Sample of serializing union type

gistfile1.fs

open System.IO
open System.Runtime.Serialization.Json
open System.Text
        
let exprTypes = GetUnionTypes baseType
let ToJson<'t> (myObj:'t) =   
    use ms = new MemoryStream() 
    DataContractJsonSerializer(typeof<'t>, exprTypes).WriteObject(ms, myObj) 
    Encoding.Default.GetString(ms.ToArray())
    
let FromJson<'t> (jsonString:string)  : 't =  
    use ms = new MemoryStream(Encoding.Default.GetBytes(jsonString)) 
    let obj = DataContractJsonSerializer(typeof<'t>, exprTypes).ReadObject(ms) 
    obj :?> 't

let value = Num 3
let json = ToJson value
let recovered : expr = FromJson json
;;

val exprTypes : seq<Type>
val ToJson : myObj:'t -> string
val FromJson : jsonString:string -> 't
val value : expr = Num 3
val json : string = "{"__type":"FSI_0002.expr.Num:#","item":3}"
val recovered : expr = Num 3