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Strongly typed heterogeneous collections
f#collections

What would be the best way to accomplish something like a dictionary where the keys are types, and the values are containers of collections of those types?

f.ex. Dictionary<T,List<List<T>>> ... but or course this can only be instantiated with a single type T...I would to have the equivalent of a type constraint on T, but allow T to be variable so that each Key is a different type, and each value is a List<List<T>> where T is the same type as the Key.

My use case is a container for different kinds of time series. I would like to retain type information to avoid downstream casting by storing all the series in a container-per-type that is stored as the value and keyed off the type.

Can I use discriminated unions? or should it just be Dictionary<Type,List<List<Type>>>?

.

I think what you are looking for are type-indexed data structures. For the most part they are fortuitously supported by the CLR if you only need one instance of the dictionary. (Sorry for the C# syntax.)

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static class MyDictionary<T>
{
  public static List<List<T>> Value;
}

// add to dictionary
MyDictionary<int>.Value = new List<List<int>>();

// access dictionary
if(MyDictionary<string>.Value != null) { ... }
By on 1/5/2009 10:17 PM ()

For example you cant store an int mapping over to a List<List<int>>, and also a string over to a List<List<string>> in the same dictionary. Static typing does not allow this without casting to obj or somelike that.
If you use a discriminated union for the type it will minimize your types options but it win't force the key to be of same type as the container.

By on 1/5/2009 1:26 PM ()

Strongly typed heterogeneous collections seem to be possible.

I'm a novice in this area, but I think we would need higher-kinded types and abstractions like type classes.

Here is some chatter on the topic for C# and scala

For now I just want to find my way to a solution that is the least bad. :-)

By on 1/5/2009 1:50 PM ()

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