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:
lengthThe minimum length of an atom that will be generated (default: 0).
-
max:
lengthThe 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
orfalse
).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:
valueThe minimum value that will be generated (default: -1e6).
-
max:
valueThe 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
andmax
values. For example, if you specifymin: 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:
valueThe minimum value that will be generated (default: -1000).
-
max:
valueThe 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
andmax
values. For example, if you specifymin: 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:
lengthMinimum length of the lists returned. Default 0
-
max:
lengthMaximum 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:
generatorSpecifies 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 themin:
andmax:
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
andpick_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 theof:
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:
lengthThe minimum length of the returned string (default 0)
-
max:
lengthThe 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:
listA 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:
sizeSet 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 usingiex> 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
Copyright © 2016 Dave Thomas [email protected]
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
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.