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POHow to design an api to a persistent collection in C#?
 

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  1. POHow to design an api to a persistent collection in C#?
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    <p>I am thinking about creating a persistent collection (lists or other) in C#, but I can't figure out a good API. </p> <p>I use '<a href="http://en.wikipedia.org/wiki/Persistent_data_structure" rel="nofollow">persistent</a>' in the <a href="http://clojure.org/data_structures#Data%20Structures-Collections" rel="nofollow">Clojure sense</a>: a persistent list is a list that behaves as if it has value semantics instead of reference semantics, but does not incur the overhead of copying large value types. Persistent collections use copy-on-write to share internal structure. Pseudocode: </p> <pre><code>l1 = PersistentList() l1.add("foo") l1.add("bar") l2 = l1 l1.add("baz") print(l1) # ==&gt; ["foo", "bar", "baz"] print(l2) # ==&gt; ["foo", "bar"] # l1 and l2 share a common structure of ["foo", "bar"] to save memory </code></pre> <p>Clojure uses such datastructures, but additionally in Clojure all data structures are immutable. There is some overhead in doing all the copy-on-write stuff so Clojure provides a workaround in the form of <a href="http://clojure.org/transients" rel="nofollow">transient</a> datastructures that you can use if you are sure you're not sharing the datastructure with anyone else. If you have the only reference to a datastructure, why not mutate it directly instead of going through all the copy-on-write overhead.</p> <p>One way to get this efficiency gain would be to keep a reference count on your datastructure (though I don't think Clojure works that way). If the refcount is 1, you're holding the only reference so do the updates destructively. If the refcount is higher, someone else is also holding a reference to it that's supposed to behave like a value type, so do copy-on-write to not disturb the other referrers. </p> <p>In the API to such a datastructure, one could expose the refcounting, which makes the API seriously less usable, or one could not do the refcounting, leading to unnecessary copy-on-write overhead if every operation is COW'ed, or the API loses it's value type behaviour and the user has to manage when to do COW manually. </p> <p>If C# had copy constructors for structs, this would be possible. One could define a struct containing a reference to the real datastructure, and do all the incref()/decref() calls in the copy constructor and destructor of the struct. </p> <p>Is there a way to do something like reference counting or struct copy constructors automatically in C#, without bothering the API users?</p> <p><strong>Edit:</strong> </p> <ul> <li>Just to be clear, I'm just asking about the API. Clojure already has an implementation of this written in Java. </li> <li>It is certainly possible to make such an interface by using a struct with a reference to the real collection that is COW'ed on every operation. The use of refcounting would be an optimisation to avoid unnecessary COWing, but apparently isn't possible with a sane API. </li> </ul>
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      1. POHow to design an api to a persistent collection in C#?
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    1. COWhat methods do you need to support? I have an efficient idea for a collection which only supports 'add' and accessing (e.g. iterating) which drops down to O(N) for remove and replace operations.
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      1. POHow to design an api to a persistent collection in C#?
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    2. COOr are you asking about just the API and not implementations?
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    3. COI hope otherwise, by I imagine you will be flummoxed trying to change such a basic assumption of how the the Common Language Runtime languages are designed (reference counted in the VM, inaccessible to the language-level user). What languages even let you change something so deep in the language, besides Lisp and C++? (or making your own language/runtime/CLR VM).
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