F# split sequence into sub lists on every nth element

Question

Say I have a sequence of 100 elements. Every 10th element I want a new list of the previous 10 elements. In this case I will end up with a list of 10 sublists.

Seq.t

Answer 1

I think that the solution from Brian is probably the most reasonable simple option. A probelm with sequences is that they cannot be easily processed with the usual pattern matching (like functional lists). One option to avoid that would be to use LazyList from F# PowerPack.

Another option is to define a computation builder for working with IEnumerator type. I wrote something like that recently - you can get it here. Then you can write something like:

let splitEach chunkSize (s:seq<_>) =
  Enumerator.toSeq (fun () -> 
    let en = s.GetEnumerator()
    let rec loop n acc = iter {
      let! item = en
      match item with 
      | Some(item) when n = 1 -> 
          yield item::acc |> List.rev 
          yield! loop chunkSize []
      | Some(item) -> 
          yield! loop (n - 1) (item::acc)
      | None -> yield acc |> List.rev }
    loop chunkSize [] )

This enables using some functional patterns for list processing - most notably, you can write this as a usual recursive function (similar to the one you would write for lists/lazy lists), but it is imperative under the cover (the let! constructo of iter takes the next element and modifies the enumerator).