Srtp and Records.fs

(*
WHAT'S GOING ON HERE?!

Sometimes you don't care about a particular type, you're interested in one field only, let's say `EntityId`.
Instead of using interface (which isn't even possible if don't own a type),
we can do structural typing in F# using SRTP and Active Patterns.
Active patterns are not required for this, but they do make code much easier to use.
*)
// So we have 2 types with field `EntityId: string`:
    type Test =
        { EntityId: string }

    type Test2 =
        { EntityId: string
          Name: string 
          Length: int }

// First we define active pattern for detecting
// whether this type has this field or not:
    let inline (|HasEntityId|) x =
        fun () -> (^a : (member EntityId: string) x) // as you can see, SRTP syntax is hardly comfortable to use

// Then we define function for retrieving this field
    let inline entityId (HasEntityId f) = f()

// Another AP, this time for detecting `Name: string`:
    let inline (|HasName|) n =
        fun () -> (^a : (member Name: string) n)

// and function for getting the name:
    let inline name (HasName f) = f()
    
// Now here's the beauty of it: we can combine them!    
    let inline printNameAndId (HasName n & HasEntityId id) = sprintf "name %s id %s" (n()) (id())
    
// Watching it in action:
    let test1 = { EntityId = "123" }
    let test2 = {Test2.EntityId = "123"; Name = "123"; Length = 2}
    
// Works both with Test and Test2 types since both have `EntityId` field
    let a = entityId test1
    let b = entityId test2
    
    let c = name test2
    
    let d = printNameAndId test2

// *Compile* error when using type without required fields:
    let error = printNameAndId test1