The OCaml syntax doesn't work with F#. Optional and named arguments can't be used with let functions, but only with members (both static and instance members) and class constructors.

type complex =
    {r: float; i: float}
    member c.Add(i, r) = {r = c.r + r; i = c.i + i}

c.Add(3., 4.)
c.Add(i = 3., r = 4.)
c.Add(r = 4., i = 3.)

type complex =
  {r: float; i: float}
  member c.Add(?i, ?r) =
     let i = defaultArg i 0.
     let r = defaultArg r 0.
     {r = c.r + r; i = c.i + i}

let c = {i = 1.; r = 1.}
> c.Add();;
val it : complex = 1.00+1.00i
> c.Add(1.);;
val it : complex = 1.00+2.00i
> c.Add(1., 2.);;
val it : complex = 3.00+2.00i
> c.Add(i = 1., r = 2.);;
val it : complex = 3.00+2.00i
> c.Add(r = 1., i = 2.);;
val it : complex = 2.00+3.00i

When used on a class constructor, named arguments are compiled into property setters:

    new MenuItem(text="Hello")  
    new Form(Width=300)

• Cours 11 - Méthodes (fr) (Laurent Le Brun - some samples come from this page)
• Language specifications