Module `QCheck`

Quickcheck inspired property-based testing

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. She 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:

'a arbitrary is used to describe how to generate random values, shrink them (make counter-examples as small as possible), print them, etc. Auxiliary modules such as Gen, Print, and Shrink can be used along with make to build one's own arbitrary instances.

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.

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

module Gen : sig ... end

Pretty printing

module Print : sig ... end

module Iter : sig ... end

module Shrink : sig ... end

Observe Values

module Observable : sig ... end

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 -> string) 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

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

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.

See Test.make to build a test, and Test.check_exn to run one test simply. For more serious testing, it is better to create a testsuite and use QCheck_runner.

module TestResult : sig ... end: Result of running a test

module Test : sig ... end

Sub-tests

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: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

Combinators for

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 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 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 to 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 chars.

val numeral_char : char arbitrary: Uniformly distributed over '0'..'9'.

val string_gen_of_size : int Gen.t -> char Gen.t -> string arbitrary
val string_gen : char Gen.t -> string arbitrary: Generates strings with a distribution of length of small_nat.

val string : string arbitrary: Generates strings with a distribution of length of small_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 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 if char.

val printable_string : string arbitrary: Generates strings with a distribution of length of small_nat and distribution of characters of printable_char.

val printable_string_of_size : int Gen.t -> string arbitrary: Generates strings with distribution of characters of printable_char.

val small_printable_string : string arbitrary
val numeral_string : string arbitrary: Generates strings with a distribution of length of small_nat and distribution of characters of numeral_char.

val numeral_string_of_size : int Gen.t -> string arbitrary: Generates strings with a distribution of characters of numeral_char.

val list : 'a arbitrary -> 'a list arbitrary: Generates lists with length generated by small_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 small_nat.

val array_of_size : int Gen.t -> 'a arbitrary -> 'a array arbitrary: Generates arrays with length from the given distribution.

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 option : 'a arbitrary -> 'a option arbitrary: Choose between returning Some random value, or None.

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 Pervasives.(=)), 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: Utils on functions

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

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

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

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.

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.

`gen : 'a Gen.t;`
`print : ('a -> string) 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