was stock

Problem #8 Eliminate consecutive duplicates of list elements.

This problem was a bit harder than usual. A couple of things were learned from doing this problem. Using [] to enclose an a’ type while get rid of type mismatch errors. Another development tip is to build the inner recursive function first and then make the wrapper function.

let compress_list clist =
  let rec rec_compress_list head tail =
   match tail with
   | [] -> []
   | (y::ys) ->  if [y] = [head] then
                   rec_compress_list y ys
                 else
                   [head] @ (rec_compress_list y ys)
 in match clist with
   | [] -> clist
   | (y::ys) -> (rec_compress_list y ys);;

Here are some test cases.

['a';'a';'a';'a';'a';'a';'b';'b';'b';'b';'a';'a';'a';'b';'b';'c';'d';'d';'d';'d';'d';'d';'e'] |> compress_list |> List.iter (printf "[%c]") ;
printf "n";
['a';'b';'b';'b';'b';'a';'a';'a';'b';'b';'c';'d';'d';'d';'d';'d';'d';'e'] |> compress_list |> List.iter (printf "[%c]") ;

–Update

Joel Lanciaux was kind enough to point out a bug. Apparently my function was chopping off the last element. D’oh!

Here is the updated solution

let compress_list clist =
  let rec rec_compress_list head tail =
   match tail with
   | [] -> [head]
   | (y::ys) ->  if [y] = [head] then
                   rec_compress_list y ys
                 else
                   [head] @ (rec_compress_list y ys)
 in match clist with
   | [] -> clist
   | (y::ys) -> (rec_compress_list y ys);;