Lazy lists for OCaml

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zlist: Lazy lists for OCaml


zlist consists of the definition of a lazy list type and a number of useful functions for manipulating and constructing lazy lists.


build status

Development is hosted at GitLab and mirrored at GitHub.

API documentation can be found online.


This implementation is heavily inspired by "Functional Programming in Scala", by Chiusano and Bjarnason (2014).


The easiest way to install zlist is through the opam repository:

$ opam install zlist


Alternatively, you can build and install zlist from its sources.

First, pin the sources using opam:

$ opam pin add -n zlist <TOP-LEVEL SOURCE DIRECTORY>
$ opam install zlist -t --deps-only

then build the package:

$ ocaml pkg/pkg.ml build

After the package is built, zlist's test suite is run by invoking

$ ocaml pkg/pkg.ml test

Lazy lists

The type of a lazy list is

type 'a t =
  | Nil
  | Cons of 'a Lazy.t * 'a t Lazy.t

That is, unlike a normal list, both the head and tail of a cons cell are lazily-evaluated. This lazy structure allows us to generate infinite lists and to apply arbitrary transformations to the list without constructing new instances in memory.


Assume this following code has been evaluated in the OCaml top-level:

#require "zlist" ;;
open Zlist ;;

We can generate an infinite list of even integers:

let evens = Lazy_list.(enum_from 0 |> map (fun x -> 2 * x)) ;;

and observe the first 10:

Lazy_list.(take 10 evens |> to_list) ;;

- : int list = [0; 2; 4; 6; 8; 10; 12; 14; 16; 18]

The fibonacci numbers can be generated via Lazy_list.unfold:

let fibs = Lazy_list.unfold (0, 1) (fun (a, b) -> Some ((b, a + b), a)) ;;
fibs |> Lazy_list.(take 10 |> to_list) ;;

- : int list = [0; 1; 1; 2; 3; 5; 8; 13; 21; 34]


zlist is copyright 2016 by Jesse Haber-Kucharsky.

zlist is released under the terms of the Apache license, version 2.0. See /LICENSE for more information.