Propagating optional arguments

问题:

The following code does not compile.

type A(?arg) =
  member __.Arg : string option = arg

type B(?arg) =
  inherit A(arg) //ERROR expected type string but has type 'a option

I assume this is because an instance of the underlying type of the option must be provided, and the compiler handles passing Some/None based on syntax.

Assuming my assumption has been correctly assumed, is there a workaround for this? Is it possible to propagate optional arguments?

回答1:

F# spec 8.13.5 Optional arguments to method members

Callers may specify values for optional arguments by using the following techniques:

• By name, such as arg2 = 1.
• By propagating an existing optional value by name, such as ?arg2=None or ?arg2=Some(3) or ?arg2=arg2. This can be useful when building one method that passes optional arguments on to another.

• By using normal, unnamed arguments matched by position.

  type A(?arg) =
      member __.Arg : string option = arg
  
  type B(?arg) =
      inherit A(?arg = arg) 
  
  printfn "1. %A" (B()).Arg // None
  printfn "2. %A" (B("1")).Arg // Some "1"
  
  printfn "3. %A" (A()).Arg // None
  printfn "4. %A" (A("1")).Arg // Some "1"
  

回答2:

Sorry had to test it first: it seems you are right - you have to do the "?" for A yourself:

type A(arg : string option) =
  new (a) = new A(Some a)
  new () = new A(None)
  member __.Arg : string option = arg

type B(?arg) =
  inherit A(arg)