Extensible record in return types

Hi, newbie question here, I was wondering why this doesn’t typecheck. Is there a way to define a function which returns records with a guarantee on its fields? Thank you.
Screen Shot 2020-03-10 at 6.33.04

The annotation for fn is saying that it can return any record provided it has a name and age field of the correct type. However, this is not the case. The function will always return a particular record, not any record. You can make it type check if you specify that:

fn : () -> PersonLike {}
fn _ =
     Person "John" 23
1 Like

Thank you very much. As a follow up, is there a way to
type a function like this?

type alias Person =
    { name : String
    , age : Int
    }


type alias Alien =
    { name : String
    , age : Int
    , planet : String
    }


type alias PersonLike a =
    { a
        | name : String
        , age : Int
    }


fn : () -> PersonLike ? 
fn _ =
    if 1 == 1 then
        Person "John" 23

    else
        Alien "John" 230 "Mars"
1 Like

No, The return value of a function need to be a concrete type. I think perhaps that you want to use a custom type here instead.

If it was like you described, how would the caller know to access the planet field? How would the caller know what kind of thing was returned?

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