F#: Need help converting C# to F#

Question

I'm trying to write a small little scripting engine for a bullet hell game and I would like to do it in F#. I wrote some C# code to conceptualize it, but I'm having trouble porting it to F#. The C# code is posted below, and I would like some help porting it to F#. I have a feeling the matching F# code will be significantly smaller. I'm open to any sort of creative solutions :)

interface IRunner
{
    Result Run(int data);
}

struct Result
{
    public Result(int data, IRunner next)
    {
        Data = data;
        Next = next;
    }
    public int Data;
    public IRunner Next;
}

class AddOne : IRunner
{
    public Result Run(int data)
    {
        return new Result(data + 1, null);
    }
}

class Idle : IRunner
{
    public Result Run(int data)
    {
        return new Result(data, null);
    }
}

class Pair : IRunner
{
    IRunner _one;
    IRunner _two;

    public Pair(IRunner one, IRunner two)
    {
        _one = one;
        _two = two;
    }

    public Result Run(int data)
    {
        var res = _one.Run(data);
        if (res.Next != null)
            return new Result(res.Data, new Pair(res.Next, _two));
        return new Result(res.Data, _two);
    }
}

class Repeat : IRunner
{
    int _counter;
    IRunner _toRun;

    public Repeat(IRunner toRun, int counter)
    {
        _toRun = toRun;
        _counter = counter;
    }

    public Result Run(int data)
    {
        var res = _toRun.Run(data);
        if (_counter > 1)
        {
            if (res.Next != null)
                return new Result(res.Data,
                            new Pair(res.Next,
                                new Repeat(_toRun, _counter - 1)));
            return new Result(res.Data, new Repeat(_toRun, _counter - 1));
        }
        return res;
    }
}

class Sequence : IRunner
{
    IEnumerator<IRunner> _runner;

    public Sequence(IEnumerator<IRunner> runner)
    {
        _runner = runner;
    }
    public Result Run(int data)
    {
        var res = _runner.Current.Run(data);
        bool next = _runner.MoveNext();
        if (res.Next != null)
        {
            return new Result(res.Data,
                        new Pair(res.Next, new Sequence(_runner)));
        }

        return new Result(res.Data, new Sequence(_runner));
    }
}

Answer 1

Here's something that's almost a direct translation of the same solution strategy.

That said, I think there may be a better/simpler representation choice, I'm still mulling it over.

type Runner = int -> Result
and Result = Result of int * option<Runner>

let AddOne = fun x -> Result(x+1, None)

let Idle = fun x -> Result(x, None)

let rec Pair(r1,r2) = fun x ->
    match r1 x with
    | Result(data,None) -> Result(data, Some(r2))
    | Result(data,Some(next)) -> Result(data,Some(Pair(next,r2)))

let rec Repeat r n = fun x ->
    if n = 0 then r x else
    match r x with
    | Result(data,None) -> Result(data, Some(Repeat r (n-1)))
    | Result(data,Some(next)) -> Result(data, Some(Pair(next, Repeat r (n-1))))

EDIT

Here's another way that's a little more refined... am still trying to see if there's a good way to work in a "list", since the results seem isomorphic to cons cells...

type Runner = Runner of (int -> int * option<Runner>)

let AddOne = Runner(fun x -> x+1, None)

let Idle = Runner(fun x -> x, None)

let rec Pair(Runner(r1),R2) = Runner(fun x ->
    match r1 x with
    | data,None -> data, Some(R2)
    | data,Some(next) -> data, Some(Pair(next,R2)))

let rec Repeat (Runner(r) as R) n = Runner(fun x ->
    if n = 0 then r x else
    match r x with
    | data,None -> data, Some(Repeat R (n-1))
    | data,Some(next) -> data, Some(Pair(next, Repeat R (n-1))))

EDIT

One more version, it uses lists, but now I've a feeling for what's weird here...

type Runner = Runner of (int -> int * list<Runner>)

let AddOne = Runner(fun x -> x+1, [])

let Idle = Runner(fun x -> x, [])

let rec Pair(Runner(r1),R2) = Runner(fun x ->
    match r1 x with
    | data,xs -> data, xs @ [R2]) // inefficient

let rec Repeat (Runner(r) as R) n = Runner(fun x ->
    if n = 0 then r x else
    match r x with
    | data,xs -> data, xs @ List.init (n-1) (fun _ -> R)) // inefficient

It's almost just like an 'Action queue', a list of int->int functions. But each guy can produce some 'suffix actions' that run immediately after him (but before the remaining work in the would-be queue), and trying to maintain the ordering with a purely functional data structure is potentially inefficient (without the right tree/queue library at hand). It would be interesting to know how this will be used/consumed, as perhaps a small change there might allow for a completely different strategy.

Answer 2

Forget the C#, go back to the design documents (or whatever) and re-implement. I mean, literally, forget the C#. The worst thing you can do in F# is to write C#. This is, of course, an instance of a generic rule: The worst thing you can do in language X is to write a program in language Y. Bind X and Y as you wish.

Answer 3

I am assuming that IRunner and Result are predefined, since if not you should redesign the system to be more focused on FP concepts without all this inheritance.

Anyway, here is a binary (I believe) counterpoint to the given example

type AddOne =
    interface IRunner with
        member this.Run(data) = new Result(data+1, null)

type Idle =
    interface IRunner with
        member this.Run(data) = new Result(data, null)

type Pair(one:IRunner, two:IRunner) =
    interface IRunner with
        member this.Run(data) =
            let res = one.Run(data)
            if res.Next <> null then
                new Result(res.Data, new Pair(res.Next, two))
            else
                new Result(res.Data, two)

type Repeat(toRun:IRunner, counter:int) =
    interface IRunner with
        member this.Run(data) =
            let res = toRun.Run(data)
            if counter > 1 then
                if res.Next <> null then
                    new Result(res.Data, new Pair(res.Next, new Repeat(toRun, counter - 1)))
                else
                    new Result(res.Data, new Repeat(toRun, counter - 1))
            else
                res

type Sequence(runner:System.Collections.Generic.IEnumerator<IRunner>) =
    interface IRunner with
        member this.Run(data) =
            let res = runner.Current.Run(data)
            let next = runner.MoveNext()
            if res.Next <> null then
                new Result(res.Data, new Pair(res.Next, new Sequence(runner)))
            else
                new Result(res.Data, new Sequence(runner))

Answer 4

You may consider using the built-in sequence support in F# (along with the very nice sequence expressions). This example is a little contrived but you could think of the program as a sequence of sequences, the "action queue" that Brian was hinting at. By adding the outer sequence it allows the inner sequence to fully control it's lifetime.

// an infinite sequence, repeat the last element of the underlying sequence
// now we can avoid using option types or null/boolean checks in our fold
let lastInfinite (a:seq<'a>) = seq {
    let last = ref Unchecked.defaultof<'a>
    for i in a do
        last := i
        yield i
    let last' = !last
    while true do
        yield last'
}

let addOne = seq [ fun x -> x + 1 ]
let idle = seq [ id<int> ]
let repeat run counter = Seq.truncate counter (lastInfinite run)
let sequence = Seq.concat
let pair one two = lastInfinite (Seq.append one two)

let program = seq [ repeat idle 5;
                    repeat addOne 100;
                    idle; ]

// use Seq.scan if you need lazy access to the intermediate results
// or perhaps Seq.map...
let result = Seq.concat program
             |> Seq.fold (fun state value -> value state) 0

printfn "result: %A" result