// Post -> User -> UserTweet
let userTweet (p : Post) (u : User) =
  // ...
​
// UserId -> Async<Result<User option, Exception>>
let getUserById (userId : UserId) = async {
  // ...
}
​
// PostId -> Async<Result<Post option, Exception>>
let getPostById (postId : PostId) = async {
  // ...
}
​
// Async<Result<Post option, Exception>>
asyncResultOption {
  let! post = getPostById samplePostId
  let! user = getUserById post.UserId
  return userTweet post user
}