A "Choice" (Either in Haskell, Result in Rust) wouldn't work for fork() as it can have 3 results, and you'd want the `Child` case cleanly and easily separated from `Pid`.
I think the parent meant only a sum type, not a concrete example of it such as Either of Haskell, which would surely not suffice here. In Haskell you would probably define a new sum type for this occasion, e.g.:
data ForkResult = Failure | Parent Int | Child
match fork() -> | Error(errno) -> ... | Pid(pid) -> ...
Or a general "Choice" sum, perhaps using phantom types so int<err> isn't compatible with int<pid>. But then all of a sudden, instead of a single word being returned, a tag and possibly variably-sized result has to be returned, and that's quite a hassle which doesn't fit well with C.