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PO
 

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    <p>Since no one else is jumping in, I'll add my speculation. First of all, the majority of the <code>IDictionary&lt;'k,'v&gt;</code> interface still makes sense for an immutable dictionary; it's really only adding or removing individual elements that won't work. Secondly, there's a decent amount of code that has already been written which relies on <code>IDictionary</code>, so being able to use F# values with that code is a nice plus.</p> <p>On the other hand, several of <code>ISet&lt;'t&gt;</code>'s methods require mutation, including not just adding individual elements, but also several set mutating operators such as union and intersection. Additionally, many set use cases are already subsumed by the <code>IEnumerable&lt;'t&gt;</code> interface - I think that it will be relatively rare for people to rely on <code>ISet&lt;'t&gt;</code> implementations for immutable set based code.</p> <p>I don't think that genericity has anything to do with it - note that despite the MSDN documentation, F# maps implement the generic <code>IDictionary</code> interface, not the non-generic one.</p>
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    1. POWhy doesn't the F# Set implement ISet<T>?
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    1. COThis sounds plausible. Still, it's frustrating if true. It seems like a mistake to me to include all those interface methods that imply the mutability of the implementing class. (Likewise, I think it would be a mistake to include methods implying the class should be _immutable_.) It would be nice if `ISet<T>` included only "read" members. I think the interface as-is is too opinionated, and now I _still_ don't really have a way to communicate simply that "this is a collection of unique items." (At least so far as F# interop is concerned.)
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      1. USSean Devlin
    2. COLooks like not quite the majority: by my count, only five of the 11 proper members of `ISet<T>` require mutation. If you include inherited members from `ICollection<T>`, it becomes eight of 18. (And of these, at least `void Add(T item)` will probably be implemented explicitly.)
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    3. COI was thinking about writing a similar answer, but then I looked at the `ISet<T>` interface and found out that it has quite a few operations that can still be implemented (namely `SetEquals`, `IsSubsetOf`, `IsSupersetOf`, `Overlaps`, `IsProperSubsetOf`, `IsProperSupersetOf`), maybe it's fewer than `IDictionary`, but still quite a few... Another reasoning may be that there are no .NET methods that would take `ISet<T>`, so there is no application-driven reason for implementing it (I guess there may be some good use case for `IDictionary`).
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      1. USTomas Petricek
 

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