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How to cast a System.Object as a list<'a>
f#

In the following snippet my intention is to convert a System.Object

(which could be an FSharpList) to a list of whatever generic type it is

holding.

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    match o with
    | :? list<'a>              -> addChildList(o :?> list<'a>)
    | _                       -> addChild(o)

Unfortunately only list<obj> is ever matched as a list. I would like list<Foo> to also be matched as a list.

.

Also see [link:stackoverflow.com]

By on 1/26/2010 9:38 AM ()

type variables (ex: 'a) can not be used in pattern matching. In fact, the really can be used much at run-time. The compiler is automatically replacing your 'a with type obj, hence your matching issues. One approach might be a match against specific types of lists. For example...

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match o with 
| :? int list -> printfn "its a list of int" 
| :? Foo list -> printfn "its a list of Foo" 
| _ -> printfn "I'm not sure what it is"

Of course, this type of solution may not be sufficiently "general" for the problem you are trying to solve.

By on 1/26/2010 9:10 AM ()

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