I'm pretty new to OCaml, so any feedback is appreciated. The goal is to implement a trie that efficiently stores and sorts strings.

module CharMap = Map.Make(Char)

(* count of members of the set that end at this node * mapping from
   next char => children *)
type trie = Node of int * trie CharMap.t

let empty = Node (0, CharMap.empty)

(* Add a string to the trie *)
let rec add (Node (count, children) : trie) = function 
  | "" -> Node (count + 1, children)
  | str -> 
    let firstChar = String.get str 0 in
    let lastChars = String.sub str 1 ((String.length str) - 1) in
    let newTrie = if CharMap.mem firstChar children
      then add (CharMap.find firstChar children) lastChars
      else add empty lastChars in
    let newChildren = CharMap.add firstChar newTrie children in   
    Node (count, newChildren)

let addAll (trie : trie) (strs : string list) : trie =
  List.fold_left (fun trieAcc str -> add trieAcc str) empty strs 

(* Get all the strings of a trie *)
let traverse (trie : trie) : string list =
  let rec helper (Node (count, children) : trie) (prevChar : char option)
      : string list =
    let string_of_char chr = 
      match chr with 
        | None -> ""
        | Some ch -> String.make 1 ch in
    let perChild = List.map (fun (chr, child) -> 
      (helper child (Some chr))) (CharMap.bindings children) in
      let fromChildren = List.flatten perChild in
    let withPrev =
      List.map (fun str -> (string_of_char prevChar) ^ str) fromChildren in
    let rec clone str count = 
      if count = 0 then [] else str::(clone str (count - 1))
    in (clone (string_of_char prevChar) count) @ withPrev in  
  helper trie None

let sort (strs: string list): string list =
  traverse (addAll empty strs)

What do you think?


2 Answers 2


You're pretty new to OCaml? You're using OCaml 3.12 features and probably did a lot of functional programming before. Let's see if I can show what idioms I've seen before. I'm not an OCaml expert though, far from it.

Empty trie Consider using something like Empty to denote the empty trie. type trie = Empty | Node of int * trie CharMap.t This is better than your empty hack, and will allow you to take advantage of OCaml's pattern matching.

string_of_char is a nice OCaml name, but this is not really string_of_char, but string_of_char_option. Since it's not important, I'd tend to make it look smaller in the code.

let string_of_char_option = function None -> "" | Some ch -> String.make 1 ch

Auxiliary function name You don't need to use the name helper: traverse is fine. It's not ambiguous since traverse is not recursive. This applies to all functions with auxiliary functions.

Indentation I'm not sure I understand how you choose to put your ins. Also, fromChildren has a wrong indentation.

  • \$\begingroup\$ Thanks for the feedback! Good catch on string_of_char. I feel like string_of_char_opt is a good compromise. I know it's not ambiguous, but shadowing variables from enclosing scopes (like having traverse within traverse) feels too "clever". Indentation is weird, I know. I'm trying to keep it as readable as possible with 80 chars/line. \$\endgroup\$ Commented Mar 19, 2012 at 23:04
  • 1
    \$\begingroup\$ I'm not sure about Empty. Adding Empty to type trie means that there are now more ways to encode emptiness, increasing complexity. Also, there's never a time that I check if some trie = empty, so it's not clear when I'd take advantage of the pattern matching. \$\endgroup\$ Commented Mar 19, 2012 at 23:06

This is how I would write the traverse function:

(* Get all the strings of a trie *)
let traverse (trie : trie) : string list =
  let string_of_char chr = String.make 1 chr in
  let rec traverse (Node (count, children) : trie) (prevString : string) : string list =
    let fromChildren = List.flatten (List.map (fun (chr, child) -> 
      (traverse child (prevString^(string_of_char chr)))) (CharMap.bindings children))
    let rec clone count =
      if count = 0 then fromChildren
      else prevString::(clone (count - 1))
    clone count
  traverse trie ""

I think you use too much auxiliary variables, there is no need for them here.

The next step when you have a fonction like this is to use minimum space on the stack, for that you must remove the List.flatten function:

let traverse_tr trie =
  let string_of_char chr = String.make 1 chr in
  let rec traverse (Node(count, children)) prevString acc =
    let rec clone count acc =
      if count = 0 then acc
      else (clone (count-1) (prevString :: acc))
    CharMap.fold (fun chr t -> traverse t (prevString^(string_of_char chr)) )
      children (clone count acc)
  traverse trie "" []

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