Monthly Downloads: 185,505
Programming language: Elixir
License: Apache License 2.0
Tags: Testing     Data     Property-based Testing    
Latest version: v0.5.0

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StreamData is an Elixir library for data generation and property-based testing.

Read the announcement on the Elixir website.


Add stream_data to your list of dependencies:

defp deps() do
  [{:stream_data, "~> 0.5", only: :test}]

and run mix deps.get. StreamData is usually added only to the :test environment since it's used in tests and test data generation. To also import StreamData's formatter configuration, add the :dev environment as well as :test for stream_data and add :stream_data to your .formatter.exs:

  import_deps: [:stream_data]


The documentation is available online.

StreamData is made of two main components: data generation and property-based testing. The StreamData module provides tools to work with data generation. The ExUnitProperties module takes care of the property-based testing functionality.

Data generation

All data generation functionality is provided in the StreamData module. StreamData provides "generators" and functions to combine those generators and create new ones. Since generators implement the Enumerable protocol, it's easy to use them as infinite streams of data:

StreamData.integer() |> Stream.map(&abs/1) |> Enum.take(3)
#=> [1, 0, 2]

StreamData provides all the necessary tools to create arbitrarily complex custom generators:

require ExUnitProperties

domains = [

email_generator =
  ExUnitProperties.gen all name <- StreamData.string(:alphanumeric),
                           name != "",
                           domain <- StreamData.member_of(domains) do
    name <> "@" <> domain

Enum.take(StreamData.resize(email_generator, 20), 2)
#=> ["[email protected]", "[email protected]"]

Property testing

Property testing aims at randomizing test data in order to make tests more robust. Instead of writing a bunch of inputs and expected outputs by hand, with property-based testing we write a property of our code that should hold for a set of data, and then we generate data in this set, in attempt to falsify that property. To generate this data, we can use the above-mentioned StreamData module.

use ExUnitProperties

property "bin1 <> bin2 always starts with bin1" do
  check all bin1 <- binary(),
            bin2 <- binary() do
    assert String.starts_with?(bin1 <> bin2, bin1)

To know more about property-based testing, read the ExUnitProperties documentation. Another great resource about property-based testing in Erlang (but with most ideas that apply to Elixir as well) is Fred Hebert's website propertesting.com.

The property-based testing side of this library is heavily inspired by the original QuickCheck paper (which targeted Haskell) as well as Clojure's take on property-based testing, test.check.

Differences from other property-based testing frameworks

There are a handful of property-based testing frameworks for the BEAM ecosystem (Erlang, Elixir, and so on). For Elixir, the main alternative to StreamData is PropCheck. PropCheck is a wrapper around PropEr, which is a property-based testing framework for Erlang. There are a few fundamental differences between StreamData and PropEr. They are listed below to help you choose between the two.

PropEr (via PropCheck):

  • It provides stateful property-based testing. If you need to test a system with state by building a model of the system to test against, you'll have to go with PropCheck since StreamData doesn't support this yet.

  • It can store counter-examples: StreamData doesn't support storing counter-examples in a file (you have to reuse the seed that caused the failure in order to reproduce it).


  • Provides functionality for generating data as the base for property-based testing. StreamData generators can be used outside of property-based testing as normal Elixir streams that produce random data.

  • It is native to Elixir. It's written entirely in Elixir and has an idiomatic Elixir API (for example, all generators are Elixir enumerables).


Copyright 2017 Andrea Leopardi and Josรฉ Valim

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.

*Note that all licence references and agreements mentioned in the StreamData README section above are relevant to that project's source code only.