Module QCheck.Gen

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.

type 'a t = Stdlib.Random.State.t -> 'a

A random generator for values of type 'a.

type 'a sized = int -> Stdlib.Random.State.t -> 'a

Random generator with a size bound.

val return : 'a -> 'a t

Create a constant generator.

val pure : 'a -> 'a t

Synonym for return

  • since 0.8
val bind : 'a t -> ('a -> 'b t) -> 'b t

Monadic bind for writing dependent generators. First generates an 'a and then passes it to the given function, to generate a 'b.

  • since 0.90
val (>>=) : 'a t -> ('a -> 'b t) -> 'b t

Synonym for bind since 0.90

val ap : ('a -> 'b) t -> 'a t -> 'b t

Applicative operator for composing a function generator and an argument generator into a result generator.

val (<*>) : ('a -> 'b) t -> 'a t -> 'b t

Synonym for ap since 0.90

val map : ('a -> 'b) -> 'a t -> 'b t

map f g transforms a generator g by applying f to each generated element.

val map2 : ('a -> 'b -> 'c) -> 'a t -> 'b t -> 'c t

map2 f g1 g2 transforms two generators g1 and g2 by applying f to each pair of generated elements.

val map3 : ('a -> 'b -> 'c -> 'd) -> 'a t -> 'b t -> 'c t -> 'd t

map3 f g1 g2 g3 transforms three generators g1, g2, and g3 by applying f to each triple of generated elements.

val map4 : ('a -> 'b -> 'c -> 'd -> 'e) -> 'a t -> 'b t -> 'c t -> 'd t -> 'e t

map4 f g1 g2 g3 g4 transforms four generators g1, g2, g3, and g4 by applying f to each quadruple of generated elements.

  • since 0.25
val map5 : ('a -> 'b -> 'c -> 'd -> 'e -> 'f) -> 'a t -> 'b t -> 'c t -> 'd t -> 'e t -> 'f t

map5 f g1 g2 g3 g4 g5 transforms five generators g1, g2, g3, g4, and g5 by applying f to each quintuple of generated elements.

  • since 0.25
val map_keep_input : ('a -> 'b) -> 'a t -> ('a * 'b) t

map_keep_input f g transforms a generator g by applying f to each generated element. Returns both the generated element from g and the output from f.

val (>|=) : 'a t -> ('a -> 'b) -> 'b t

An infix synonym for map.

val (<$>) : ('a -> 'b) -> 'a t -> 'b t

An infix synonym for map

  • since 0.13
val oneof : 'a t list -> 'a t

Constructs a generator that selects among a given list of generators.

  • raises Invalid_argument

    or Failure if list is empty

val oneof_list : 'a list -> 'a t

Constructs a generator that selects among a given list of values.

  • raises Invalid_argument

    or Failure if list is empty

  • since 0.90
val oneofl : 'a list -> 'a t

Constructs a generator that selects among a given list of values.

  • raises Invalid_argument

    or Failure if list is empty

val oneof_array : 'a array -> 'a t

Constructs a generator that selects among a given array of values.

  • raises Invalid_argument

    or Failure if list is empty

  • since 0.90
val oneofa : 'a array -> 'a t

Constructs a generator that selects among a given array of values.

  • raises Invalid_argument

    or Failure if list is empty

val oneof_weighted : (int * 'a t) list -> 'a t

Constructs a generator that selects among a given list of generators. Each of the given generators are chosen based on a positive integer weight.

  • since 0.90
val frequency : (int * 'a t) list -> 'a t

Constructs a generator that selects among a given list of generators. Each of the given generators are chosen based on a positive integer weight.

val oneof_list_weighted : (int * 'a) list -> 'a t

Constructs a generator that selects among a given list of values. Each of the given values are chosen based on a positive integer weight.

  • since 0.90
val frequencyl : (int * 'a) list -> 'a t

Constructs a generator that selects among a given list of values. Each of the given values are chosen based on a positive integer weight.

val oneof_array_weighted : (int * 'a) array -> 'a t

Constructs a generator that selects among a given array of values. Each of the array entries are chosen based on a positive integer weight.

  • since 0.90
val frequencya : (int * 'a) array -> 'a t

Constructs a generator that selects among a given array of values. Each of the array entries are chosen based on a positive integer weight.

val shuffle_array : 'a array -> 'a array t

Creates a generator of shuffled arrays.

  • since 0.90
val shuffle_a : 'a array -> unit t

Shuffles the array in place.

val shuffle_list : 'a list -> 'a list t

Creates a generator of shuffled lists.

  • since 0.90
val shuffle_l : 'a list -> 'a list t

Creates a generator of shuffled lists.

val shuffle_list_weighted : (int * 'a) list -> 'a list t

Creates a generator of weighted shuffled lists. A given list is shuffled on each generation according to the weights of its elements. An element with a larger weight is more likely to be at the front of the list than an element with a smaller weight. If we want to pick random elements from the (head of) list but need to prioritize some elements over others, this generator can be useful.

Example: given a weighted list [1, "one"; 5, "five"; 10, "ten"], the generator is more likely to generate ["ten"; "five"; "one"] or ["five"; "ten"; "one"] than ["one"; "ten"; "five"] because "ten" and "five" have larger weights than "one".

  • since 0.90
val shuffle_w_l : (int * 'a) list -> 'a list t

An alias for shuffle_list_weighted since 0.90.

  • since 0.11
val range_subset : size:int -> int -> int -> int array t

range_subset ~size:k low high generates an array of length k of sorted distinct integers in the range low..high (included).

Complexity O(k log k), drawing k random integers.

  • raises Invalid_argument

    outside the valid region 0 <= k <= high-low+1.

  • since 0.18
val array_subset : int -> 'a array -> 'a array t

array_subset k arr generates a sub-array of k elements at distinct positions in the input array arr, in the same order.

Complexity O(k log k), drawing k random integers.

  • raises Invalid_argument

    outside the valid region 0 <= size <= Array.length arr.

  • since 0.18

Primitive generators

val unit : unit t

The unit generator.

val bool : bool t

The boolean generator.

val float : float t

Generates floating point numbers.

val float_pos : float t

Generates positive floating point numbers (0. included).

val float_neg : float t

Generates negative floating point numbers (-0. included).

val pfloat : float t

Generates positive floating point numbers (0. included).

val nfloat : float t

Generates negative floating point numbers (-0. included).

val float_bound_inclusive : float -> float t

float_bound_inclusive bound returns a random floating-point number between 0 and bound (inclusive). 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 t

float_bound_exclusive bound returns a random floating-point number between 0 and bound (exclusive). 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 t

float_range low high generates floating-point numbers within low and high (inclusive)

  • raises Invalid_argument

    if high < low or if the range is larger than max_float.

  • since 0.11
val (--.) : float -> float -> float t

Synonym for float_range

  • since 0.11
val float_exp : float -> float t

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

  • raises Invalid_argument

    if m is NaN.

  • since 0.90
val exponential : float -> float t

Synonym for float_exp.

  • since 0.23
val nat : int t

Generates small natural numbers.

val int_pos_mid : int t

Generates small natural numbers. Synonym for nat.

  • since 0.90
val big_nat : int t

Generates natural numbers, possibly large.

  • since 0.10
  • deprecated

    use map abs int instead.

val int_neg : int t

Generates strictly negative integers uniformly (0 excluded).

  • since 0.90
val neg_int : int t

Generates non-strictly negative integers (0 included).

  • deprecated

    use int_neg or map (fun i -> -i) nat instead.

val int_pos : int t

Generates non-strictly positive integers uniformly (0 included).

  • since 0.90
val pint : int t

Generates non-strictly positive integers uniformly (0 included).

val int : int t

Generates integers uniformly.

val int_small : int t

Generated small signed integers.

  • since 0.90
val int_pos_small : int t

Small integers (< 100)

  • since 0.90
val nat_small : int t

Small integers (< 100) Synonym for int_pos_small.

  • since 0.90
val small_nat : int t

Small integers (< 100)

  • since 0.5.1
val small_int : int t

Small UNSIGNED integers, for retrocompatibility.

val small_signed_int : int t

Small SIGNED integers, based on small_nat.

  • since 0.5.2
val int_bound : int -> int t

Uniform integer generator producing integers between 0 and bound (inclusive). For bound < 2^{30} - 1 uses Random.State.int for integer generation.

  • raises Invalid_argument

    if the argument is negative.

val int_range : int -> int -> int t

Uniform integer generator producing integers within low,high (inclusive).

  • raises Invalid_argument

    if low > high.

val graft_corners : 'a t -> 'a list -> unit -> 'a t

graft_corners gen l () makes a new generator that enumerates the corner cases in l and then behaves like g.

Note that graft_corners gen l () is stateful, meaning that once the elements of l have been emitted, subsequent calls will not reproduce them. It is therefore recommended that separate tests each use a fresh generator.

  • since 0.6
val int_pos_corners : int list

Non-negative corner cases for int.

  • since 0.6
  • deprecated

    define your own list of corner cases instead

val int_corners : int list

All corner cases for int.

  • since 0.6
  • deprecated

    define your own list of corner cases instead

val int_small_corners : unit -> int t

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

Note that int_small_corners () is stateful, meaning that once the list of corner cases has been emitted, subsequent calls will not reproduce them.

  • since 0.90
val (--) : int -> int -> int t

Synonym for int_range.

val int32 : int32 t

Generates int32 values uniformly.

  • since 0.24
val int64 : int64 t

Generates int64 values uniformly.

  • since 0.24
val ui32 : int32 t

Generates int32 values.

  • deprecated

    use int32 instead, the name is wrong, values are signed.

val ui64 : int64 t

Generates int64 values.

  • deprecated

    use int64 instead, the name is wrong, values are signed.

val list : 'a t -> 'a list t

Builds a list generator from an element generator. List size is generated by nat.

val list_size : int t -> 'a t -> 'a list t

Builds a list generator from a (non-negative) size generator and an element generator.

val list_repeat : int -> 'a t -> 'a list t

list_repeat i g builds a list generator from exactly i elements generated by g.

  • deprecated

    use list_size (return i) g instead.

val array : 'a t -> 'a array t

Builds an array generator from an element generator. Array size is generated by nat.

val array_size : int t -> 'a t -> 'a array t

Builds an array generator from a (non-negative) size generator and an element generator.

val array_repeat : int -> 'a t -> 'a array t

array_repeat i g builds an array generator from exactly i elements generated by g.

  • deprecated

    use array_size (return i) g instead.

val option : ?ratio:float -> 'a t -> 'a option t

An option generator, with optional ratio.

  • parameter ratio

    a float between 0. and 1. indicating the probability of a sample to be Some _ rather than None.

  • since 0.19 (renamed from [opt])
val opt : ?ratio:float -> 'a t -> 'a option t

opt is an alias of option for backward compatibility.

  • since 0.18 ([?ratio] parameter)
  • deprecated

    use option instead.

val result : ?ratio:float -> 'a t -> 'e t -> ('a, 'e) Stdlib.result t

A result generator, with optional ratio.

  • parameter ratio

    a float between 0. and 1. indicating the probability of a sample to be Ok _ rather than Error _.

  • since 0.24
val char : char t

Generates characters upto character code 255.

val char_printable : char t

Generates printable ascii characters - either '\n' or in the range 32 to 126, inclusive.

  • since 0.90
val printable : char t

Synonym for char_printable.

val char_numeral : char t

Generates numeral characters uniformly distributed over '0'..'9'.

  • since 0.90
val numeral : char t

Synonym for char_numeral.

val char_range : char -> char -> char t

Generates chars between the two bounds, inclusive. Example: char_range 'a' 'z' for all lower case ascii letters.

  • since 0.13
val bytes_size : ?gen:char t -> int t -> bytes t

Builds a bytes generator from a (non-negative) size generator. Accepts an optional character generator (the default is char).

  • since 0.20
val bytes_size_of : int t -> char t -> bytes t

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

  • since 0.90
val bytes : bytes t

Builds a bytes generator. Bytes size is generated by nat and characters are generated by char.

Note: This had an optional ?gen parameter removed in 0.90. Use bytes_of instead to pass a char generator.

  • since 0.20
val bytes_of : char t -> bytes t

Builds a bytes generator using the given character generator.

  • since 0.20
val bytes_printable : bytes t

Generator using the printable character generator.

  • since 0.20
val bytes_small : bytes t

Builds a bytes generator using the char character generator, length is nat_small

  • since 0.20
val bytes_small_of : char t -> bytes t

Builds a bytes generator using the given character generator, length is nat_small.

  • since 0.20
val string_size : ?gen:char t -> int t -> string t

Builds a string generator from a (non-negative) size generator. Accepts an optional character generator (the default is char).

val string_size_of : int t -> char t -> string t

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

  • since 0.90
val string : string t

Builds a string generator. String size is generated by nat and characters are generated by char.

Note: This had an optional ?gen parameter removed in 0.90. Use string_of instead to pass a char generator.

val string_of : char t -> string t

Builds a string generator using the given character generator.

  • since 0.11
val string_readable : string t

Builds a string generator using the printable character generator.

  • since 0.11
val string_printable : string t

Builds a string generator using the printable character generator.

  • since 0.18
val small_string : ?gen:char t -> string t

Builds a string generator, length is nat_small Accepts an optional character generator (the default is char).

val string_small : string t

Builds a string generator using the char character generator, length is nat_small

  • since 0.20
val string_small_of : char t -> string t

Builds a string generator using the given character generator, length is nat_small.

  • since 0.20
val list_small : 'a t -> 'a list t

Generates lists of small size (see nat_small).

  • since 0.90
val small_list : 'a t -> 'a list t

Generates lists of small size (see nat_small).

  • since 0.5.3
val flatten_list : 'a t list -> 'a list t

Generate a list of elements from individual generators

  • since 0.90
val flatten_l : 'a t list -> 'a list t

Generate a list of elements from individual generators

  • since 0.13
val flatten_array : 'a t array -> 'a array t

Generate an array of elements from individual generators

  • since 0.90
val flatten_a : 'a t array -> 'a array t

Generate an array of elements from individual generators

  • since 0.13
val flatten_option : 'a t option -> 'a option t

Generate an option from an optional generator

  • since 0.90
val flatten_opt : 'a t option -> 'a option t

Generate an option from an optional generator

  • since 0.13
val flatten_result : ('a t, 'e) Stdlib.result -> ('a, 'e) Stdlib.result t

Generate a result from Ok g, an error from Error e

  • since 0.90
val flatten_res : ('a t, 'e) Stdlib.result -> ('a, 'e) Stdlib.result t

Generate a result from Ok g, an error from Error e

  • since 0.13
val array_small : 'a t -> 'a array t

Generates arrays of small size (see nat_small).

  • since 0.90
val small_array : 'a t -> 'a array t

Generates arrays of small size (see nat_small).

  • since 0.10

Tuple generators

Create tuple generators by composing individual element generators. For example, Gen.(tup3 int char bool) creates a (int * char * bool) triple generator by composing the int, char, and bool generators.

val pair : 'a t -> 'b t -> ('a * 'b) t

Generates pairs.

val triple : 'a t -> 'b t -> 'c t -> ('a * 'b * 'c) t

Generates triples.

val quad : 'a t -> 'b t -> 'c t -> 'd t -> ('a * 'b * 'c * 'd) t

Generates quadruples.

  • since 0.5.1
val tup2 : 'a t -> 'b t -> ('a * 'b) t

Combines two generators into a 2-tuple generator.

val tup3 : 'a t -> 'b t -> 'c t -> ('a * 'b * 'c) t

Combines three generators into a 3-tuple generator.

val tup4 : 'a t -> 'b t -> 'c t -> 'd t -> ('a * 'b * 'c * 'd) t

Combines four generators into a 4-tuple generator.

val tup5 : 'a t -> 'b t -> 'c t -> 'd t -> 'e t -> ('a * 'b * 'c * 'd * 'e) t

Combines five generators into a 5-tuple generator.

val tup6 : 'a t -> 'b t -> 'c t -> 'd t -> 'e t -> 'f t -> ('a * 'b * 'c * 'd * 'e * 'f) t

Combines six generators into a 6-tuple generator.

val tup7 : 'a t -> 'b t -> 'c t -> 'd t -> 'e t -> 'f t -> 'g t -> ('a * 'b * 'c * 'd * 'e * 'f * 'g) t

Combines seven generators into a 7-tuple generator.

val tup8 : 'a t -> 'b t -> 'c t -> 'd t -> 'e t -> 'f t -> 'g t -> 'h t -> ('a * 'b * 'c * 'd * 'e * 'f * 'g * 'h) t

Combines eight generators into an 8-tuple generator.

val tup9 : 'a t -> 'b t -> 'c t -> 'd t -> 'e t -> 'f t -> 'g t -> 'h t -> 'i t -> ('a * 'b * 'c * 'd * 'e * 'f * 'g * 'h * 'i) t

Combines nine generators into a 9-tuple generator.

val join : 'a t t -> 'a t

Collapses a generator of generators to simply a generator.

  • since 0.5
val sized : 'a sized -> 'a t

Creates a generator from a size-bounded generator by first generating a size using nat and passing the result to the size-bounded generator.

val sized_size : int t -> 'a sized -> 'a t

Creates a generator from a size-bounded generator by first generating a size using the integer generator and passing the result to the size-bounded generator.

  • since 0.5
val fix : (('a -> 'b t) -> 'a -> 'b t) -> 'a -> 'b t

Parametrized fixpoint combinator for generating recursive values.

The fixpoint is parametrized over an arbitrary state ('a), and the fixpoint computation may change the value of this state in the recursive calls.

In particular, this can be used for size-bounded generators ('a is int). The passed size-parameter should decrease to ensure termination.

Example:

  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 ->
                             oneof_weighted
                               [1, map leaf nat;
                                2, map2 node (self (n/2)) (self (n/2))]
                        ))
val nat_split2 : int -> (int * int) t

nat_split2 n generates pairs (n1, n2) of natural numbers with n1 + n2 = n.

This is useful to split sizes to combine sized generators.

  • since 0.18
val pos_split2 : int -> (int * int) t

pos_split2 n generates pairs (n1, n2) of strictly positive (nonzero) natural numbers with n1 + n2 = n.

  • raises Invalid_argument

    unless n >= 2.

This is useful to split sizes to combine sized generators.

  • since 0.18
val nat_split : size:int -> int -> int array t

nat_split ~size:k n generates k-sized arrays n1,n2,..nk of natural numbers in [0;n] with n1 + n2 + ... + nk = n.

This is useful to split sizes to combine sized generators.

Complexity O(k log k).

  • since 0.18
val pos_split : size:int -> int -> int array t

pos_split ~size:k n generates k-sized arrays n1,n2,..nk of strictly positive (non-zero) natural numbers with n1 + n2 + ... + nk = n.

This is useful to split sizes to combine sized generators.

Complexity O(k log k).

  • raises Invalid_argument

    unless 0 < k <= n or 0 = k = n.

  • since 0.18
val delay : (unit -> 'a t) -> 'a t

Delay execution of some code until the generator is actually called. This can be used to manually implement recursion or control flow in a generator.

  • since 0.17
val (let+) : 'a t -> ('a -> 'b) -> 'b t

Binding operator alias for map.

val (and+) : 'a t -> 'b t -> ('a * 'b) t

Binding operator alias for pair.

val (let*) : 'a t -> ('a -> 'b t) -> 'b t

Binding operator alias for (>>=).

val (and*) : 'a t -> 'b t -> ('a * 'b) t

Binding operator alias for pair.

Debug generators

These functions should not be used in tests: they are provided for convenience to debug/investigate what values a generator produces.

val generate : ?rand:Stdlib.Random.State.t -> n:int -> 'a t -> 'a list

generate ~n g generates n instances of g.

val generate1 : ?rand:Stdlib.Random.State.t -> 'a t -> 'a

generate1 g generates one instance of g.