('a -> 'b) -> ('c -> 'b) -> Async<Result<'a, 'c>> -> Async<'b>

type HttpResponse<'a, 'b> =
  | OK of 'a
  | InternalError of 'b
​
// CreatePostRequest -> Async<Result<PostId, Exception>>
let createPost (req : CreatePostRequest) = async {
  // ...
}    
​
// Async<HttpResponse>
let handler (httpReq : HttpRequest) = 
  // ... 
​
  // Async<Result<PostId, Exception>>
  let createPostAR = createPost httpReq
​
  createPostAR
  |> AsyncResult.fold Ok InternalError