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Package patricia-tree

This library contains a single module: PatriciaTree.

This is version 0.9.0 of the library. It is known to work with OCaml versions ranging from 4.14 to 5.2.

This is an OCaml library that implements sets and maps as Patricia Trees, as described in Okasaki and Gill's 1998 paper Fast mergeable integer maps. It is a space-efficient prefix trie over the big-endian representation of the key's integer identifier.

The source code of this library is available on Github under an LGPL-2.1 license.


This library can be installed with opam:

opam install patricia-tree

Alternatively, you can clone the source repository and install with dune:

git clone
opam install . --deps-only
cd patricia-tree
dune build
dune install
# To build documentation
opam install odoc
dune build @doc


  • Similar to OCaml's Map and Set, using the same function names when possible and the same convention for order of arguments. This should allow switching to and from Patricia Tree with minimal effort.
  • The functor parameters (PatriciaTree.KEY module) requires an injective to_int : t -> int function instead of a compare function. PatriciaTree.KEY.to_int should be fast, injective, and only return positive integers. This works well with hash-consed types.
  • The Patricia Tree representation is stable, contrary to maps, inserting nodes in any order will return the same shape. This allows different versions of a map to share more subtrees in memory, and the operations over two maps to benefit from this sharing. The functions in this library attempt to maximally preserve sharing and benefit from sharing, allowing very important improvements in complexity and running time when combining maps or sets is a frequent operation.

    To do so, these functions often have extra requirements on their argument (e.g. inter f m1 m2 can be optimized by not inspecting common subtrees when f is idempotent). To avoid accidental errors, they are renamed (e.g. to idempotent_inter for the efficient version and nonidempotent_inter_no_share for the general one)

  • Since our Patricia Tree use big-endian order on keys, the maps and sets are sorted in increasing order of keys. We only support positive integer keys. This also avoids a bug in Okasaki's paper discussed in QuickChecking Patricia Trees by Jan Mitgaard.
  • Supports generic maps and sets: a 'm map that maps 'k key to ('k, 'm) value. This is especially useful when using GADTs for the type of keys. This is also sometimes called a dependent map.
  • Allows easy and fast operations across different types of maps and set (e.g. an intersection between a map and a set), since all sets and maps, no matter their key type, are really positive integer sets or maps.
  • Multiple choices for internal representation (PatriciaTree.NODE), which allows for efficient storage (no need to store a value for sets), or using weak nodes only (values removed from the tree if no other pointer to it exists). This system can also be extended to store size information in nodes if needed.
  • Exposes a common interface (PatriciaTree.NODE.view) to allow users to write their own pattern matching on the tree structure without depending on the PatriciaTree.NODE being used.

Quick overview


This library contains a single module, PatriciaTree. The functors used to build maps and sets are the following:

  • For homogeneous (non-generic) maps and sets: PatriciaTree.MakeMap and PatriciaTree.MakeSet. These are similar to the standard library's maps and sets.

    module MakeMap(Key: KEY) : MAP with type key = Key.t
    module MakeSet(Key: KEY) : SET with type elt = Key.t
  • For Heterogeneous (generic) maps and sets: PatriciaTree.MakeHeterogeneousMap and PatriciaTree.MakeHeterogeneousSet.

    module MakeHeterogeneousMap(Key: HETEROGENEOUS_KEY)(Value: VALUE) : HETEROGENEOUS_MAP
      with type 'a key = 'a Key.t
       and type ('k,'m) value = ('k,'m) Value.t
    module MakeHeterogeneousSet(Key: HETEROGENEOUS_KEY) : HETEROGENEOUS_SET
      with type 'a elt = 'a Key.t


Here is a brief overview of the various module types of our library:


Homogeneous map

Here is a small example of a non-generic map:

(** Create a key struct *)
module Int (*: PatriciaTree.KEY*) = struct
  type t = int
  let to_int x = x

(** Call the map and/or set functors *)
module IMap = PatriciaTree.MakeMap(Int)
module ISet = PatriciaTree.MakeSet(Int)

(** Use all the usual map operations *)
let map =
  IMap.empty |>
  IMap.add 1 "hello" |>
  IMap.add 2 "world" |>
  IMap.add 3 "how do you do?"
  (* Also has an [of_list] and [of_seq] operation for initialization *)

let _ = IMap.find 1 map (* "hello" *)
let _ = IMap.cardinal map (* 3 *)

(** The strength of Patricia Tree is the speedup of operation on multiple maps
    with common subtrees. *)
let map2 =
  IMap.idempotent_inter_filter (fun _key _l _r -> None)
  (IMap.add 4 "something" map) (IMap.add 5 "something else" map)
let _ = map == map2 (* true *)
(* physical equality is preserved as much as possible, although some intersections
   may need to build new nodes and won't be fully physically equal, they will
   still share subtrees if possible. *)

(** Many operations preserve physical equality whenever possible *)
let _ = (IMap.add 1 "hello" map) == map (* true: already present *)

(** Example of cross map/set operation: only keep the bindings of [map]
    whose keys are in a given set *)
let set = ISet.of_list [1; 3]
module CrossOperations = IMap.WithForeign(ISet.BaseMap)
let restricted_map = CrossOperations.nonidempotent_inter
  { f = fun _key value () -> value } map set

Heterogeneous map

(** Very small typed expression language *)
type 'a expr =
  | G_Const_Int : int -> int expr
  | G_Const_Bool : bool -> bool expr
  | G_Addition : int expr * int expr -> int expr
  | G_Equal : 'a expr * 'a expr -> bool expr

module Expr : PatriciaTree.HETEROGENEOUS_KEY with type 'a t = 'a expr = struct
  type 'a t = 'a expr

  (** Injective, so long as expression are small enough
      (encodes the constructor discriminant in two lowest bits).
      Ideally, use a hash-consed type, to_int needs to be fast *)
  let rec to_int : type a. a expr -> int = function
    | G_Const_Int i ->   0 + 4*i
    | G_Const_Bool b ->  1 + 4*(if b then 1 else 0)
    | G_Addition(l,r) -> 2 + 4*(to_int l mod 10000 + 10000*(to_int r))
    | G_Equal(l,r) ->    3 + 4*(to_int l mod 10000 + 10000*(to_int r))

  (** Full polymorphic equality *)
  let rec polyeq : type a b. a expr -> b expr -> (a, b) PatriciaTree.cmp =
    fun l r -> match l, r with
    | G_Const_Int l, G_Const_Int r -> if l = r then Eq else Diff
    | G_Const_Bool l, G_Const_Bool r -> if l = r then Eq else Diff
    | G_Addition(ll, lr), G_Addition(rl, rr) -> (
        match polyeq ll rl with
        | Eq -> polyeq lr rr
        | Diff -> Diff)
    | G_Equal(ll, lr), G_Equal(rl, rr) ->    (
        match polyeq ll rl with
        | Eq -> (match polyeq lr rr with Eq -> Eq | Diff -> Diff) (* Match required by typechecker *)
        | Diff -> Diff)
    | _ -> Diff

(** Map from expression to their values: here the value only depends on the type
    of the key, not that of the map *)
module EMap = PatriciaTree.MakeHeterogeneousMap(Expr)(struct type ('a, _) t = 'a end)

(** You can use all the usual map operations *)
let map : unit EMap.t =
  EMap.empty |>
  EMap.add (G_Const_Bool false) false |>
  EMap.add (G_Const_Int 5) 5 |>
  EMap.add (G_Addition (G_Const_Int 3, G_Const_Int 6)) 9 |>
  EMap.add (G_Equal (G_Const_Bool false, G_Equal (G_Const_Int 5, G_Const_Int 7))) true

let _ = EMap.find (G_Const_Bool false) map (* false *)
let _ = EMap.cardinal map (* 4 *)

(** Fast operations on multiple maps with common subtrees. *)
let map2 =
    { f = fun _key _l _r -> None } (* polymorphic 1rst order functions are wrapped in records *)
    (EMap.add (G_Const_Int 0) 8 map)
    (EMap.add (G_Const_Int 0) 9 map)

Release status

This should be close to a stable release. It is already being used as part of a larger project successfully, and this usage as helped us mature the interface. As is, we believe the project is usable, and we don't anticipate any major change before 1.0.0. We didn't commit to a stable release straight away as we would like a bit more time using this library before doing so.

Known issues

There is a bug in the OCaml typechecker which prevents us from directly defining non-generic maps as instances of generic maps. To avoid this, non-generic maps use a separate value type (instead of just using `'b`)

type (_, 'b) snd = Snd of 'b [@@unboxed]

It should not incur any extra performance cost as it is unboxed, but can appear when manipulating non-generic maps.

For more details about this issue, see the OCaml discourse discussion.

Comparison to other OCaml libraries

ptmap and ptset

There are other implementations of Patricia Tree in OCaml, namely ptmap and ptset, both by J.C. Filliatre. These are smaller and closer to OCaml's built-in Map and Set, however:

  • Our library allows using any type key that comes with an injective to_int function, instead of requiring key = int.
  • We support generic types for keys/elements.
  • We support operation between sets and maps of different types.
  • We use a big-endian representation, allowing easy access to min/max elements of maps and trees.
  • Our interface and implementation tries to maximize the sharing between different versions of the tree, and to benefit from this memory sharing. Theirs do not.
  • These libraries work with older version of OCaml (>= 4.05 I believe), whereas ours requires OCaml >= 4.14 (for the new interface of Ephemeron used in PatriciaTree.WeakNode).
  • Our keys are limited to positive integers.


Additionally, there is a dependent map library: dmap. It allows creating type safe dependent maps similar to our heterogeneous maps. However, its maps aren't Patricia trees. They are binary trees build using a (polymorphic) comparison function, similarly to the maps of the standard library. Another difference is that the type of values in the map is independent from the type of the keys, (allowing keys to be associated with different values in different maps).

dmap also works with OCaml >= 4.12, whereas we require OCaml >= 4.14.

Contributions and bug reports

Any contributions are welcome!

You can report any bug, issues, or desired features using the Github issue tracker. Please include OCaml, dune, and library version information in you bug reports.

If you want to contribute code, feel free to fork the repository on Github and open a pull request. By doing so you agree to release your code under this project's license (LGPL-2.1).

There is no imposed coding style for this repository, here are just a few guidelines and conventions:

  • Module type names should use SCREAMING_SNAKE_CASE.
  • Module and functor names use PascalCase, functors names start with Make.
  • Even though the library implements homogeneous maps as a specialization of heterogeneous ones, the naming convention is that no prefix means homogeneous, and all heterogeneous objects are prefixed with heterogeneous.
  • Please document any new functions in the interface, using ocamldoc style comments.
  • Please consider adding test for new features/fixed bugs if at all possible. This library uses a QuickCheck framework for tests.