Document objects, row polymorphism and constraints

[amiralies]

For example a document explaining these would be useful

type userLike<'a> = {.."name": string} as 'a

let extractName: userLike<_> => string = user => user["name"]

let name = extractName({"name": "John", "age": 20})
let name' = extractName({"name": "John", "age": 20, "username": "siever"})
 
let name'' = extractName({"age": 20, "username": "siever"}) // type error shuold have "name" field

type user = {"name": string}
let user = {"name": "John"}

type userWithAge = {"name": string, "age": int}

let userWithAge = {"name": "John", "age": 21}

// Printing name for a type with subtypes
[user, (userWithAge :> user)]
    ->Array.map(user => user["name"])
    ->Array.forEach(Js.log)

[bobzhang]

Note this is not subtyping. It is row polymorphism

[amiralies]

@bobzhang Do you think we should document these features? are we going to preserve these object related features in language or they are likely to be removed in the future?

[bobzhang]

we will keep this

[ryyppy]

This should probably go in an advanced section