Slice/Group a sequence of equal chars in F#

Question

I need to extract the sequence of equal chars in a text.

For example: The string \"aaaBbbcccccccDaBBBzcc11211\" should be converted to a list of strings

Answer 1

When you're folding, you'll need to carry along both the previous value and the accumulator holding the temporary results. The previous value is wrapped as option to account for the first iteration. Afterwards, the final result is extracted and reversed.

"aaaBbbcccccccDaBBBzcc11211"
|> Seq.map string
|> Seq.fold (fun state ca ->
    Some ca,
    match state with
    | Some cb, x::xs when ca = cb -> x + ca::xs
    | _, xss ->                      ca::xss )
    (None, [])
|> snd 
|> List.rev
// val it : string list =
//   ["aaa"; "B"; "bb"; "ccccccc"; "D"; "a"; "BBB"; "z"; "cc"; "11"; "2"; "11"]