Generated values are provided by instances of the sealed class Gen. You can think of a Gen as kind of like an input stream but for property test values. Each Gen will provide a (usually) infinite stream of these values for one particular type.

Kotest has two types of generators - Arb for generating arbitrary (random) values and Exhaustive for generating a finite set of values in a closed space.

Both types of gens can be mixed and matched in property tests. For example, you could test a function with 100 random positive integers (an arb) alongside every even number from 0 to 200 (exhaustive).

Some generators are only available on the JVM. See the full list here.


Arbs generate two types of values - a hard coded set of edge cases and an infinite stream of randomly chosen samples.

The samples may be repeated, and some values may never be generated at all. For example generating 1000 integers between 0 and Int.MAX cannot return all possible values, and some values may happen to be generated more than once. Similarly, generating 1000 random integers between 0 and 500 will definitely result in some values appearing more than once.

Some common arbitraries include numbers with or without a range, strings in the unicode set, random lists, data classes with random parameters, emails, codepoints, chars and so on.

In addition to the random values, arbs may provide edge cases. One of the design features of Kotest's property testing is that values for some types will always include "common" edge cases that you probably want to be included in your tests.

For example, when testing a function that accepts an integer, you probably want to ensure that, at the very least, it is tested with zero, a positive number and a negative number. If only random values were provided, the chances of zero appearing would be fairly low, so Kotest will always provide some "edge cases" for integers (unless you specify otherwise).

When executing tests, the framework will alternate randomly between samples and edgecases. The split is determined by a configuration value which defaults to 2% edgecases.

Not all arbs have edge cases, but the arbs for the most common types do. Here are some examples of edge cases used by some arbs:

  • ints: 0, 1, -1, Int.MAX_VALUE, Int.MIN_VALUE
  • doubles: 0, 1, -1, Double.MAX_VALUE, Double.MIN_VALUE, Double.POSITIVE_INFINITY, Double.NEGATIVE_INFINITY, Double.NaN
  • strings: empty string, string of min length, lowest codepoint
  • lists: empty list, list of a single element, list with duplicate elements
  • maps: empty map
  • nullable values: null


Exhaustives generate all values from a given space. This is useful when you want to ensure every value in that space is used. For example, for enum values, it is usually more helpful to ensure each enum is used, rather than picking randomly from the enums values and potentially missing some and duplicating others.

Typical exhaustives include small collections, enums, boolean values, powerset of a list or set, pre-defined integer ranges, and predefined string ranges.

Once an exhaustive has provided all it's values, it will loop and start again, so an exhaustive can be used in a test that requires any number of inputs.

For example:

enum class Season { Winter, Fall, Spring Summer }
forAll<Int, Season>(100) { a, season -> ... }

Here we asked for 100 iterations, so each value of Season would be provided 25 times.