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Facet Pattern for Principle of Least Authority (POLA) with F# Anonymous Object <= "error FS0670: This code is not sufficiently generic."
f#polaprinciple of least authorityfacet patternanonymous object

I get the following error, I've tried with inline, but it has not worked for me. Any suggestions?

error FS0670: This code is not sufficiently generic. The type variable 'a could not be generalized because it would escape its scope.

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// Facet Pattern 
// Used for Principle of Least Authority (POLA)
// Inspired by: "The Lazy Programmer's Guide to Secure Computing"
//              http://www.youtube.com/watch?v=eL5o4PFuxTY
type IFacet = 
    abstract Set    : 'a   -> unit
    abstract Get    : unit -> 'a Option
    abstract Revoke : unit -> unit

type OnOff = On | Off

let allowWrite access f = if access=On then f()
let allowRead  access f = if access=On then Some( f() ) else None

/// Revocable Facet Maker
let makeFacet x =
    let mutable access = On
    { new IFacet with 
        member this.Revoke() =            access <- Off             
        member this.Get()    = allowRead  access (fun () -> !x )        // <== error
        member this.Set(v)   = allowWrite access (fun () ->  x := v)
    } 

// usage pseudo code
let my = ref "Revocable Facet"
let facet = makeFacet my
:
Bob(  facet.Get)
Carol(facet.Set)
Delay 1000
facet.Revoke()
.
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type IFacet<'a> =
    ...
By on 12/15/2012 11:39 AM ()

Yes<'a>! That helps and some modifications later it works. Complete code:

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// Facet Pattern 
// Used for Principle of Least Authority (POLA)
// Inspired by: "The Lazy Programmer's Guide to Secure Computing"
//              http://www.youtube.com/watch?v=eL5o4PFuxTY
type IFacet<'a> =
    abstract Set    : 'a   -> unit
    abstract Get    : unit -> 'a Option
    abstract Revoke : unit -> unit

type OnOff = On | Off

let allowWrite access f = if access=On then f()
let allowRead  access f = if access=On then Some( f() ) else None

/// Revocable Facet Maker
let makeFacet x =
    let access = ref On
    { new IFacet<_> with 
        member this.Revoke() =             access := Off             
        member this.Get()    = allowRead  !access (fun () -> !x )
        member this.Set(v)   = allowWrite !access (fun () ->  x := v)
    } 

// usage
let my = ref "Revocable Facet"
let facet = makeFacet my

facet.Get()             = Some "Revocable Facet"
facet.Set( "changed" )
facet.Get()             = Some "changed"
facet.Revoke()        
facet.Get()             = None
facet.Set( "changed2" )
facet.Get()             = None
my.Value                = "changed"
By on 12/15/2012 11:56 AM ()

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