type Face =  | Ace | King | Queen | Jack  | Number of int  type Color =  | Spades | Hearts | Diamonds | Clubs  type Card =  Face * Color  let aceOfHearts = Ace, Hearts let tenOfSpades = (Number 10), Spades  match card with | Ace, Hearts -> printfn "Ace Of Hearts!" | _, Hearts -> printfn "A lovely heart" | (Number n), Spades -> printfn "%d of Spades" n | _, (Diamonds|Clubs) -> printfn "Diamonds or clubs"  // Warning: // Incomplete pattern matches on this expression. // For example, the value '(_,Spades)' may indicate // a case not covered by the pattern(s).

// JS promises made easy promise {  let! res = Fetch.fetch url []  let! txt = res.text()  return txt.Length }  // Declare your own computation expression type OptionBuilder() =  member _.Bind(opt, binder) =  match opt with  | Some value -> binder value  | None -> None  member _.Return(value) =  Some value  let option = OptionBuilder()  option {  let! x = trySomething()  let! y = trySomethingElse()  let! z = andYetTrySomethingElse()  // Code will only hit this point if the three  // operations above return Some  return x + y + z }

[<Measure>] type m [<Measure>] type s  let distance = 12.0<m> let time = 6.0<s>  let thisWillFail = distance + time // ERROR: The unit of measure 'm' does // not match the unit of measure 's'  let thisWorks = distance / time // 2.0<m/s>

[<Literal>] let JSON_URL = "https://jsonplaceholder.typicode.com/todos"  // Type is created automatically from the url type Todos = Fable.JsonProvider.Generator<JSON_URL>  async {  let! (_, res) = Fable.SimpleHttp.Http.get url  let todos = Todos.ParseArray res  for todo in todos do  // Compilation fails if the JSON schema changes  printfn $"USER: {todo.userId}, TITLE {todo.title}, COMPLETED {todo.completed}" }