Module QCheck

module QCheck: sig .. end

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:

Examples:

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

QCheck.Test.check_exn test;;

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;;

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 QCheck.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 QCheck.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 QCheck.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 QCheck.(==>), 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
Generate Random Values

Pretty printing


module Print: sig .. end
Show Values
module Iter: sig .. end
Iterators
module Shrink: sig .. end
Shrink Values

Observe Values



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.

Inspired from https://blogs.janestreet.com/quickcheck-for-core/ and Koen Claessen's "Shrinking and Showing functions".

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 QCheck.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 = {
   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 QCheck.Test.

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.
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_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 QCheck.Test.make to build a test, and QCheck.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



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 QCheck.Test.make.
Since 0.6
Raises No_example_found if no example is found within count tries.
name : description of the example to find (used in the exception).
count : number of attempts.
f : the property that the example must satisfy.
val find_example_gen : ?rand:Random.State.t ->
?name:string -> ?count:int -> f:('a -> bool) -> 'a Gen.t -> 'a
Toplevel version of QCheck.find_example. find_example_gen ~f arb ~n is roughly the same as Gen.generate1 (find_example ~f arb |> gen).
Since 0.6
Raises No_example_found if no example was found within count tries.
rand : the random state to use to generate inputs.

Combinators for QCheck.arbitrary


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 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
Deprecated.use QCheck.small_signed_int.
Small unsigned integers. See QCheck.Gen.small_int.
val small_signed_int : int arbitrary
Small signed integers.
Since 0.5.2
val (--) : int -> int -> int arbitrary
Synonym to QCheck.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. Uniformly distributed.
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. 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 QCheck.string but with a small length (that is, 0--10).
val small_list : 'a arbitrary -> 'a list arbitrary
Generates lists of small size (range 0 -- 10).
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 QCheck.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 QCheck.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 QCheck.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
Deprecated.use QCheck.fun_ instead.
Generator of functions of arity 1. The functions are always pure and total functions:
Since 0.6
val fun2_unsafe : 'a arbitrary ->
'b arbitrary ->
'c arbitrary -> ('a -> 'b -> 'c) arbitrary
Deprecated.use QCheck.fun_ instead.
Generator of functions of arity 2. The remark about fun1 also apply here. renamed from QCheck.fun2.
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 QCheck.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 QCheck.Tuple or QCheck.Observable.pair rather than applying QCheck.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 QCheck.oneof but with frequencies.
val frequencyl : ?print:'a Print.t ->
?small:('a -> int) -> (int * 'a) list -> 'a arbitrary
Same as QCheck.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 QCheck.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.
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.
print : optional printer for the f's output.