Question

I have these types:

  type ShouldRetry = ShouldRetry of (RetryCount * LastException -> bool * RetryDelay)
  and RetryCount = int
  and LastException = exn
  and RetryDelay = TimeSpan

  type RetryPolicy = RetryPolicy of ShouldRetry

Now I want composability of the retries; something like this:

let serverOverloaded = [| exnRetry<TimeoutException>;
                          exnRetry<ServerBusyException> |]
                       |> Array.map (fun fn -> fn (TimeSpan.FromSeconds(4.0)))

let badNetwork = [||] // etc

let compose p1 p2 =
  // http://fssnip.net/7h
  RetryPolicy(ShouldRetry( (fun (c,e) ->
    let RetryPolicy(ShouldRetry(fn))  = p1
    let RetryPolicy(ShouldRetry(fn')) = p2
    let (cont, delay) = fn c,e
    if cont then cont, delay 
    else
      let (cont', delay') = fn' c,e
      cont', delay') ))

let finalPolicy = serverOverloaded |> Array.scan compose (RetryPolicies.NoRetry())

But I’m getting compiler errors on fn, delay and fn', saying “The value or constructor ‘fn’ is not defined”.

Answer 1

I can see two problems in your compose function.

When decomposing p1 and p2, the pattern needs to be wrapped in parentheses (otherwise, the compiler interprets the code as a definition of RetryPolicy function, instead of pattern matching):

let (RetryPolicy(ShouldRetry(fn)))  = p1 
let (RetryPolicy(ShouldRetry(fn'))) = p2 

When calling fn' a bit later, you need to pass it the arguments in a tuple (otherwise, the compiler thinks that you’re calling fn' with just a single argument c and then building a tuple):

let (cont', delay') = fn' (c,e) 

I didn’t check (or tried to run) the whole example, so I don’t know if the rest of the code does what you want.