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A ppx deriving sample-functions for Popper








Popper (after Karl) is an OCaml testing library that can
be used for writing simple unit-tests as well as property-based ones. Its
underlying design is inspired by the Python library

See the documentation page for information on
how to get started.


High-level features of Popper include:

  • A uniform API for defining regular unit- and property-based tests.

  • Embedded shrinking — invariants that were used when constructing samples for property-based tests are always respected.

  • Compositional design — tests may be bundled and nested arbitrarily.

  • Ships with a ppx for automatically deriving comparator and sample functions for custom data types.

  • Deterministic (and reproducible) results.

  • Colorful output (cred goes to Alcotest, couldn't resist some inspiration here).

  • Support for line-number reporting, timing information and logging.


See CONTRIBUTING.md for how to build and contribute to


Show me an example

Here's what test output might look like:

It was generated from the following code:

open Popper
open Sample.Syntax

type exp =
  | Lit of bool
  | And of exp * exp
  | Or of exp * exp
  | Not of exp
[@@deriving show, ord, popper]

(* A buggy evaluator function *)
let rec eval = function
  | Lit b -> b
  | And (e1, e2) -> eval e1 || eval e2
  | Or (e1, e2) -> eval e1 || eval e2
  | Not b -> not @@ eval b

(* A simple unit test *)
let test_hello_world =
  test @@ fun () ->
    equal Comparator.string "hello world" (String.lowercase_ascii "Hello World")

(* Another unit test *)
let test_lit_true = test @@ fun () -> is_true (eval (Lit true) = true)

(* A property-based test *)
let test_false_ident_or =
  test @@ fun () ->
  let* e = exp_sample in
  is_true (eval e = eval (Or (Lit false, e)))

(* Another property-based test *)
let test_true_ident_and =
  test @@ fun () ->
    let* e = Sample.with_log "e" pp_exp exp_sample in
    is_true ~loc:__LOC__ (eval e = eval (And (Lit true, e)))

let suite =
    [ ("Hello World", test_hello_world)
    ; ("Lit true", test_lit_true)
    ; ("False ident or", test_false_ident_or)
    ; ("True ident and", test_true_ident_and)

let () = run suite

Comparing with other libraries

Popper is designed with the following objectives in mind:

  1. Make it as seamless as possible to write property-based tests — for instance
    by using a ppx to derive sample functions for custom data-types.

  2. Use embedded shrinking (ala
    Hypothesis) and eliminate the
    need for writing shrinkers manually.

The property-based aspects overlap with the OCaml libraries
QCheck and

Popper also supports writing simple unit tests and the ability to compose tests
into suites. This API and the output is partly inspired by the testing
library Alcotest.

Here's a table comparing features across different OCaml testing libraries:

Library Test suites Property-based Embeded shrinking PPX generators Fuzzying
Alcotest - -
OUnit - -

It might be possible to write some adaptors to be able to integrate with
these libraries but nothing such exists at the moment.