Solve stack overflow in zipIndex function.

zipIndex.fsx

#r @"c:/packages/FsControl.1.0.9/lib/net40/FsControl.Core.dll"
 
open FsControl.Core.TypeMethods
open FsControl.Core.Types
open FsControl.Operators
 
type MonadBuilder() =
    member inline b.Return(x)    = result x
    member inline b.Bind(p,rest) = p >>= rest
    member        b.Let (p,rest) = rest p
    member    b.ReturnFrom(expr) = expr
 
let monad     = new MonadBuilder()

 
let f a :StateT<_,Cont<_,_>> = monad {
        let! n =  get ()
        do! put (n + 1)
        return n, a
    }
 
let zipIndex (xs: 'a list) : (int * 'a) list = Cont.run ((StateT.run (traverse f xs)) 0) fst
 
// or using eval functions
type Cont<'t,'u> with static member eval (Cont x) = x id
type StateT<'t,'u> with static member inline eval (StateT m) s = monad {
                            let! t = m s
                            return (fst t)}
 
let zipIndex' (xs: 'a list) : (int * 'a) list = Cont.eval ((StateT.eval (traverse f xs)) 0)