probsnotpossible.raku

class User {
    has $.id       is potentially_exists;
    has $.username is potentially_exists;
    has $.bio      is potentially_exists;
}

# . . .

sub get_user(id) returns User {
    my fields = &samewith.of.^used_fields; # Or something lol
    User(api_call("/user/$id?fields={$fields.join: ','}"))
}

# . . .

my user = get_user(1234);


# Because the type system sees that the attributes are used,
# it automagically backwards propigates to the return type of the specific call of get_user
# e.g. for this specific user, 
# User(api_call("/user/$id?fields={$fields.join: ','}")) becomes a call to
# /user/1234?fields=username,bio"
say user.username;
say user.bio;