I don't see the difference between sending a Person instance and sending a map of keywords about a person. The coupling is the same.
Sometimes it feels like a real burden for not much gain over just passing around the basic data (a name, an age) I wanted to deal with to start with. Clojure's proposition is that in many many cases, not getting fancy with the data or over-engineering your problem representation will lead to simpler programs that are easier to maintain, giving you an alternative route to safety and maintenance instead of type-checking.
The same way you would create a keyword in Clojure to represent the age of an entity (e.g. ':entity/age') that can be put in a map describing a person or a cat.
In both cases you minimized the interface between your modules and you have less coupling.
More generally, though, it seems like row-types might be a form of static typing that would fit Rich's preferred style of programming.
let printAge object = print object.age
And it'll just check if anything passed to printAge has a field "age".
If your function only needs to know the age, then why would it take a Person or a Cat at all, instead of just accepting an age parameter? But assuming you have a reason, who says you do need to coerce anything or add any dummy data? You don't even have to go very niche to get that functionality, eg in Typescript:
class Person {
age: number
constructor (age: number) { this.age = age }
}
class Cat {
age: number
constructor (age: number) { this.age = age }
}
const printNextAge = (thing: { age: number }) => {
console.log(thing.age + 1)
}
// These all work
printNextAge(new Person(12))
printNextAge(new Cat(23))
const someRandomObject = { age: 10, colour: 'green', weight: 'heavy' }
printNextAge(someRandomObject)
// These don't:
const lady = { name: 'carol' }
printNextAge(lady)
// error TS2345: Argument of type '{ name: string; }' is not assignable to parameter of type '{ age: number; }'.
// Property 'age' is missing in type '{ name: string; }'.
const caveman = { age: 'stone' }
printNextAge(caveman)
// error TS2345: Argument of type '{ age: string; }' is not assignable to parameter of type '{ age: number; }'.
// Types of property 'age' are incompatible.
// Type 'string' is not assignable to type 'number'.
interface Aged {
int getAge();
}