Use of phantom types for static validation of list length

phantom.fs

// Phantom types - http://web.archive.org/web/20100615031841/http://blog.matthewdoig.com/?p=138
 
// Rather than DUs, just use simple marker interfaces
type Zero = interface end
type Succ<'length> = interface end

type Phantom<'a,'length> = Phantom of 'a list
let toList (Phantom l) = l

let nil = Phantom [] : Phantom<_,Zero>

// 'a -> Phantom<'a,'length> -> Phantom<'a,Succ<'length>>
let cons (e : 'a) (l : Phantom<'a,'length>) =
    let cons1 = e :: (toList l)
    Phantom cons1 : Phantom<_,Succ<'length>>

// Phantom<'a,'length> -> Phantom<'b,'length> -> Phantom<'a*'b,'length>
let combine (al : Phantom<'a,'length>) (bl : Phantom<'b,'length>)=
    let combine1 = List.zip (toList al) (toList bl)
    Phantom combine1 : Phantom<_,'length>

//val a : Phantom<int,Succ<Succ<Zero>>>
let a = cons 1 (cons 2 nil)
 
//[1; 2]
let r = toList a
 
//val b : Phantom<string,Succ<Succ<Zero>>>
let b = cons "a" (cons "b" nil)
 
//[(1,"a"); (2,"b")]
let r1 = toList (combine a b)

// Type mismatch. Expecting a Phantom<string,Succ<Succ<Zero>>>
// but given Phantom<string,Succ<Zero>>.
// The type Succ<Zero> does not match the type ‘Zero’.
let c = cons "a" nil
let r2 = toList (combine a c)