使用不可变对象的F＃待办事项列表

Question

I'm trying to figure out how to do a to do list in F# using immutable objects. The to do list (not necessarily an F# list) might be pulled from a database or collected from user input or read from XML or JSON, etc. That part is not so important.

Pseudo code:

do for some length of time:
   for each item in the to do list:
      if item is ready to do:
         do item
         if it worked:
            remove from the todo list

   wait a bit before trying again
report on items that weren't ready or that failed.   

The to do list will be some collection of F# records which will have at least an instruction ("Send Email", "Start a process", "Copy a File", "Ask for a raise") along with parameters as a sub-collection.

Can such a thing be done with immutable objects alone? Or must I use a .NET List or some other mutable object?

I don't need fully-fleshed out working code, just some ideas about how I'd put such a thing together.

UPDATE: First attempt at (half-)coding this thing:

let processtodo list waittime deadline = 
    let rec inner list newlist =
        match list with
        | [] when not List.isEmpty newlist ->
                inner newlist []

        | head :: tail when head.isReady->
                let res = head.action
                inner tail ( if res = true then tail else list)

        | head :: tail when not head.isReady ->
                inner tail list

        | _ when deadline not passed ->
            // wait for the waittime
            inner list
        | _ -> report on unfinished list

    inner list []

I tried to write this in the typical fashion seen in many examples. I assumed that the items support the "isReady" and "action" methods. The thing I don't like is its not tail-call recursive, so will consume stack space for each recursion.

Answer 1

Recursion and/or continuations are the typical strategies to transform code with mutable structures in loops to immutable structures. If you know how to write a recursive "List.filter" then you'll probably have some ideas to be on the right track.