Quixir: Pure Elixir Property-based Testing

Property-based testing is a technique for testing your code by considering general properties of the functions you write. Rather than using explicit values in your tests, you instead try to define the types of the values to feed it, and the properties of the results produced.

For example, given a list, you know that reversing it should produce a list with the same number of elements. You can specify this in Quixir like this:

ptest some_list: list do
  reversed = my_reverse(some_list)
  assert length(reversed) == length(some_list)
end

This says that we're going to run a property test. It will run the block with a large number of different lists, and inside the block you can refer to each list as some_list. Inside the block, we have normal ExUnit test code: we produce a reversed copy of the list, then assert its length is the same as the original.

But what list do we actually pass in? The simple answer is "lots of them." In this particular case, we'll generate a hundred lists. These will vary in length, and vary in content, but we guarantee to include at least one empty list and one list containing a single element (as these are both common boundary cases that can break code). The overall test passes if the assertion it contains is true for all these lists.

What's The Big Deal?

Property-based testing delivers two major benefits.

First, it tests things you might not have considered when writing tests manually. It can run tens or hundreds of thousands of tests, using a range of inputs, and verify that the properties you specify are honored.

Second, and more important, writing property-based tests forces you to think about the invariants in your code: what should be true no matter what I feed this function? And invariants are the cornerstone of all good design. Most likely you use them every day, but they're often implicit in what you do. Property-based testing surfaces these invariants—they will drive (and improve) the design of your code.

’nuf hype. Here are the details. But first…

Alternatives

For a different approach, see ExCheck, built on triq.

Installation

def deps do
  [
    ...
    { :quixir, "~> 0.9", only: :test },
    ...
  ]
end

Including in Tests

Quixir tests run inside regular ExUnit tests, and can take advantage of all the ExUnit features, including tagging, setup, and describe blocks.

Here's a full test file:

defmodule TestReverse do
  use ExUnit.Case
  use Quixir

  import MyList, only: [ reverse: 1 ]

  test "a reversed list has the same length as the original" do
    ptest original: list do
      reversed = reverse(original)
      assert length(reversed) == length(original)
    end
  end

  test "reversing a list twice returns the original" do
    ptest original: list do
      new_list = original |> reverse |> reverse
      assert new_list == original
    end
  end

  test "reversing a list of length 1 does nothing" do
    ptest original: list(1) do
      assert reverse(original) == original
    end
  end

  test "reversing a list of length 2 swaps the elements" do
    ptest original: list(2) do
      [ b, a ] = reverse(original)
      assert [ a, b ] == original
    end
  end

  test "reversing a list of length 3 swaps the extremes" do
    ptest original: list(3) do
      [ c, b, a ] = reverse(original)
      assert [ a, b, c ] == original
    end
  end
end

Anatomy of a Property Test

The general form of a property test is

ptest [name1: type, name2: type, …], [option,…] do
  # code including assertions
  # this code can reference the values in name1 and name2
end

As the options are generally omitted, this simplifies to

ptest name1: type, name2: type, …  do
  # code including assertions
end

Options

repeat_for: n

Number of times to run the block, using different values each time. Defaults to 100.

trace: true

Dumps the values used in each iteration of the block.

For example:

ptest [ a: int, b: int ], trace: true, repeat_for: 50 do
  assert a + b == b + a
end

Type Specifications

A type specification is the name of a Quixir type generator, optionally followed by a keyword list of constraints.

int
int(min: 20, max: 50)
int(must_have: [ 0, 10, 100 ])

There's a full list of these generators, their constraints, and their defaults, below.

Sometimes type specifications can be nested. For example, this specifies (possibly empty) lists of positive integers.

list(of: int(min: 1))

And this is a generator for keyword lists:

list(of: tuple(like: { atom, string })

Back references to values

Occasionally you want to make the constraints of one type depend on the value generated for a prior type. You do this using the pin operator, ^. For example, the following generates sets of two integers where the second is guaranteed to be greater the first:

ptest a: int, b: int(min: ^a + 1) do
  assert a < b
end

Examples

(These examples don't show the test "xxxx" do/end wrappers.)

ptest numbers: list(choose(from: [ int, float ])) do
  # numbers will be a randomly sized list containing
  # a mixture of ints and floats
end

ptest x: positive_int(y: value(^x * ^x)) do
  # x is a random positive integer, and y is the square
  # of that integer
end

ptest x: positive_int, y: int(min: ^x+1), z: int(min: ^y+1)  do
  # x is a random positive integer, y is larger than x,
  # and z is larger than y
end

ptest options: map(of: { atom, string}, min: 3, max: 7) do
  # options will be a map with between 3 and 7 entries.
  # each entry will have an atom as a key and a string
  # as a value.
end

ptest options: map(like: %{ name: string, age: int(min:0, max: 130) }) do
  # options will be a map with two elements, a name and an age.
  # The name will be a string, and the age an integer
  # betweem 0 and 130
end

ptest options: list(of: { atom, string}, min: 3, max: 7) do
  # options will be a keyword list with between 3 and 7 entries.
end

defmodule Person do
  defstruct name: "", age: 0
end

ptest person: struct(Person) do
  # person will be instances of struct person. Because the
  # default name is a string, the name in this test struct
  # will be a random string. Similarly, age will be a random
  # integer
end

ptest person: struct(%Person{ name: string(chars: :ascii),
                              age:  int(min: 1, max: 125)) do
  # This time, the name will be a random string of 7-bit ascii,
  # and the age will be an integer from 1 to 125.
end

List of Type Generators

Quixir uses the Pollution library to create the streams of values that are injected into the tests. These generators are documented in HexDocs. Here's a (poorly formatted) version:

any()

Generates a stream of values of any of the types: atom, float, int, list, map, string, and tuple. Structs are not included, as they require additional information to create.

If you need finer control over the types and values returned, see the choose/2 function.
atom(options \\ [])

Return a stream of atoms. The characters in the atom are drawn from the ASCII printable set (space through ~).

Example:
```
iex> import Pollution.{Generator, VG}
iex> atom(max: 10) |> as_stream |> Enum.take(5)
[:"", :"Kv0{LGp", :"?0HX"y", :ad, :"DrS=t(Q"]
```
Options
- min: length
  
  The minimum length of an atom that will be generated (default: 0).
- max: length
  
  The maximum length of an atom that will be generated (default: 255).
- must_have: [ value, … ]
  
  Values that must be included in the results. There are no must-have vaules by default.

`bool()`

Return a stream of random booleans (true or false).

Example

  iex> import Pollution.{Generator, VG}
  iex> bool |> as_stream |> Enum.take(5)
  [true, false, true, true, false]

`choose(options)`

Each time a value is needed, randomly choose a generator from the list and invoke it.

Example

  iex> import Pollution.{Generator, VG}
  iex> choose(from: [ int(min: 3, max: 7), bool ]) |> as_stream |> Enum.take(5)
  [6, false, 4, true, true]

float(options \\ [])

Return a stream of random floating point numbers.

Example
```
  iex> import Pollution.{Generator, VG}
  iex> float |> as_stream |> Enum.take(5)
  [0.0, -1.0, 1.0, 5.0e-324, -5.0e-324]
```
Options
- min: value
  
  The minimum value that will be generated (default: -1e6).
- max: value
  
  The maximum value that will be generated (default: 1e6).
- must_have: [ value, … ]
  
  Values that must be included in the results. The default is
  
  [ 0.0, -1.0, 1.0, epsilon, -epsilon ]
  
  (where epsilon is the smallest expressible float)
  
  Must have values are automatically adjusted to account for the min and max values. For example, if you specify min: 0.5 then only the 1.0 must-have value will be generated.
See also

• positive_float() • negative_float • nonnegative_float
int(options \\ [])

Return a stream of random integers.

Example
```
  iex> import Pollution.{Generator, VG}
  iex> int |> as_stream |> Enum.take(5)
  [0, -1, 1, 215, -401]
```
Options
- min: value
  
  The minimum value that will be generated (default: -1000).
- max: value
  
  The maximum value that will be generated (default: 1000).
- must_have: [ value, … ]
  
  Values that must be included in the results. The default is
  
  [ 0, -1, 1 ]
  
  Must have values are automatically adjusted to account for the min and max values. For example, if you specify min: 0 then only the 0 and 1 must-have values will be generated.
See also

• positive_int() • negative_int • nonnegative_int

`list()`

Return a stream of lists. Each list will have a random length (within limits), and each element in each list will be randomly chosen from the specified types.

Example

iex> import Pollution.{Generator, VG}
iex> list(of: bool, max: 7) |> as_stream|> Enum.take(5)
[
 [],
 [false, false, false],
 [false, true, true, false, true],
 [false, true, true, true, true, false, true],
 [true, true, false, false, false]
]

There are a few special-case constructors:

list(length)

Return lists of the given length
list(generator)

Return lists whose elements are created by generator
```
iex> list(bool) |> as_stream|> Enum.take(5)
```

Otherwise, pass options:

min: length

Minimum length of the lists returned. Default 0
max: length

Maximum length of the lists returned. Default 100
must_have: [ value, … ]

Values that must be returned. Defaults to returning an empty list (so the parameter is must_have: [ [] ] if the minimum length is zero, nothing otherwise.

of: generator

Specifies the generator used to populate the lists.

Examples

iex> import Pollution.{Generator, VG}

iex> list(of: int, min: 1, max: 5) |> as_stream |> Enum.take(4)
[[0, -1, 1, -546], [442], [150], [-836, 540, -979]]

iex> list(of: int, min: 1, max: 5) |> as_stream |> Enum.take(4)
[[0], [-1, 1, 984, -206], [-246], [433, 125, -757]]

iex> list(of: choose(from: [value(1), value(2)]), min: 1, max: 5)
...>         |> as_stream |> Enum.take(4)
[[2], [1, 1, 2], [2, 2, 1, 1, 1], [2, 2, 1]]

iex> list(of: seq(of: [value(1), value(2)]), min: 1, max: 5)
...>         |> as_stream |> Enum.take(4)
[[1, 2], [1, 2, 1, 2], [1], [2, 1]]

list(size)
list(min, max)
map(options \\ [])

Create maps that either mirror a particular structure or that contain random numbers of elements.

To create a stream of maps with a given structure, use the like: option:
```
map(like: %{ name: string, age: int(min:0, max: 130) })
```
In this example, the keys are static atoms—each generated map will have these two keys. You can also use generators as keys:
```
map(like: %{ atom: string })
```
This will generate single element maps, where each element has a random atom as a key and a random string as a value.

To create a stream of variable size maps, use of:, optionally with the min: and max: options.
```
map(of: { atom, string }, min: 3, max: 6)
```
This will generate a stream of maps of between 3 and 6 elements each, when each element has an atom as a key and a string as a value.

You can use generators such as choose and pick_one to make things more interesting:
```
map(of: { atom, choose(from: [string, integer]) }, min: 3, max: 6)
```
With this example, some elements will have a string value, and some will have an integer value.
negative_float()

Return a stream of floats not greater than -1.0. (Arguably this should be "not greater than -epsilon"). Same as float(max: -1.0)
negative_int()

Return a stream of integers less than 0. Same as int(max: -1)
nonnegative_float()

Return a stream of floats greater than or equal to zero. Same as float(min: 0.0)
nonnegative_int()

Return a stream of integers greater than or equal to 0. Same as int(min: 0)

`pick_one(options)`

Randomly chooses a generator from the list, and then returns a stream of values that it produces. This choice is made only once—call pick_one again to get a different result.

Examples

iex> import Pollution.{Generator, VG}
iex> stream = pick_one(from: [int, bool]) |> as_stream
iex> Enum.take(stream, 5)
[0, -1, 1, -223, 72]
iex> Enum.take(stream, 5)
[0, -1, 1, -553, 847]
iex> Enum.take(stream, 5)
[0, -1, 1, -518, -692]
iex> Enum.take(stream, 5)
[0, -1, 1, 580, 668]
iex> Enum.take(stream, 5)
[0, -1, 1, -989, -353]
iex> stream = pick_one(from: [int, bool]) |> as_stream
iex> Enum.take(stream, 5)
[true, false, false, false, false]
iex> Enum.take(stream, 5)
[false, true, false, false, false]

positive_float()

Return a stream of floats not less than 1.0. (Arguably this should be "not less than epsilon"). Same as float(min: 1.0)
positive_int()

Return a stream of integers not less than 1. Same as int(min: 1)

`seq(options)`

Give seq a list of generators (using the of: option). It will cycle through these as it streams values.

Examples

iex> import Pollution.{Generator, VG}
iex> seq(of: [int, bool, float]) |> as_stream |> Enum.take(10)
[0, true, 0.0, -1, true, -1.0, 1, true, 1.0, -702]

string(options \\ [])

Return a stream of strings of randomly varying length.

Examples
```
iex> import Pollution.{Generator, VG}
iex> string(max: 4) |> as_stream |> Enum.take(5)
["", " ", "墍勧", "㘃牸ྥ姷", ""]
iex> string(chars: :digits, max: 4) |> as_stream |> Enum.take(5)
["33", "", "7", "6223", "55"]
```
Options
- min: length
  
  The minimum length of the returned string (default 0)
- max: length
  
  The maximum length of the returned string (default 300)
- chars: :ascii | :digits | :lower | :printable | :upper | :utf
  
  The set of characters that may be included in the result:
  
  | :ascii | 0..127 | | :digits | ?0..?9 | | :lower | ?a..?z | | :printable | 32..126 | | :upper | ?A..?Z | | :utf | 0..0xd7af |
  
  The default is :utf8.
- must_have: list
  
  A list of strings that must be in the result stream. Defaults to ["", "␠"], filtered by the maximum and minimum lengths.
struct(template)

Generate a stream of structs. Before starting, the generator reflects on the struct that is passed in, looking at the types of the values of each field. It then maps this onto a map() generator, using appropriate subgenerators for each of those fields.

For example, given:
```
 iex> defmodule MyStruct
 iex>    defstruct an_atom: :a, an_int: 0, other: nil
 iex> end
```
You could call
```
iex> struct(MyStruct)
```
As well as passing in the name of a struct, you can pass in an instance:
```
iex> struct(%MyStruct{})
```
In either case, the result would be a stream of MyStructs, as if you had called
```
map(like: %{ an_atom: atom,
             an_int:  int,
             other:   any,
             __struct__: MyStruct)
```
If you supply generators to the struct you pass in, these will be used in place of generators for the defaults:
```
struct(%MyStruct{an_int: int(min: 20), other: string})
```
tuple(options \\ [])

Generate a stream of tuples. The default is to create tuples of varying sizes with varying content, which is unlikely to be useful. You'll more likely want to use the like: option, which sets a template for the tuples.

Example
```
iex> import Pollution.{Generator, VG}
iex> tuple(like: { value("insert"), string(chars: :upper, max: 10)}) |>
...> as_stream |> Enum.take(3)
[{"insert", "M"}, {"insert", "GFOHZNDER"}, {"insert", "FCDO"}]
```
Options
- min: size • max: size
  
  Set the minimum and maximum sizes of the returned tuples. The defaults are 0 and 6, but this is overridden by the actual size if the like: option is specified.
- like: { template }
  
  A template of generators used to fill the tuple. The generated tuples will have the same size as the template, and each element wil be generated from the corresponding generator in the template. For example, a Keyword list could be generated using
```
iex> list(of: tuple(like: { atom, string(chars: lower, max: 10) })) |> as_stream |> Enum.take(5)
```

`value(val)`

Generates an infinite stream where each element is its parameter.

Example

iex> import Pollution.{Generator, VG}
iex> value("nom") |> as_stream |> Enum.take(3)
["nom", "nom", "nom"]

Shrinking

One of the perils of feeding random data into the code under test is that sometimes you'll get a report that your code failed when fed some obscure value, say -8768476943812378, but in reality it would also have failed if given plain -1.

Shrinking is an attempt to remedy this. When a test fails, Quixir automatically looks at each generated paramater in turn. For each, it tries generating successively "simpler" values, reporting the simplest value that still causes the code to fail.

This process is not guaranteed to find the minimal test case, but it still does a fairly good job of sorting out what values are important.

Copyright and License

Licensed under the Apache License, Version 2.0 (the “License”); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an AS IS BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

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Quixir: Pure Elixir Property-based Testing

What's The Big Deal?

Alternatives

Installation

Including in Tests

Anatomy of a Property Test

Options

Type Specifications

Back references to values

Examples

List of Type Generators

any()

atom(options \\ [])

Example:

Options

bool()

Example

choose(options)

Example

float(options \\ [])

Example

Options

See also

int(options \\ [])

Example

Options

See also

list()

Example

Examples

list(size)

list(min, max)

map(options \\ [])

negative_float()

negative_int()

nonnegative_float()

nonnegative_int()

pick_one(options)

Examples

positive_float()

positive_int()

seq(options)

Examples

string(options \\ [])

Examples

Options

struct(template)

tuple(options \\ [])

Example

Options

value(val)

Example

Shrinking

Copyright and License

`any()`

`atom(options \\ [])`

`bool()`

`choose(options)`

`float(options \\ [])`

`int(options \\ [])`

`list()`

`list(size)`

`list(min, max)`

`map(options \\ [])`

`negative_float()`

`negative_int()`

`nonnegative_float()`

`nonnegative_int()`

`pick_one(options)`

`positive_float()`

`positive_int()`

`seq(options)`

`string(options \\ [])`

`struct(template)`

`tuple(options \\ [])`

`value(val)`