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Casting a F# list into Generic.IList
f#

Hi,

My apologies, but this is a newbie question. I have a function which requires a list of type System.Collections.Generic.IList<Subscription>. I create a list using [] of Subscription objects, but it doesn't seem to be able to cast it to the IList type. I get the following error:

The type ''a list' is not compatible with the type 'System.Collections.Generic.IList<Subscription>'

Any advice would be much much appreciated. Thanks.

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Empty list is polymorphic, but an explicit type annotation should do the trick:

[] : int list :> IList<int >

By on 6/23/2009 11:24 AM ()Reply
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My understanding is [] in F# is quite 'internal' to F#.

This should do it

let x = (Array.of_list [1..1]) :> System.Collections.Generic.IList<int>

By on 6/22/2009 7:07 PM ()Reply
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Thanks, Gary. This did the trick. (I appended "< int >" to the end and it worked like a charm.)

By on 6/23/2009 9:39 AM ()Reply
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There are several differences between an F# list and a typical IList. For one, they have different performance properties: an F# list is actually a linked list, meaning that e.g. accessing the i'th element of the list takes O(n) time. An IList is typically implemented using an underlying array, which means that access is O(1). On the other hand appending to the front of an F# list is O(1).Maybe more importantly, F# lists are immutable: e.g. you can use them in multithreaded code without locking.So anyway, since these differences are important, it makes sense to make conversions explicit.

By on 6/23/2009 3:01 AM ()Reply
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A follow-up question: Is there a way to define a function/operator, that would work like [/[| but would return ILists (of the appropriate type)? It would make my code look much cleaner. Thanks.

By on 6/23/2009 2:21 PM ()Reply
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No, there's not.

I suggest you to declare a function doing this conversion:

let ilist (x: _ array) = x :> System.Collections.Generic.IList<_>

let a = ilist [|1..10|]

It looks clean to me. If that's still too verbose, you might want to have an infix operator for this:

let (!!) = ilist

let b = !![|1; 2; 3|]

Laurent.

By on 6/23/2009 2:38 PM ()Reply
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Thanks, Laurent. I ended up using your ilist suggestion, but had to append "<_>" at the end to get it to work.

By on 6/24/2009 3:12 PM ()Reply

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