stuff.fsx

let rec pairs list =
    match list with
    | [] -> []
    | x::ys -> [for y in ys -> x, y] @ pairs ys

pairs [1..4]

let rec pairslc list = [
    match list with
    | [] -> ()
    | x::ys -> for y in ys do yield x, y
               yield! pairslc ys ]

pairslc [1..4]

let rec firstFastPairs list acc =
    match list with
    | [] -> acc
    | x::ys ->
        firstFastPairs ys (List.fold (fun acc y -> (x,y)::acc) acc ys)

firstFastPairs [1..4] [] |> List.rev

let fastPairs list =
    let rec fastPairs acc list =
        match list with
        | [] -> acc
        | x::ys -> fastPairs (List.fold (fun acc y -> (x,y)::acc) acc ys) ys
    fastPairs [] list

fastPairs [1..4] |> List.rev

let rec dealPairs candidates =
    match candidates with
    | [] -> []
    | [a] -> [a,None]
    | a::b::rest -> (a, Some b)::(dealPairs rest)

dealPairs [1..4]