
    [Symbol.for('@@value')]: x,
    fold: f => f(x),
    map: f => of(f(x)),
    inspect: () => `${name}(${x})`
  })
  
  return {of}
}

Works for me 👍, but maybe we want to take it a step forward and really define the property in the object as a non-enumerable, non-configurable, non-writable property? Completely frozen and unchangeable.

Also maybe freeze it: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/freeze

Sure.

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

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

Will add freeze

👍