EOPL - 2.3.2 - Exercise 2.16

Easy one, in two versions.

Version 1

let rec list_find_last_position sym los = 
    match los with
      [] -> None
    | (s :: los') when s = sym -> 
        (match list_find_last_position sym los' with
             None -> Some 0
         |   Some i -> Some (i + 1))
    | (s :: los') -> 
        (match list_find_last_position sym los' with
             None -> None
         |   Some i -> Some (i + 1))

Version 2

let rec list_find_last_position sym los = 
    match los with
      [] -> None
    | (s :: los') -> 
        match list_find_last_position sym los' with
          None -> if s = sym then Some 0 else None
        | Some i -> Some (i + 1)

Version 2 was obtained by observing similarities in Version 1. The idea appeared in previous examples: pattern guards do not always lead to shorter code.