Creating a tagged union using F# Struct with Explicit LayoutKind -

i'm trying create f# equivalent of tagged union. need in hot path of application code, discriminated unions cause of heap allocations.

here's example:

[<struct; structlayout(layoutkind.explicit)>] type result =     [<defaultvalue; fieldoffset 0>]     val mutable isasync : bool     [<defaultvalue; fieldoffset 1>]      val mutable async : async<obj>     [<defaultvalue; fieldoffset 1>]      val mutable sync : obj 

however, problems start when want provide kind of creation methods it.

example 1

static member async(a:async<obj>) = result(isasync = true; async=a) static member sync(s:obj) = result(isasync = false; sync=s) 

throws the member or object constructor 'result' takes 0 argument(s) here given 1. required signature 'result()' compilation error.

example 2

new(a:async<obj>) = { isasync = true; async = a; } new(s:obj) = { isasync = false; sync=s } 

throws extraneous fields have been given values compilation error.

you have 3 problems:

1. you need separate constructor arguments commas (,), not semicolons (;).
2. object references must aligned on pointer-sized boundaries. therefore, on x64 offset object references must 8.
3. static , instance member names shouldn't overlap.

this works me:

[<struct; structlayout(layoutkind.explicit)>] type result =     [<defaultvalue; fieldoffset 0>]     val mutable isasync : bool     [<defaultvalue(false); fieldoffset 8>]      val mutable async : async<obj>     [<defaultvalue; fieldoffset 8>]      val mutable sync : obj      static member makeasync(a:async<obj>) = result(isasync = true, async=a)     static member makesync(s:obj) = result(isasync = false, sync=s)