Is it possible to implement a recursive “SelectMany”?

Question

As we all know, Enumerable.SelectMany flattens a sequence of sequences into a single sequence. What if we wanted a method that could flatten sequences of sequen

Answer 1

This is trivial in F# with recursive sequence expressions.

let rec flatten (items: IEnumerable) =
  seq {
    for x in items do
      match x with
      | :? 'T as v -> yield v
      | :? IEnumerable as e -> yield! flatten e
      | _ -> failwithf "Expected IEnumerable or %A" typeof<'T>
  }

A test:

// forces 'T list to obj list
let (!) (l: obj list) = l
let y = ![["1";"2"];"3";[!["4";["5"];["6"]];["7"]];"8"]
let z : string list = flatten y |> Seq.toList
// val z : string list = ["1"; "2"; "3"; "4"; "5"; "6"; "7"; "8"]