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    <p>This is a very interesting problem :-). Sadly, I don't think there is any F# library that allows you to solve that easily. As far as I can tell, <a href="http://bindablelinq.codeplex.com/" rel="nofollow">Bindable LINQ</a> was one attempt to implement LINQ query pattern (i.e. <code>Select</code>, <code>SelectMany</code> and <code>Where</code> methods) for <code>ObservableCollection&lt;T&gt;</code>, which is essentially what you need. To use it from F#, you could wrap LINQ operations in functions like <code>map</code> etc.</p> <p>As you say, functions like <code>Seq.map</code> work by on <code>IEnumerable</code> and so they are not incremental. What you'd need here are functions like <code>map</code>, <code>filter</code> and <code>collect</code>, but implemented over <code>ObservableCollection&lt;'T&gt;</code> in such a way that they do the changes incrementally - when new element is added to the source collection, the <code>map</code> function would process it and add a new element to the resulting collection. </p> <p>I experimented with that a bit and here is an implementation of <code>map</code> that is incremental:</p> <pre class="lang-ml prettyprint-override"><code>open System.Collections.Specialized open System.Collections.ObjectModel module ObservableCollection = /// Initialize observable collection let init n f = ObservableCollection&lt;_&gt;(List.init n f) /// Incremental map for observable collections let map f (oc:ObservableCollection&lt;'T&gt;) = // Create a resulting collection based on current elements let res = ObservableCollection&lt;_&gt;(Seq.map f oc) // Watch for changes in the source collection oc.CollectionChanged.Add(fun change -&gt; printfn "%A" change.Action match change.Action with | NotifyCollectionChangedAction.Add -&gt; // Apply 'f' to all new elements and add them to the result change.NewItems |&gt; Seq.cast&lt;'T&gt; |&gt; Seq.iteri (fun index item -&gt; res.Insert(change.NewStartingIndex + index, f item)) | NotifyCollectionChangedAction.Move -&gt; // Move element in the resulting collection res.Move(change.OldStartingIndex, change.NewStartingIndex) | NotifyCollectionChangedAction.Remove -&gt; // Remove element in the result res.RemoveAt(change.OldStartingIndex) | NotifyCollectionChangedAction.Replace -&gt; // Replace element with a new one (processed using 'f') change.NewItems |&gt; Seq.cast&lt;'T&gt; |&gt; Seq.iteri (fun index item -&gt; res.[change.NewStartingIndex + index] &lt;- f item) | NotifyCollectionChangedAction.Reset -&gt; // Clear everything res.Clear() | _ -&gt; failwith "Unexpected action!" ) res </code></pre> <p>Implementing <code>map</code> was quite easy, but I'm afraid that functions like <code>collect</code> and <code>groupBy</code> will be quite tricky. Anyway, here is a sample showing how you can use it:</p> <pre class="lang-ml prettyprint-override"><code>let src = ObservableCollection.init 5 (fun n -&gt; n) let res = ObservableCollection.map (fun x -&gt; printfn "processing %d" x; x * 10) src src.Move(0, 1) src.Remove(0) src.Clear() src.Add(5) src.Insert(0, 3) src.[0] &lt;- 1 // Compare the original and the result printfn "%A" (Seq.zip src res |&gt; List.ofSeq) </code></pre>
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    1. POHow to implement a non-trivial map between collections in Reactive Extensions?
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    1. USTomas Petricek
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    1. POHow to implement a non-trivial map between collections in Reactive Extensions?
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    1. COThank you Tomas! Now I am developing it in a small library. For example: map : ('T -> 'A) -> RxSeq<'T> -> RxSeq<'A> where type RxSeq<'T> = IObservableWithState<RxSeqUpdate<'T>, ObservableCollection<'T>>
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    2. CO@Alfa07 Nice! Are you going to share the library somewhere? If so, perhaps I could contribute a few more functions.
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    3. COYes, sure! It would be great! As soon as I get in reasonable shape I will share it and send the link to you.
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