Question

let undefined = ["string"; ""; "string"; "boolean";"";"innermost"]

I have a list and I want to write a function that return a list without duplicate and empty string list. For example the undefined list above will return :

["string"; "boolean"; "innermost"]

I write this function it return for me without duplicate but how can I add the condition with testing an empty string.

let rec uniquify = function
| [] -> []
| x::xs -> x :: uniquify (List.filter ((<>) x) xs)

Thank you very much

Answer 1

Just pipe the result to List.filter (fun s -> s <> "") to remove the empty string afterwards. That’s the simple, compositional way, yo u could also hack your function to drop it silently

let rec uniquify = function
| [] -> []
| x::xs ->
  (if x = "" then [] else [x]) @ uniquify (List.filter ((<>) x) xs)

Note that your function is quadratic, you can have better complexity by sorting the list first, or by converting to a set and back. Batteries has functions to do that for you.

let do_stuff list =
  let open Batteries in
  List.remove (List.sort_unique String.compare list) ""