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POModifying RX Computational Expression Builder to hold previous values
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2013-08-06T15:48:13.653
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I'm using a slightly modified version of the RX builder presented here: <a href="http://mnajder.blogspot.com/2011/09/when-reactive-framework-meets-f-30.html" rel="nofollow">http://mnajder.blogspot.com/2011/09/when-reactive-framework-meets-f-30.html</a> Rather than taking <code>IObservable<'T></code> directly my computational expression has a type of: <pre><code>type MyType<'a,'b> = MyType of (IObservable<'a> -> IObservable<'b>) let extract (MyType t) = t </code></pre> Combinators then take on the form: <pre><code>let where (f: 'b -> bool) (m:MyType<_,'b>) = MyType(fun input -> (extract m input).Where(f)) </code></pre> Within the expression itself, I often need to reference back to previous values that have been fed into the stream. In order to do so, I've defined a <code>MyType</code> which maintains a rolling immutable list of the <code>n</code> most recent values. <pre><code>let history n = MyType(fun input -> Observable.Create(fun (o:IObserver<_>) -> let buffer = new History<_>(n) o.OnNext(HistoryReadOnly(buffer)) input.Subscribe(buffer.Push, o.OnError, o.OnCompleted) ) ) </code></pre> With this, I can now do something like: <pre><code>let f = obs { let! history = history 20 // Run some other types, and possibly do something with history } </code></pre> I am finding that I am using this history quite frequently, ideally I would want to have this embedded directly into <code>IObservable<'a></code>. Obviously I can't do that. So my question is, what is a reasonable way to introduce this concept of history that I have here. Should I be extending <code>IObservable<'T></code> (not sure how to do that), wrapping the <code>IObservable<'T></code>? I appreciate any suggestions. Edit: Added full example code. <pre><code>open System open System.Collections.Generic open System.Reactive.Subjects open System.Reactive.Linq // Container function type MyType<'a,'b> = MyType of (IObservable<'a> -> IObservable<'b>) let extract (MyType t) = t // Mini Builder let internal mbind (myTypeB:MyType<'a,'b>) (f:'b -> MyType<'a,'c>) = MyType(fun input -> let obsB = extract myTypeB input let myTypeC= fun resB -> extract (f resB) input obsB.SelectMany(myTypeC) ) type MyTypeBuilder() = member x.Bind (m,f) = mbind m f member x.Combine (a,b) = MyType(fun input -> (extract a input).Concat(extract b input)) member x.Yield (r) = MyType(fun input -> Observable.Return(r)) member x.YieldFrom (m:MyType<_,_>) = m member x.Zero() = MyType(fun input -> Observable.Empty()) member x.Delay(f:unit -> MyType<'a,'b>) = f() let mtypeBuilder = new MyTypeBuilder() // Combinators let simplehistory = MyType(fun input -> Observable.Create(fun (o:IObserver<_>) -> let buffer = new List<_>() o.OnNext(buffer) input.Subscribe(buffer.Add, o.OnError, o.OnCompleted) ) ) let where (f: 'b -> bool) m = MyType(fun input -> (extract m input).Where(f)) let take (n:int) m = MyType(fun input -> (extract m input).Take(n)) let buffer m = MyType(fun input -> (extract m input).Buffer(1)) let stream = MyType(id) // Example let myTypeResult (t:MyType<'a,'b>) (input:'a[]) = (extract t (input.ToObservable().Publish().RefCount())).ToArray().Single() let dat = [|1 .. 20|] let example = mtypeBuilder { let! history = simplehistory let! someEven = stream |> where(fun v -> v % 2 = 0) // Foreach Even let! firstValAfterPrevMatch = stream |> take 1 // Potentially where a buffer operation would run, all values here are after i.e. we cant get values before last match let! odd = stream |> where(fun v -> v % 2 = 1) |> take 2 // Take 2 odds that follow it yield (history.[history.Count - 1], history.[0], someEven,firstValAfterPrevMatch, odd) // Return the last visited item in our stream, the very first item, an even, the first value after the even and an odd } let result = myTypeResult example dat val result : (int * int * int * int * int) [] = [|(5, 1, 2, 3, 5); (7, 1, 2, 3, 7); (7, 1, 4, 5, 7); (9, 1, 4, 5, 9); (9, 1, 6, 7, 9); (11, 1, 6, 7, 11); (11, 1, 8, 9, 11); (13, 1, 8, 9, 13); (13, 1, 10, 11, 13); (15, 1, 10, 11, 15); (15, 1, 12, 13, 15); (17, 1, 12, 13, 17); (17, 1, 14, 15, 17); (19, 1, 14, 15, 19); (19, 1, 16, 17, 19)|] </code></pre>
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Modifying RX Computational Expression Builder to hold previous values
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1. COGiven you added the bounty after some good answers below, can you provide some specific examples where those options don't give you what you need?
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2. CO@DaxFohl Added a fully working example.
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