How can I refactor out the required else clause?

Question

I have a C# method that looks a bit like this:

bool Eval() {
  // do some work
  if (conditionA) {
     // do some work
     if (conditionB) {
       // do s         


        

Answer 1

Using the higher-order functions in the Option module can make this flow very cleanly without any mutable state:

let Eval () =
    // do some work
    if not conditionA then None else
        // do some work
        Some state
    |> Option.bind (fun state ->
        if not conditionB then None else
            // do some work
            Some state')
    |> Option.bind (fun state ->
        if not conditionC then None else
            // do some work
            Some true)
    |> defaultArg <| false

Or for further clarity, using named functions rather than lambdas:

let Eval () =
    let a () =
        if not conditionA then None else
            // do some work
            Some state
    let b state =
        if not conditionB then None else
            // do some work
            Some state'
    let c state =
        if not conditionC then None else
            // do some work
            Some true
    // do some work
    a () |> Option.bind b |> Option.bind c |> defaultArg <| false