Eric Lippert's challenge “comma-quibbling”, best answer?

Question

I wanted to bring this challenge to the attention of the stackoverflow community. The original problem and answers are here. BTW, if you did not follow it before, you should

Answer 1

Here's a simple F# solution, that only does one forward iteration:

let CommaQuibble items =
    let sb = System.Text.StringBuilder("{")
    // pp is 2 previous, p is previous
    let pp,p = items |> Seq.fold (fun (pp:string option,p) s -> 
        if pp <> None then
            sb.Append(pp.Value).Append(", ") |> ignore
        (p, Some(s))) (None,None)
    if pp <> None then
        sb.Append(pp.Value).Append(" and ") |> ignore
    if p <> None then
        sb.Append(p.Value) |> ignore
    sb.Append("}").ToString()

(EDIT: Turns out this is very similar to Skeet's.)

The test code:

let Test l =
    printfn "%s" (CommaQuibble l)

Test []
Test ["ABC"]        
Test ["ABC";"DEF"]        
Test ["ABC";"DEF";"G"]        
Test ["ABC";"DEF";"G";"H"]        
Test ["ABC";null;"G";"H"]