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define container type (e.g. list or array) via a generic argument
f#

I have a type which has a container of items.

In some cases, I want this container to be a list but in some other cases I need to be an array.

I tried to make the container as a generic argument as follows

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type MyType<'container> = {

    Items : 'container<int>

   ...

}

but I got the error message

"Type parameter cannot be used as type constructor "

Is there any other way to achieve a similar result?

.

<int>Is there any other way to achieve a similar result?

Add generic parameter to your type
</int>

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type MyType< 'Container > = 
    {
        Items : 'Container
       ...    }
By on 3/10/2011 8:34 AM ()

but now it's not a enumeration/collection anymore ;)

As a sidenote:

this is why I really want real typeclasses for F#! - Of course you can handle this situation just by sticking to seq/IEnumerable but just imagine the beauty of a collection-type class with map/fold/filter/... and even better imagine you could define all those functions in terms of fold(r) like you can in Haskell ;)

By on 3/10/2011 9:04 AM ()

Hi,

what about using an algebraic type:

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type 'a MyItemType = 
    | MyItemTypeSeq of 'a seq
    | MyItemTypeArr of 'a array
    | MyItemTypeList of 'a list
    with
    member me.Length = 
        match v with
        | MyItemTypeSeq s -> Seq.length s
        | MyItemTypeArr a -> Array.length a
        | MyItemTypeList l -> List.length l
 
    static member map f v = 
        match v with
        | MyItemTypeSeq s -> MyItemTypeSeq (Seq.map f s)
        | MyItemTypeArr a -> MyItemTypeArr (Array.map f a)
        | MyItemTypeList l -> MyItemTypeList (List.map f l)

You can easily implement all the stuff you need (see the map example) and use it just like Seq or List or whatever

If not I guess you can use the fact that every array, list, ... is a Seq to - so you just define your Items as 'a seq

By on 3/10/2011 4:28 AM ()

Hi,

Thanks a lot !

This is very close to what I was looking for.

By on 3/10/2011 7:57 AM ()

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