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Quickcheck inspired property-based testing
Introduction
Examples
Assumptions
Generate Random Values
Printing Values
Shrinking Values
1. Iterators
2. Shrinkers
Arbitrary
Tests
Generating Functions

package qcheck-core

qcheck-core
- QCheck
  - Fn
  - Gen
  - Iter
  - Observable
  - Print
  - Shrink
  - Test
  - TestResult
  - Tuple
    
    Infix
- QCheck2
  - Fn
  - Gen
  - Observable
  - Print
  - Shrink
    
    Number
  - Test
  - TestResult
  - Test_exceptions
  - Tree
  - Tuple
    
    Infix
qcheck-core.runner
- QCheck_base_runner
  - Color
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Quickcheck inspired property-based testing

Introduction

The library takes inspiration from Haskell's QuickCheck library. The rough idea is that the programmer describes invariants that values of a certain type need to satisfy ("properties"), as functions from this type to bool. The programmer also needs to describe how to generate random values of the type, so that the property is tried and checked on a number of random instances.

This explains the organization of this module:

The 'a arbitrary record type describes how to generate random values, shrink them (reduce counter-examples to a minimum), print them, etc. It is the generator type expected by Test.make.

Auxiliary modules such as Gen, Print, and Shrink can be used along with make to build custom generators.

Test is used to describe a single test, that is, a property of type 'a -> bool combined with an 'a arbitrary that is used to generate the test cases for this property. Optional parameters allow to specify the random generator state, number of instances to generate and test, etc.

Examples

List.rev is involutive:

let test =
  QCheck.(Test.make ~count:1000
            (list int) (fun l -> List.rev (List.rev l) = l));;

QCheck.Test.check_exn test;;

Not all lists are sorted (false property that will fail. The 15 smallest counter-example lists will be printed):

let test = QCheck.(
    Test.make
      ~count:10_000 ~max_fail:3
      (list small_nat)
      (fun l -> l = List.sort compare l));;
QCheck.Test.check_exn test;;

generate 20 random trees using Gen.fix :

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

let leaf x = Leaf x
let node x y = Node (x,y)

let g = 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))]
                      ))

    Gen.generate ~n:20 g;;

More complex and powerful combinators can be found in Gabriel Scherer's Generator module. Its documentation can be found here.

Assumptions

val (==>) : bool -> bool -> bool

b1 ==> b2 is the logical implication b1 => b2 ie not b1 || b2 (except that it is strict and will interact better with Test.check_exn and the likes, because they will know the precondition was not satisfied.).

WARNING: this function should only be used in a property (see Test.make), because it raises a special exception in case of failure of the first argument, to distinguish between failed test and failed precondition. Because of OCaml's evaluation order, both b1 and b2 are always evaluated; if b2 should only be evaluated when b1 holds, see assume.

val assume : bool -> unit

assume cond checks the precondition cond, and does nothing if cond=true. If cond=false, it interrupts the current test.

WARNING This function, like (==>), should only be used in a test, not outside. Example:

Test.make (list int) (fun l ->
    assume (l <> []);
    List.hd l :: List.tl l = l)

since 0.5.1

val assume_fail : unit -> 'a

assume_fail () is like assume false, but can take any type since we know it always fails (like assert false). This is useful to ignore some branches in if or match.

Example:

Test.make (list int) (function
    | [] -> assume_fail ()
    | _::_ as l -> List.hd l :: List.tl l = l)

since 0.5.1

Generate Random Values

module Gen : sig ... end

The Gen module offers combinators to build custom generators. Unlike the the 'a arbitrary record type, which comes with printers, shrinkers, etc. Gen.t represents a type for generation only.

Printing Values

module Print : sig ... end

The Print module offers combinators for printing generated values.

Shrinking Values

Shrinking is used to reduce the size of a counter-example. It tries to make the counter-example smaller, e.g., by decreasing an integer, or removing elements of a list, until the property to test holds again; it then returns the smallest value that still made the test fail.

Shrinking is defined as a type Shrink.t that takes an argument to shrink and produces an iterator of type Iter.t of shrinking candidates.

Iterators

module Iter : sig ... end

Iter is compatible with the library "sequence". An iterator i is simply a function that accepts another function f (of type 'a -> unit) and calls f on a sequence of elements f x1; f x2; ...; f xn.

Shrinkers

module Shrink : sig ... end

The Shrink module contains combinators to build up composite shrinkers for user-defined types

Arbitrary

A value of type 'a arbitrary glues together a random generator, and optional functions for shrinking, printing, computing the size, etc. It is the "normal" way of describing how to generate values of a given type, to be then used in tests (see Test).

type 'a stat = string * ('a -> int)

A statistic on a distribution of values of type 'a. The function MUST return a positive integer.

type 'a arbitrary = private {

gen : 'a Gen.t;
print : 'a Print.t option;
(*
print values
*)
small : ('a -> int) option;
(*
size of example
*)
shrink : 'a Shrink.t option;
(*
shrink to smaller examples
*)
collect : ('a -> string) option;
(*
map value to tag, and group by tag
*)
stats : 'a stat list;
(*
statistics to collect and print
*)

}

A value of type 'a arbitrary is an object with a method for generating random values of type 'a, and additional methods to compute the size of values, print them, and possibly shrink them into smaller counter-examples.

NOTE the collect field is unstable and might be removed, or moved into Test.

Made private since 0.8

val make : 
  ?print:'a Print.t ->
  ?small:('a -> int) ->
  ?shrink:'a Shrink.t ->
  ?collect:('a -> string) ->
  ?stats:'a stat list ->
  'a Gen.t ->
  'a arbitrary

Builder for arbitrary. Default is to only have a generator, but other arguments can be added.

parameter print
printer for values (counter-examples)

parameter collect
for statistics

parameter shrink
to shrink counter-examples

Adjusting arbitrary generators

There is a range to get and set fields on an arbitrary record type.

val set_print : 'a Print.t -> 'a arbitrary -> 'a arbitrary

val set_small : ('a -> int) -> 'a arbitrary -> 'a arbitrary

val set_shrink : 'a Shrink.t -> 'a arbitrary -> 'a arbitrary

val set_collect : ('a -> string) -> 'a arbitrary -> 'a arbitrary

val set_stats : 'a stat list -> 'a arbitrary -> 'a arbitrary

since 0.6

val add_shrink_invariant : ('a -> bool) -> 'a arbitrary -> 'a arbitrary

Update shrinker by only keeping smaller values satisfying the given invariant.

since 0.8

val set_gen : 'a Gen.t -> 'a arbitrary -> 'a arbitrary

Change the generator

since 0.7

val add_stat : 'a stat -> 'a arbitrary -> 'a arbitrary

Add a statistic to the arbitrary instance.

since 0.6

val gen : 'a arbitrary -> 'a Gen.t

Access the underlying random generator of this arbitrary object.

since 0.6

val get_gen : 'a arbitrary -> 'a Gen.t

Access the underlying random generator of this arbitrary object.

since 0.6

val get_print : 'a arbitrary -> 'a Print.t option

Primitive combinators for arbitrary

val unit : unit arbitrary

Always generates (), obviously.

val bool : bool arbitrary

Uniform boolean generator.

val float : float arbitrary

Generates regular floats (no nan and no infinities).

val pos_float : float arbitrary

Positive float generator (no nan and no infinities).

val neg_float : float arbitrary

Negative float generator (no nan and no infinities).

val float_bound_inclusive : float -> float arbitrary

float_bound_inclusive n is uniform between 0 and n included. If bound is negative, the result is negative or zero. If bound is 0, the result is 0.

since 0.11

val float_bound_exclusive : float -> float arbitrary

float_bound_exclusive n is uniform between 0 included and n excluded. If bound is negative, the result is negative or zero.

raises Invalid_argument
if bound is zero.

since 0.11

val float_range : float -> float -> float arbitrary

float_range low high is uniform between low included and high included.

raises Invalid_argument
if low > high or if the range is larger than max_float.

since 0.11

val exponential : float -> float arbitrary

exponential m generates floating-point numbers following an exponential distribution with a mean of m.

raises Invalid_argument
if m is NaN.

since 0.23

val int : int arbitrary

Int generator. Uniformly distributed.

val int_bound : int -> int arbitrary

int_bound n is uniform between 0 and n included.

val int_range : int -> int -> int arbitrary

int_range a b is uniform between a and b included. b must be larger than a.

val small_nat : int arbitrary

Small unsigned integers.

since 0.5.1

val small_int : int arbitrary

Small unsigned integers. See Gen.small_int.

deprecated
use small_signed_int.

val small_signed_int : int arbitrary

Small signed integers.

since 0.5.2

val (--) : int -> int -> int arbitrary

Synonym for int_range.

val int32 : int32 arbitrary

Int32 generator. Uniformly distributed.

val int64 : int64 arbitrary

Int64 generator. Uniformly distributed.

val pos_int : int arbitrary

Positive int generator (0 included). Uniformly distributed. See Gen.pint

val small_int_corners : unit -> int arbitrary

As small_int, but each newly created generator starts with a list of corner cases before falling back on random generation.

val neg_int : int arbitrary

Negative int generator (0 included, see Gen.neg_int). The distribution is similar to that of small_int, not of pos_int.

val char : char arbitrary

Uniformly distributed on all the chars (not just ascii or valid latin-1).

val printable_char : char arbitrary

Uniformly distributed over a subset of printable ascii chars. Ascii character codes 32 to 126, inclusive - or '\n' with code 10.

val numeral_char : char arbitrary

Uniformly distributed over '0'..'9'.

val bytes_gen_of_size : int Gen.t -> char Gen.t -> bytes arbitrary

Builds a bytes generator from a (non-negative) size generator and a character generator.

since 0.20

val bytes_of : char Gen.t -> bytes arbitrary

Generates bytes with a distribution of length of Gen.nat.

since 0.20

val bytes : bytes arbitrary

Generates bytes with a distribution of length of Gen.nat and distribution of characters of char.

since 0.20

val bytes_small : bytes arbitrary

Same as bytes but with a small length (ie Gen.small_nat ).

since 0.20

val bytes_small_of : char Gen.t -> bytes arbitrary

Same as bytes_of but with a small length (ie Gen.small_nat ).

since 0.20

val bytes_of_size : int Gen.t -> bytes arbitrary

Generates bytes with distribution of characters of char.

since 0.20

val bytes_printable : bytes arbitrary

Generates bytes with a distribution of length of Gen.nat and distribution of characters of printable_char.

since 0.20

val string_gen_of_size : int Gen.t -> char Gen.t -> string arbitrary

Builds a string generator from a (non-negative) size generator and a character generator.

val string_gen : char Gen.t -> string arbitrary

Generates strings with a distribution of length of Gen.nat.

val string_of : char Gen.t -> string arbitrary

Synonym for string_gen added for convenience.

since 0.20

val string : string arbitrary

Generates strings with a distribution of length of Gen.nat and distribution of characters of char.

val small_string : string arbitrary

Same as string but with a small length (ie Gen.small_nat ).

val string_small : string arbitrary

Synonym for small_string added for convenience.

since 0.20

val string_small_of : char Gen.t -> string arbitrary

Same as string_of but with a small length (ie Gen.small_nat ).

since 0.20

val small_list : 'a arbitrary -> 'a list arbitrary

Generates lists of small size (see Gen.small_nat).

since 0.5.3

val string_of_size : int Gen.t -> string arbitrary

Generates strings with distribution of characters of char.

val printable_string : string arbitrary

Generates strings with a distribution of length of Gen.nat and distribution of characters of printable_char.

val string_printable : string arbitrary

Synonym for printable_string added for convenience.

since 0.20

val printable_string_of_size : int Gen.t -> string arbitrary

Generates strings with distribution of characters of printable_char.

val string_printable_of_size : int Gen.t -> string arbitrary

Synonym for printable_string_of_size added for convenience.

since 0.20

val small_printable_string : string arbitrary

Generates strings with a length of small_nat and distribution of characters of printable_char.

val string_small_printable : string arbitrary

Synonym for small_printable_string added for convenience.

since 0.20

val numeral_string : string arbitrary

Generates strings with a distribution of length of Gen.nat and distribution of characters of numeral_char.

val string_numeral : string arbitrary

Synonym for numeral_string added for convenience.

since 0.20

val numeral_string_of_size : int Gen.t -> string arbitrary

Generates strings with a distribution of characters of numeral_char.

val string_numeral_of_size : int Gen.t -> string arbitrary

Synonym for numeral_string_of_size added for convenience.

since 0.20

val list : 'a arbitrary -> 'a list arbitrary

Generates lists with length generated by Gen.nat.

val list_of_size : int Gen.t -> 'a arbitrary -> 'a list arbitrary

Generates lists with length from the given distribution.

val array : 'a arbitrary -> 'a array arbitrary

Generates arrays with length generated by Gen.nat.

val array_of_size : int Gen.t -> 'a arbitrary -> 'a array arbitrary

Generates arrays with length from the given distribution.

val option : ?ratio:float -> 'a arbitrary -> 'a option arbitrary

Choose between returning Some random value with optional ratio, or None.

Tuples of arbitrary generators

These shrink on gen1, then gen2, then ...

val pair : 'a arbitrary -> 'b arbitrary -> ('a * 'b) arbitrary

Combines two generators into a generator of pairs. Order of elements can matter (w.r.t shrinking, see Shrink.pair)

val triple : 
  'a arbitrary ->
  'b arbitrary ->
  'c arbitrary ->
  ('a * 'b * 'c) arbitrary

Combines three generators into a generator of 3-tuples. Order matters for shrinking, see Shrink.pair and the likes

val quad : 
  'a arbitrary ->
  'b arbitrary ->
  'c arbitrary ->
  'd arbitrary ->
  ('a * 'b * 'c * 'd) arbitrary

Combines four generators into a generator of 4-tuples. Order matters for shrinking, see Shrink.pair and the likes

val tup2 : 'a arbitrary -> 'b arbitrary -> ('a * 'b) arbitrary

Combines two generators into a 2-tuple generator. Order of elements can matter (w.r.t shrinking, see Shrink.tup2) Prints as many elements as available printers

val tup3 : 
  'a arbitrary ->
  'b arbitrary ->
  'c arbitrary ->
  ('a * 'b * 'c) arbitrary

Combines three generators into a 3-tuple generator. Order of elements can matter (w.r.t shrinking, see Shrink.tup2) Prints as many elements as available printers

val tup4 : 
  'a arbitrary ->
  'b arbitrary ->
  'c arbitrary ->
  'd arbitrary ->
  ('a * 'b * 'c * 'd) arbitrary

Combines four generators into a 4-tuple generator. Order of elements can matter (w.r.t shrinking, see Shrink.tup2) Prints as many elements as available printers

val tup5 : 
  'a arbitrary ->
  'b arbitrary ->
  'c arbitrary ->
  'd arbitrary ->
  'e arbitrary ->
  ('a * 'b * 'c * 'd * 'e) arbitrary

Combines five generators into a 5-tuple generator. Order of elements can matter (w.r.t shrinking, see Shrink.tup2) Prints as many elements as available printers

val tup6 : 
  'a arbitrary ->
  'b arbitrary ->
  'c arbitrary ->
  'd arbitrary ->
  'e arbitrary ->
  'f arbitrary ->
  ('a * 'b * 'c * 'd * 'e * 'f) arbitrary

Combines six generators into a 6-tuple generator. Order of elements can matter (w.r.t shrinking, see Shrink.tup2) Prints as many elements as available printers

val tup7 : 
  'a arbitrary ->
  'b arbitrary ->
  'c arbitrary ->
  'd arbitrary ->
  'e arbitrary ->
  'f arbitrary ->
  'g arbitrary ->
  ('a * 'b * 'c * 'd * 'e * 'f * 'g) arbitrary

Combines seven generators into a 7-tuple generator. Order of elements can matter (w.r.t shrinking, see Shrink.tup2) Prints as many elements as available printers

val tup8 : 
  'a arbitrary ->
  'b arbitrary ->
  'c arbitrary ->
  'd arbitrary ->
  'e arbitrary ->
  'f arbitrary ->
  'g arbitrary ->
  'h arbitrary ->
  ('a * 'b * 'c * 'd * 'e * 'f * 'g * 'h) arbitrary

Combines eight generators into a 8-tuple generator. Order of elements can matter (w.r.t shrinking, see Shrink.tup2) Prints as many elements as available printers

val tup9 : 
  'a arbitrary ->
  'b arbitrary ->
  'c arbitrary ->
  'd arbitrary ->
  'e arbitrary ->
  'f arbitrary ->
  'g arbitrary ->
  'h arbitrary ->
  'i arbitrary ->
  ('a * 'b * 'c * 'd * 'e * 'f * 'g * 'h * 'i) arbitrary

Combines nine generators into a 9-tuple generator. Order of elements can matter (w.r.t shrinking, see Shrink.tup2) Prints as many elements as available printers

Combinatoric arbitrary combinators

val choose : 'a arbitrary list -> 'a arbitrary

Choose among the given list of generators. The list must not be empty; if it is Invalid_argument is raised.

val oneofl : 
  ?print:'a Print.t ->
  ?collect:('a -> string) ->
  'a list ->
  'a arbitrary

Pick an element randomly in the list.

val oneofa : 
  ?print:'a Print.t ->
  ?collect:('a -> string) ->
  'a array ->
  'a arbitrary

Pick an element randomly in the array.

val oneof : 'a arbitrary list -> 'a arbitrary

Pick a generator among the list, randomly.

deprecated
this function is badly specified and will not use shrinkers appropriately. Consider using Gen.oneof and then make to build a well behaved arbitrary instance.

val always : ?print:'a Print.t -> 'a -> 'a arbitrary

Always return the same element.

val frequency : 
  ?print:'a Print.t ->
  ?small:('a -> int) ->
  ?shrink:'a Shrink.t ->
  ?collect:('a -> string) ->
  (int * 'a arbitrary) list ->
  'a arbitrary

Similar to oneof but with frequencies.

val frequencyl : 
  ?print:'a Print.t ->
  ?small:('a -> int) ->
  (int * 'a) list ->
  'a arbitrary

Same as oneofl, but each element is paired with its frequency in the probability distribution (the higher, the more likely).

val frequencya : 
  ?print:'a Print.t ->
  ?small:('a -> int) ->
  (int * 'a) array ->
  'a arbitrary

Same as frequencyl, but with an array.

val map : ?rev:('b -> 'a) -> ('a -> 'b) -> 'a arbitrary -> 'b arbitrary

map f a returns a new arbitrary instance that generates values using a#gen and then transforms them through f.

parameter rev
if provided, maps values back to type 'a so that the printer, shrinker, etc. of a can be used. We assume f is monotonic in this case (that is, smaller inputs are transformed into smaller outputs).

val map_same_type : ('a -> 'a) -> 'a arbitrary -> 'a arbitrary

Specialization of map when the transformation preserves the type, which makes shrinker, printer, etc. still relevant.

val map_keep_input : 
  ?print:'b Print.t ->
  ?small:('b -> int) ->
  ('a -> 'b) ->
  'a arbitrary ->
  ('a * 'b) arbitrary

map_keep_input f a generates random values from a, and maps them into values of type 'b using the function f, but it also keeps the original value. For shrinking, it is assumed that f is monotonic and that smaller input values will map into smaller values.

parameter print
optional printer for the f's output.

Tests

A test is a universal property of type foo -> bool for some type foo, with an object of type foo arbitrary used to generate, print, etc. values of type foo.

The main features of this module are:

Test.make to build a test,
Test.make_neg to build a negative test that is expected not to satisfy the tested property,
Test.check_exn to run a single test with a simple runner.

A test fails if the property does not hold for a given input. The simple form or the rich form) offer more elaborate forms to fail a test.

For more serious testing, it is recommended to create a testsuite and use a full-fledged runner:

QCheck_base_runner is a QCheck-only runner (useful if you don't have or don't need another test framework)
QCheck_alcotest interfaces to the Alcotest framework
QCheck_ounit interfaces to the to OUnit framework

Test Results

module TestResult : sig ... end

Module to represent the result of running a test

Defining Tests

module Test : sig ... end

Module related to individual tests. Since 0.18 most of it moved to QCheck2, and the type 'a cell was made a private implementation detail.

Sub-tests

The infrastructure used to find counter-examples to properties can also be used to find data satisfying a predicate, within a property being tested.

See https://github.com/c-cube/qcheck/issues/31

exception No_example_found of string

val find_example : 
  ?name:string ->
  ?count:int ->
  f:('a -> bool) ->
  'a Gen.t ->
  'a Gen.t

find_example ~f gen uses gen to generate some values of type 'a, and checks them against f. If such a value is found, it is returned. Otherwise an exception is raised. NOTE this should only be used from within a property in Test.make.

parameter count
number of attempts.

parameter name
description of the example to find (used in the exception).

parameter f
the property that the example must satisfy.

raises No_example_found
if no example is found within count tries.

since 0.6

val find_example_gen : 
  ?rand:Stdlib.Random.State.t ->
  ?name:string ->
  ?count:int ->
  f:('a -> bool) ->
  'a Gen.t ->
  'a

Toplevel version of find_example. find_example_gen ~f arb ~n is roughly the same as Gen.generate1 (find_example ~f arb |> gen).

parameter rand
the random state to use to generate inputs.

raises No_example_found
if no example was found within count tries.

since 0.6

Generating Functions

The QCheck module supports generation of pure function values. The implementation is inspired from https://blogs.janestreet.com/quickcheck-for-core/ and Koen Claessen's "Shrinking and Showing Functions".

Generated function arguments are of type Observable.t and function results are of type arbitrary.

Underneath the hood, generated function values have a table-based representation. They therefore need to be applied in a special way, e.g., with Fn.apply.

Observing arguments

module Observable : sig ... end

Observables are usable as arguments for random functions. The random function will observe its arguments in a way that is determined from the observable instance.

Deprecated function generator combinators

val fun1_unsafe : 'a arbitrary -> 'b arbitrary -> ('a -> 'b) arbitrary

Generator of functions of arity 1. The functions are always pure and total functions:

when given the same argument (as decided by Stdlib.(=)), it returns the same value
it never does side effects, like printing or never raise exceptions etc. The functions generated are really printable.

renamed from fun1 since 0.6

deprecated
use fun_ instead.

since 0.6

val fun2_unsafe : 
  'a arbitrary ->
  'b arbitrary ->
  'c arbitrary ->
  ('a -> 'b -> 'c) arbitrary

Generator of functions of arity 2. The remark about fun1 also apply here. renamed from fun2 since 0.6

deprecated
use fun_ instead since 0.6

type _ fun_repr

Internal data for functions. A 'f fun_ is a function of type 'f, fundamentally.

type _ fun_ =

| Fun : 'f fun_repr * 'f -> 'f fun_

A function packed with the data required to print/shrink it. See Fn to see how to apply, print, etc. such a function.

One can also directly pattern match on it to obtain the executable function.

For example:

QCheck.Test.make
  QCheck.(pair (fun1 Observable.int bool) (small_list int))
  (fun (Fun (_,f), l) -> l=(List.rev_map f l |> List.rev l))

module Fn : sig ... end

A utility module of helpers for printing, shrinking, and applying generated function values.

Defining function generators

val fun1 : 'a Observable.t -> 'b arbitrary -> ('a -> 'b) fun_ arbitrary

fun1 o ret makes random functions that take an argument observable via o and map to random values generated from ret. To write functions with multiple arguments, it's better to use Tuple or Observable.pair rather than applying fun_ several times (shrinking will be faster).

since 0.6

val fun2 : 
  'a Observable.t ->
  'b Observable.t ->
  'c arbitrary ->
  ('a -> 'b -> 'c) fun_ arbitrary

since 0.6

val fun3 : 
  'a Observable.t ->
  'b Observable.t ->
  'c Observable.t ->
  'd arbitrary ->
  ('a -> 'b -> 'c -> 'd) fun_ arbitrary

since 0.6

val fun4 : 
  'a Observable.t ->
  'b Observable.t ->
  'c Observable.t ->
  'd Observable.t ->
  'e arbitrary ->
  ('a -> 'b -> 'c -> 'd -> 'e) fun_ arbitrary

since 0.6

Tuples of observables

To circumvent the arity boundaries of fun1, ..., fun4, one can instead define uncurried functions, instead accepting a tuple argument. A resulting function then needs to be applied with fun_nary.

module Tuple : sig ... end

val fun_nary : 
  'a Tuple.obs ->
  'b arbitrary ->
  ('a Tuple.t -> 'b) fun_ arbitrary

fun_nary makes random n-ary functions. Example:

let module O = Observable in
fun_nary Tuple.(O.int @-> O.float @-> O.string @-> o_nil) bool

since 0.6