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How to determine if a value is an option
f#

Hi,
How do I determine if a value is an option and at such to extract the value that it contains

My function looks like this

member x.Encode (data:obj) =
match data with
| :? null -> do x
| :? ValueType -> do y
| :? string -> do s
| etc etc etc
...How do I check if it is an option?

I tried
match data with
| Some(t) -> x.Analyze t
| None -> x.Encode null
| null -> do m
| :? string -> do t
but I had errors

I am examining the properties of the passed in object and then encoding it using the properties values. In the case of an option, the properties are Item and Value with end up being encoded into wrong values as it should have been either the item or null.

Thank you.

.

You first need to recognise that it's a option, then you can match against the case. How about:

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let encode (data: obj)=
    match data with
    | :? Option<_> as opt -> 
        match opt with
        | Some x -> printfn "%A" x
        | None -> printfn "None"
    | null -> printfn "null"
    | :? string -> printfn "string"
    | _ -> printfn "other"
By on 1/3/2009 10:51 PM ()

Oops. My tests failed. This is the sample I used

module test =
let aOpt = Some(100)
let a = 100
let isOption (x:obj) =
match x with
| :? Option<_> as opt -> true
| _ -> false
let u = isOption aOpt
let v = isOption a

after running the test, both u and v is false which should not be so.

Also, for the line " | :? Option<_> ... , the _ has the warning message
Warning 21 This construct causes code to be less generic than indicated by its type annotations. The type variable implied by the use of a '#', '_' or other type annotation at or near 'C:\Documents and Settings\Me\My Documents\Visual Studio 2008\Projects\GiKo\Somee\Tester.fs(10,20)-(10,21)' has been constrained to be type 'obj'.

I also realised that when I changed it from Option<_> to Option<int>, I had the correct result. The issue here is that the object can be of any type.

Thank you.

By on 1/4/2009 2:52 AM ()

Yes, it seems the code I suggested did not work. I think the problem is that kind of type comparison doesn't work well with generics. The code I wrote didn't produce a warning, I think this is a compiler bug.

I think the best option here is to use the F# reflection API, you could do a test using the function: Reflection.FSharpType.IsUnion and the get the union cases and check if there's a some and a none.

Cheers,

Rob

By on 1/5/2009 3:00 AM ()

Thanks.This is the final code I ended up with. It uses reflection just as you said.

///Determines if a given value is an option. None is interpreted as null
let isOption x =
match box x with
| null -> false
| _ ->
match FSharpType.IsUnion <| x.GetType() with
| true ->
FSharpType.GetUnionCases <| x.GetType()
|> Seq.exists (fun t->t.Name="Some" || t.Name="None")
| _ -> false
///Gets the value of an option. The value is returned if the type is not an option
let optionValue x =
match isOption x with
| true ->
let _,v = FSharpValue.GetUnionFields(x,x.GetType())
v.[0]
| _ -> x

By on 1/5/2009 12:56 PM ()

If I recall correctly, Option uses null to encode None.

By on 1/7/2009 7:44 PM ()

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