https://github.com/armcn/quickcheck

Property Based Testing in R

https://github.com/armcn/quickcheck

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Property Based Testing in R

README.Rmd

---
output: github_document
---



```{r, include = FALSE}
knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>",
  fig.path = "man/figures/README-",
  out.width = "100%"
)
```

# quickcheck 


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# Overview

Property based testing in R, inspired by [QuickCheck](https://en.wikipedia.org/wiki/QuickCheck). This package builds on the property based testing framework provided by [`hedgehog`](https://github.com/hedgehogqa/r-hedgehog) and is designed to seamlessly integrate with [`testthat`](https://testthat.r-lib.org).

## Installation

You can install the released version of `quickcheck` from [CRAN](https://CRAN.R-project.org) with:

```{r, eval=FALSE}
install.packages("quickcheck")
```

And the development version from [GitHub](https://github.com/) with:

```{r, eval=FALSE}
# install.packages("remotes")
remotes::install_github("armcn/quickcheck")
```

# Usage

The following example uses `quickcheck` to test the properties of the base R `+` function. [Here](https://fsharpforfunandprofit.com/posts/property-based-testing/) is an introduction to the concept of property based testing, and an explanation of the mathematical properties of addition can be found [here](https://www.khanacademy.org/math/cc-sixth-grade-math/cc-6th-factors-and-multiples/properties-of-numbers/a/properties-of-addition).

```{r}
library(testthat)
library(quickcheck)

test_that("0 is the additive identity of +", {
  for_all(
    a = numeric_(len = 1),
    property = function(a) expect_equal(a, a + 0)
  )
})

test_that("+ is commutative", {
  for_all(
    a = numeric_(len = 1),
    b = numeric_(len = 1),
    property = function(a, b) expect_equal(a + b, b + a)
  )
})

test_that("+ is associative", {
  for_all(
    a = numeric_(len = 1),
    b = numeric_(len = 1),
    c = numeric_(len = 1),
    property = function(a, b, c) expect_equal(a + (b + c), (a + b) + c)
  )
})
```

Here we test the properties of the [`distinct`](https://dplyr.tidyverse.org/reference/distinct.html) 
function from the [`dplyr`](https://dplyr.tidyverse.org/index.html) package.

```{r}
library(dplyr, warn.conflicts = FALSE)

test_that("distinct does nothing with a single row", {
  for_all(
    a = any_tibble(rows = 1L),
    property = function(a) {
      distinct(a) %>% expect_equal(a)
    }
  )
})

test_that("distinct returns single row if rows are repeated", {
  for_all(
    a = any_tibble(rows = 1L),
    property = function(a) {
      bind_rows(a, a) %>%
        distinct() %>%
        expect_equal(a)
    }
  )
})

test_that("distinct does nothing if rows are unique", {
  for_all(
    a = tibble_of(integer_positive(), rows = 1L, cols = 1L),
    b = tibble_of(integer_negative(), rows = 1L, cols = 1L),
    property = function(a, b) {
      unique_rows <- bind_rows(a, b)
      distinct(unique_rows) %>% expect_equal(unique_rows)
    }
  )
})
```

## Quickcheck generators

Many generators are provided with `quickcheck`. Here are a few examples.

### Atomic vectors

```{r}
integer_(len = 10) %>% show_example()
character_alphanumeric(len = 10) %>% show_example()
posixct_(len = 10, any_na = TRUE) %>% show_example()
```

### Lists

```{r}
list_(a = constant(NULL), b = any_undefined()) %>% show_example()
flat_list_of(logical_(), len = 3) %>% show_example()
```

### Tibbles

```{r}
tibble_(a = date_(), b = hms_(), rows = 5) %>% show_example()
tibble_of(double_bounded(-10, 10), rows = 3, cols = 3) %>% show_example()
any_tibble(rows = 3, cols = 3) %>% show_example()
```

## Hedgehog generators

`quickcheck` is meant to work with `hedgehog`, not replace it. `hedgehog` generators
can be used by wrapping them in `from_hedgehog`. 

```{r}
library(hedgehog)

is_even <-
  function(a) a %% 2 == 0

gen_powers_of_two <-
  gen.element(1:10) %>% gen.with(function(a) 2^a)

test_that("is_even returns TRUE for powers of two", {
  for_all(
    a = from_hedgehog(gen_powers_of_two),
    property = function(a) is_even(a) %>% expect_true()
  )
})
```

Any `hedgehog` generator can be used with `quickcheck` but they can't be composed 
together to build another generator. For example this will work:

```{r}
test_that("powers of two and integers are both numeric values", {
  for_all(
    a = from_hedgehog(gen_powers_of_two),
    b = integer_(),
    property = function(a, b) {
      c(a, b) %>%
        is.numeric() %>%
        expect_true()
    }
  )
})
```

But this will cause an error:

```{r}
test_that("composing hedgehog with quickcheck generators fails", {
  tibble_of(from_hedgehog(gen_powers_of_two)) %>% expect_error()
})
```

A `quickcheck` generator can also be converted to a `hedgehog` generator which can 
then be used with other `hedgehog` functions.

```{r}
gen_powers_of_two <-
  integer_bounded(1L, 10L, len = 1L) %>%
  as_hedgehog() %>%
  gen.with(function(a) 2^a)


test_that("is_even returns TRUE for powers of two", {
  for_all(
    a = from_hedgehog(gen_powers_of_two),
    property = function(a) is_even(a) %>% expect_true()
  )
})
```

## Fuzz tests

Fuzz testing is a special case of property based testing in which the only 
property being tested is that the code doesn't fail with a range of inputs.
Here is an example of how to do fuzz testing with `quickcheck`. Let's say we want
to test that the `purrr::map` function won't fail with any vector as input.

```{r}
test_that("map won't fail with any vector as input", {
  for_all(
    a = any_vector(),
    property = function(a) purrr::map(a, identity) %>% expect_silent()
  )
})
```

## Repeat tests

Repeat tests can be used to repeatedly test that a property holds true for many
calls of a function. These are different from regular property based tests
because they don't require generators. The function `repeat_test` will call 
a function many times to ensure the expectation passes in all cases. This kind
of test can be useful for testing functions with randomness.

```{r}
test_that("runif generates random numbers between a min and max value", {
  repeat_test(
    property = function() {
      random_number <- runif(1, min = 0, max = 10)
      expect_true(random_number >= 0 && random_number <= 10)
    }
  )
})
```

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