generator-examples.md

Generator Examples

The following examples assume you have the following namespace alias:

(require '[clojure.test.check.generators :as gen])

For the most part, these are in order of simplest to most complex. They also skip over some of the built-in, basic generators.

Integers 5 through 9, inclusive

(def five-through-nine (gen/choose 5 9))
(gen/sample five-through-nine)
;; => (6 5 9 5 7 7 6 9 7 9)

A random element from a vector

(def languages (gen/elements ["clojure" "haskell" "erlang" "scala" "python"]))
(gen/sample languages)
;; => ("clojure" "scala" "clojure" "haskell" "clojure" "erlang" "erlang"
;; =>  "erlang" "haskell" "python")

An integer or nil

(def int-or-nil (gen/one-of [gen/small-integer (gen/return nil)]))
(gen/sample int-or-nil)
;; => (nil 0 -2 nil nil 3 nil nil 4 2)

An integer 90% of the time, nil 10%

(def mostly-ints (gen/frequency [[9 gen/small-integer] [1 (gen/return nil)]]))
(gen/sample mostly-ints)
;; => (0 -1 nil 0 -2 0 6 -6 8 7)

Even, positive integers

(def even-and-positive (gen/fmap #(* 2 %) gen/nat))
(gen/sample even-and-positive 20)
;; => (0 0 2 0 8 6 4 12 4 18 10 0 8 2 16 16 6 4 10 4)

Powers of two

;; generate exponents with gen/nat,
;; and then apply the function to them
(def powers-of-two (gen/fmap #(int (Math/pow 2 %)) gen/nat))
(gen/sample powers-of-two)
;; => (1 1 4 8 8 32 8 1 2 2)

Sorted seq of integers

;; apply the sort function to each generated vector
(def sorted-vec (gen/fmap sort (gen/vector gen/small-integer)))
(gen/sample sorted-vec)
;; => (() (-1) (-2 -2) (-1 2 3) (-1 2 4) (-3 2 3 3 4) (1)
;; => (-4 0 1 3 4 6) (-5 -4 -1 0 2 8) (1))

An integer and a boolean

(def int-and-boolean (gen/tuple gen/small-integer gen/boolean))
(gen/sample int-and-boolean)
;; => ([0 false] [0 true] [0 true] [3 true] [-3 false]
;; =>  [0 true] [4 true] [0 true] [-2 true] [-9 false])

Any number but 5

(def anything-but-five (gen/such-that #(not= % 5) gen/small-integer))
(gen/sample anything-but-five)
;; => (0 0 -2 1 -3 1 -4 7 -1 6)

It's important to note that such-that should only be used for predicates that are very likely to match. For example, you should not use such-that to filter out random vectors that are not sorted, as is this is exceedingly unlikely to happen randomly. If you want sorted vectors, just sort them using gen/fmap and sort.

A vector and a random element from it

(def vector-and-elem (gen/bind (gen/not-empty (gen/vector gen/small-integer))
                               #(gen/tuple (gen/return %) (gen/elements %))))
(gen/sample vector-and-elem)
;; =>([[-1] -1]
;; => [[0] 0]
;; => [[-1 -1] -1]
;; => [[2 0 -2] 2]
;; => [[0 1 1] 0]
;; => [[-2 -3 -1 1] -1]
;; => [[-1 2 -5] -5]
;; => [[5 -7 -3 7] 5]
;; => [[-1 2 2] 2]
;; => [[-8 7 -3 -2 -6] -3])

gen/bind and gen/fmap are similar: they're both binary functions that take a generator and a function as arguments (though their argument order is reversed). They differ in what the provided function's return value should be. The function provided to gen/fmap should return a value. We saw that earlier when we used gen/fmap to sort a vector. sort returns a normal value. The function provided to gen/bind should return a generator. Notice how above we're providing a function that returns a gen/tuple generator? The decision of which to use depends on whether you want to simply transform the value of a generator (sort it, multiply it by two, etc.), or create an entirely new generator out of it.

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