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Option extension method
f#

I've found it useful to use the folllowing when converting from nullable obj's to option types:
(If there's an easier baked-in way, let me know...)

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let Optional<'T> (x:obj) : 'T option = 
   if x = null then None else Some (x :?> 'T)

so:
null |> Optional<int> --> None
box 3|> Optional<int> --> Some 3

It seems to me that this could be formulated as Option.Cast<T>, but I can't seem to get it right... - here's my attempt:

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type Microsoft.FSharp.Core.option<'T> with 
    static member Cast<'T> (x:obj) : 'T option =
        if x = null then None else Some (x :?> 'T)

thanks

.

Just change the type name to Option with a capital O. type abreviations can not have augmentations.

also note that the type parameter goes onto the type name NOT the method:

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let r = box 4.0 |> option<float>.Cast

//NOT

let r = box 4.0 |> option.Cast<float> // this causes a type application error
By on 4/3/2010 9:27 PM ()

Cool beans - thanks. The extension method compiles and runs as expected:

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type Microsoft.FSharp.Core.Option<'T> with 
    static member Cast<'T> (x:obj) : 'T option =
        if x = null then None else Some (x :?> 'T)

let inline Optional<'T> (x:obj) : 'T option = 
    if x = null then None else Some (x :?> 'T)

box 3 |> Option<int>.Cast
null |> Option<int>.Cast
or
box 3 |> Optional<int>
null |> Optional<int>

The former seems to jibe with Seq.Cast and seems like a natural to include to me - tho' the latter is less verbose and inlined.

By on 4/3/2010 10:10 PM ()

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