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https://github.com/c-cube/qcheck

QuickCheck inspired property-based testing for OCaml.
https://github.com/c-cube/qcheck

alcotest monadic-interface ocaml ounit property-based-testing quickcheck random random-generator testing

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QuickCheck inspired property-based testing for OCaml.

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README 

        
= QCheck

:toc: macro

:toclevels: 4

:source-highlighter: pygments



QuickCheck inspired property-based testing for OCaml, and combinators to

generate random values to run tests on.



image::https://github.com/c-cube/qcheck/actions/workflows/main.yml/badge.svg[alt="build", link=https://github.com/c-cube/qcheck/actions/workflows/main.yml]



The documentation can be found https://c-cube.github.io/qcheck/[here].

This library spent some time in

https://github.com/vincent-hugot/iTeML[qtest], but is now

standalone again!



To construct advanced random generators, the following libraries might be

of interest:



- https://gitlab.inria.fr/fpottier/feat/[Feat]

- @gasche's https://github.com/gasche/random-generator/[generator library]



Jan Midtgaard (@jmid) has http://janmidtgaard.dk/quickcheck/index.html[a lecture] about

property-based testing that relies on QCheck.



toc::[]



== Use



See the documentation. I also wrote

https://cedeela.fr/quickcheck-for-ocaml[a blog post] that explains

how to use it and some design choices; however, be warned that the API

changed in lots of small ways (in the right direction, I hope) so the code

will not work any more.

<> is an updated version of the blog post's examples.



== Build



    $ make



You can use opam:



    $ opam install qcheck



== License



The code is now released under the BSD license.



[[examples]]

== An Introduction to the Library



First, let's see a few tests. Let's open a toplevel (e.g. utop)

and type the following to load QCheck:



[source,OCaml]

----

#require "qcheck";;

----



NOTE: alternatively, it is now possible to locally do: `dune utop src`

to load `qcheck`.



=== List Reverse is Involutive



We write a random test for checking that `List.rev (List.rev l) = l` for

any list `l`:



[source,OCaml]

----

let test =

  QCheck.Test.make ~count:1000 ~name:"list_rev_is_involutive"

   QCheck.(list small_nat)

   (fun l -> List.rev (List.rev l) = l);;



(* we can check right now the property... *)

QCheck.Test.check_exn test;;

----



In the above example, we applied the combinator `list` to

the random generator `small_nat` (ints between 0 and 100), to create a

new generator of lists of random integers. These builtin generators

come with printers and shrinkers which are handy for outputting and

minimizing a counterexample when a test fails.



Consider the buggy property `List.rev l = l`:



[source,OCaml]

----

let test =

  QCheck.Test.make ~count:1000 ~name:"my_buggy_test"

   QCheck.(list small_nat)

   (fun l -> List.rev l = l);;

----



When we run this test we are presented with a counterexample:



[source,OCaml]

----

# QCheck.Test.check_exn test;;

Exception:

QCheck.Test.Test_fail ("my_buggy_test", ["[0; 1] (after 23 shrink steps)"]).

----



In this case QCheck found the minimal counterexample `[0;1]` to the property

`List.rev l = l` and it spent 23 steps shrinking it.



Now, let's run the buggy test with a decent runner that will print the results

nicely (the exact output will change at each run, because of the random seed):



----

# QCheck_runner.run_tests [test];;



--- Failure --------------------------------------------------------------------



Test my_buggy_test failed (10 shrink steps):



[0; 1]

================================================================================

failure (1 tests failed, 0 tests errored, ran 1 tests)

- : int = 1

----



For an even nicer output `QCheck_runner.run_tests` also accepts an optional

parameter `~verbose:true`.



=== Mirrors and Trees



`QCheck` provides many useful combinators to write

generators, especially for recursive types, algebraic types,

and tuples.



Let's see how to generate random trees:



[source,OCaml]

----

type tree = Leaf of int | Node of tree * tree



let leaf x = Leaf x

let node x y = Node (x,y)



let tree_gen = QCheck.Gen.(sized @@ fix

  (fun self n -> match n with

    | 0 -> map leaf nat

    | n ->

      frequency

        [1, map leaf nat;

         2, map2 node (self (n/2)) (self (n/2))]

    ));;



(* generate a few trees, just to check what they look like: *)

QCheck.Gen.generate ~n:20 tree_gen;;



let arbitrary_tree =

  let open QCheck.Iter in

  let rec print_tree = function

    | Leaf i -> "Leaf " ^ (string_of_int i)

    | Node (a,b) -> "Node (" ^ (print_tree a) ^ "," ^ (print_tree b) ^ ")"

  in

  let rec shrink_tree = function

    | Leaf i -> QCheck.Shrink.int i >|= leaf

    | Node (a,b) ->

      of_list [a;b]

      <+>

      (shrink_tree a >|= fun a' -> node a' b)

      <+>

      (shrink_tree b >|= fun b' -> node a b')

  in

  QCheck.make tree_gen ~print:print_tree ~shrink:shrink_tree;;



----



Here we write a generator of random trees, `tree_gen`, using

the `fix` combinator. `fix` is *sized* (it is a function from `int` to

a random generator; in particular for size 0 it returns only leaves).

The `sized` combinator first generates a random size, and then applies

its argument to this size.



Other combinators include monadic abstraction, lifting functions,

generation of lists, arrays, and a choice function.



Then, we define `arbitrary_tree`, a `tree QCheck.arbitrary` value, which

contains everything needed for testing on trees:



- a random generator (mandatory), weighted with `frequency` to

  increase the chance of generating deep trees

- a printer (optional), very useful for printing counterexamples

- a *shrinker* (optional), very useful for trying to reduce big

  counterexamples to small counterexamples that are usually

  more easy to understand.



The above shrinker strategy is to



- reduce the integer leaves, and

- substitute an internal `Node` with either of its subtrees or

  by splicing in a recursively shrunk subtree.



A range of combinators in `QCheck.Shrink` and `QCheck.Iter` are available

for building shrinking functions.



We can write a failing test using this generator to see the

printer and shrinker in action:



[source,OCaml]

----

let rec mirror_tree (t:tree) : tree = match t with

  | Leaf _ -> t

  | Node (a,b) -> node (mirror_tree b) (mirror_tree a);;



let test_buggy =

  QCheck.Test.make ~name:"buggy_mirror" ~count:200

    arbitrary_tree (fun t -> t = mirror_tree t);;



QCheck_runner.run_tests [test_buggy];;

----



This test fails with:



[source,OCaml]

----



--- Failure --------------------------------------------------------------------



Test mirror_buggy failed (6 shrink steps):



Node (Leaf 0,Leaf 1)

================================================================================

failure (1 tests failed, 0 tests errored, ran 1 tests)

- : int = 1

----



With the (new found) understanding that mirroring a tree

changes its structure, we can formulate another property

that involves sequentializing its elements in a traversal:



[source,OCaml]

----

let tree_infix (t:tree): int list =

  let rec aux acc t = match t with

    | Leaf i -> i :: acc

    | Node (a,b) ->

      aux (aux acc b) a

  in

  aux [] t;;



let test_mirror =

  QCheck.Test.make ~name:"mirror_tree" ~count:200

    arbitrary_tree

    (fun t -> List.rev (tree_infix t) = tree_infix (mirror_tree t));;



QCheck_runner.run_tests [test_mirror];;



----



=== Preconditions



The functions `QCheck.assume` and `QCheck.(==>)` can be used for

tests with preconditions.

For instance, `List.hd l :: List.tl l = l` only holds for non-empty lists.

Without the precondition, the property is false and will even raise

an exception in some cases.



[source,OCaml]

----

let test_hd_tl =

  QCheck.(Test.make

    (list int) (fun l ->

      assume (l <> []);

      l = List.hd l :: List.tl l));;



QCheck_runner.run_tests [test_hd_tl];;

----



=== Long tests



It is often useful to have two version of a testsuite: a short one that runs

reasonably fast (so that it is effectively run each time a projet is built),

and a long one that might be more exhaustive (but whose running time makes it

impossible to run at each build). To that end, each test has a 'long' version.

In the long version of a test, the number of tests to run is multiplied by

the `~long_factor` argument of `QCheck.Test.make`.



=== Runners



The module `QCheck_runner` defines several functions to run tests, including

compatibility with `OUnit`.

The easiest one is probably `run_tests`, but if you write your tests in

a separate executable you can also use `run_tests_main` which parses

command line arguments and exits with `0` in case of success,

or an error number otherwise.



=== Integration within OUnit



https://github.com/gildor478/ounit[OUnit] is a popular unit-testing framework

for OCaml.

QCheck provides a sub-library `qcheck-ounit` with some helpers, in `QCheck_ounit`,

to convert its random tests into OUnit tests that can be part of a wider

test-suite.



[source,OCaml]

----

let passing =

  QCheck.Test.make ~count:1000

    ~name:"list_rev_is_involutive"

    QCheck.(list small_nat)

    (fun l -> List.rev (List.rev l) = l);;



let failing =

  QCheck.Test.make ~count:10

    ~name:"fail_sort_id"

    QCheck.(list small_nat)

    (fun l -> l = List.sort compare l);;



let _ =

  let open OUnit in

  run_test_tt_main

    ("tests" >:::

       List.map QCheck_ounit.to_ounit_test [passing; failing])



----



NOTE: the package `qcheck` contains the module `QCheck_runner`

which contains both custom runners and OUnit-based runners.



=== Integration within alcotest



https://github.com/mirage/alcotest/[Alcotest] is a simple and colorful test framework for

OCaml. QCheck now provides a sub-library `qcheck-alcotest` to

easily integrate into an alcotest test suite:



[source,OCaml]

----



let passing =

  QCheck.Test.make ~count:1000

    ~name:"list_rev_is_involutive"

    QCheck.(list small_int)

    (fun l -> List.rev (List.rev l) = l);;



let failing =

  QCheck.Test.make ~count:10

    ~name:"fail_sort_id"

    QCheck.(list small_int)

    (fun l -> l = List.sort compare l);;



let () =

  let suite =

    List.map QCheck_alcotest.to_alcotest

      [ passing; failing]

  in

  Alcotest.run "my test" [

    "suite", suite

  ]



----



=== Integration within Rely

https://reason-native.com/docs/rely/[Rely] is a Jest-inspire native reason testing framework.

@reason-native/qcheck-rely is available via NPM and provides matchers for the easy

use of qCheck within Rely.



[source, Reason]

----

open TestFramework;

open QCheckRely;



let {describe} = extendDescribe(QCheckRely.Matchers.matchers);



describe("qcheck-rely", ({test}) => {

  test("passing test", ({expect}) => {

    let passing =

      QCheck.Test.make(

        ~count=1000,

        ~name="list_rev_is_involutive",

        QCheck.(list(small_int)),

        l =>

        List.rev(List.rev(l)) == l

      );

    expect.ext.qCheckTest(passing);

    ();

  });

  test("failing test", ({expect}) => {

    let failing =

      QCheck.Test.make(

        ~count=10, ~name="fail_sort_id", QCheck.(list(small_int)), l =>

        l == List.sort(compare, l)

      );



    expect.ext.qCheckTest(failing);

    ();

  });

});



----



=== Deriver



A ppx_deriver is provided to derive QCheck generators from a type declaration.



[source,OCaml]

----

type tree = Leaf of int | Node of tree * tree

[@@deriving qcheck]

----



See the according https://github.com/c-cube/qcheck/tree/master/src/ppx_deriving_qcheck/[README]

for more information and examples.



=== Compatibility notes



Starting with 0.9, the library is split into several components:



- `qcheck-core` depends only on unix and bytes. It contains the module

  `QCheck` and a `QCheck_base_runner` module with our custom runners.

- `qcheck-ounit` provides an integration layer for `OUnit`

- `qcheck` provides a compatibility API with older versions of qcheck,

  using both `qcheck-core` and `qcheck-ounit`.

  It provides `QCheck_runner` which is similar to older versions and contains

  both custom and Ounit-based runners.

- `qcheck-alcotest` provides an integration layer with `alcotest`



Normally, for contributors,

`opam pin https://github.com/c-cube/qcheck` will pin all these packages.