UnionUtils.fs

open System
open System.Collections.Concurrent
open Microsoft.FSharp.Reflection
open Microsoft.FSharp.Quotations.Patterns

type UnionUtils =

    static member private isTypeUnionCache = ConcurrentDictionary<Type, bool>()
    static member private tagGetterCache = ConcurrentDictionary<Type, obj -> int>()

    static member private getUnionCaseInfo expr =
        match expr with
        | Lambda(_, expr) -> UnionUtils.getUnionCaseInfo expr
        | Let(_, _, expr) -> UnionUtils.getUnionCaseInfo expr
        | NewUnionCase(unionCaseInfo, _) -> unionCaseInfo
        | _ -> failwith "no way"
   
    static member private IsSameCase<'a>(x: 'a, unionCaseInfo: UnionCaseInfo) =
        
        let t = typeof<'a>
        let isUnion = UnionUtils.isTypeUnionCache.GetOrAdd(t, FSharpType.IsUnion)
        if not isUnion then failwithf "Type %s is not union!" t.Name
        
        let getTag = UnionUtils.tagGetterCache.GetOrAdd(key = t, valueFactory = Func<_,_>(fun _ ->
            let m = t.GetProperty("Tag").GetGetMethod()
            fun o -> m.Invoke(o, null) :?> int
            ))
        
        getTag x = unionCaseInfo.Tag
        
    static member IsSameCase<'a>(x: 'a, y: Quotations.Expr<'a>) =
        UnionUtils.IsSameCase(x, UnionUtils.getUnionCaseInfo y)
        
        
    static member IsSameCase<'a,'b>(x: 'a, y: Quotations.Expr<'b -> 'a>) =
        UnionUtils.IsSameCase(x, UnionUtils.getUnionCaseInfo y)

type Foo =
    | A of string * int
    | B
    
UnionUtils.IsSameCase(B, <@A@>) |> printfn "%b"