FunSpec

Trait that facilitates a “behavior-driven” style of development (BDD), in which tests are combined with text that specifies the behavior the tests verify. (Note: In BDD, the word example is usually used instead of test. The word test will not appear in your code if you use WordSpec, so if you prefer the word example you can use it. However, in this documentation the word test will be used, for clarity and to be consistent with the rest of ScalaTest.) Here's an example FunSpec:

import org.scalatest.FunSpec
import scala.collection.mutable.Stack

class StackSpec extends FunSpec {

  describe("A Stack") {

    it("should pop values in last-in-first-out order") {
      val stack = new Stack[Int]
      stack.push(1)
      stack.push(2)
      assert(stack.pop() === 2)
      assert(stack.pop() === 1)
    }

    it("should throw NoSuchElementException if an empty stack is popped") {
      val emptyStack = new Stack[String]
      intercept[NoSuchElementException] {
        emptyStack.pop()
      }
    }
  }
}

A FunSpec contains describe clauses and tests. You define a describe clause with describe, and a test with it. Both describe and it are methods, defined in FunSpec, which will be invoked by the primary constructor of StackSpec. A describe clause names, or gives more information about, the subject (class or other entity) you are specifying and testing. In the previous example, "A Stack" is the subject under specification and test. With each test you provide a string (the spec text) that specifies one bit of behavior of the subject, and a block of code that tests that behavior. You place the spec text between the parentheses, followed by the test code between curly braces. The test code will be wrapped up as a function passed as a by-name parameter to it, which will register the test for later execution.

A FunSpec's lifecycle has two phases: the registration phase and the ready phase. It starts in registration phase and enters ready phase the first time run is called on it. It then remains in ready phase for the remainder of its lifetime.

Tests can only be registered with the it method while the FunSpec is in its registration phase. Any attempt to register a test after the FunSpec has entered its ready phase, i.e., after run has been invoked on the FunSpec, will be met with a thrown TestRegistrationClosedException. The recommended style of using FunSpec is to register tests during object construction as is done in all the examples shown here. If you keep to the recommended style, you should never see a TestRegistrationClosedException.

When you execute a FunSpec, it will send Formatters in the events it sends to the Reporter. ScalaTest's built-in reporters will report these events in such a way that the output is easy to read as an informal specification of the subject being tested. For example, if you ran StackSpec from within the Scala interpreter:

scala> (new StackSpec).execute()

You would see:

A Stack
- should pop values in last-in-first-out order
- should throw NoSuchElementException if an empty stack is popped

Note: Trait FunSpec's syntax is in great part inspired by RSpec, a Ruby BDD framework.

See also: Getting started with FunSpec.

Ignored tests

To support the common use case of “temporarily” disabling a test, with the good intention of resurrecting the test at a later time, FunSpec provides registration methods that start with ignore instead of it. For example, to temporarily disable the test with the name "should pop values in last-in-first-out order", just change “it” into “ignore,” like this:

import org.scalatest.FunSpec
import scala.collection.mutable.Stack

class StackSpec extends FunSpec {

  describe("A Stack") {

    ignore("should pop values in last-in-first-out order") {
      val stack = new Stack[Int]
      stack.push(1)
      stack.push(2)
      assert(stack.pop() === 2)
      assert(stack.pop() === 1)
    }

    it("should throw NoSuchElementException if an empty stack is popped") {
      val emptyStack = new Stack[String]
      intercept[NoSuchElementException] {
        emptyStack.pop()
      }
    }
  }
}

If you run this version of StackSpec with:

scala> (new StackSpec).execute()

It will run only the second test and report that the first test was ignored:

A Stack
- should pop values in last-in-first-out order !!! IGNORED !!!
- should throw NoSuchElementException if an empty stack is popped

Informers

One of the parameters to the run method is a Reporter, which will collect and report information about the running suite of tests. Information about suites and tests that were run, whether tests succeeded or failed, and tests that were ignored will be passed to the Reporter as the suite runs. Most often the reporting done by default by FunSuite's methods will be sufficient, but occasionally you may wish to provide custom information to the Reporter from a test. For this purpose, an Informer that will forward information to the current Reporter is provided via the info parameterless method. You can pass the extra information to the Informer via one of its apply methods. The Informer will then pass the information to the Reporter via an InfoProvided event. Here's an example:

import org.scalatest.FunSpec

class ExampleSpec extends FunSpec {

  describe("The Scala language") {

    it("should add correctly") {
      val sum = 1 + 1
      assert(sum === 2)
      info("Addition seems to work")
    }

    it("should subtract correctly") {
      val diff = 7 - 2
      assert(diff === 5)
    }
  }
}

If you run this FunSpec from the interpreter, you will see the following message included in the printed report:

ExampleSpec:
The Scala language
- should add correctly
  + Addition seems to work
- should subtract correctly

Pending tests

A pending test is one that has been given a name but is not yet implemented. The purpose of pending tests is to facilitate a style of testing in which documentation of behavior is sketched out before tests are written to verify that behavior (and often, before the behavior of the system being tested is itself implemented). Such sketches form a kind of specification of what tests and functionality to implement later.

To support this style of testing, a test can be given a name that specifies one bit of behavior required by the system being tested. The test can also include some code that sends more information about the behavior to the reporter when the tests run. At the end of the test, it can call method pending, which will cause it to complete abruptly with TestPendingException.

Because tests in ScalaTest can be designated as pending with TestPendingException, both the test name and any information sent to the reporter when running the test can appear in the report of a test run. (In other words, the code of a pending test is executed just like any other test.) However, because the test completes abruptly with TestPendingException, the test will be reported as pending, to indicate the actual test, and possibly the functionality, has not yet been implemented.

You can mark a test as pending in FunSpec by placing "(pending)" after the test name, like this:

import org.scalatest.FunSpec
import scala.collection.mutable.Stack

class StackSpec extends FunSpec {

  describe("A Stack") {

    it("should pop values in last-in-first-out order") {
      val stack = new Stack[Int]
      stack.push(1)
      stack.push(2)
      assert(stack.pop() === 2)
      assert(stack.pop() === 1)
    }

    it("should throw NoSuchElementException if an empty stack is popped") (pending)
  }
}

(Note: "(pending)" is the body of the test. Thus the test contains just one statement, an invocation of the pending method, which throws TestPendingException.) If you run this version of StackSpec with:

scala> (new StackSpec).execute()

It will run both tests, but report that the test named "A stack should pop values in last-in-first-out order" is pending. You'll see:

A Stack
- should pop values in last-in-first-out order
- should throw NoSuchElementException if an empty stack is popped (pending)

Tagging tests

A FunSpec's tests may be classified into groups by tagging them with string names. As with any suite, when executing a FunSpec, groups of tests can optionally be included and/or excluded. To tag a FunSpec's tests, you pass objects that extend abstract class org.scalatest.Tag to the methods that register tests, it and ignore. Class Tag takes one parameter, a string name. If you have created Java annotation interfaces for use as group names in direct subclasses of org.scalatest.Suite, then you will probably want to use group names on your FunSpecs that match. To do so, simply pass the fully qualified names of the Java interfaces to the Tag constructor. For example, if you've defined Java annotation interfaces with fully qualified names, com.mycompany.tags.SlowTest and com.mycompany.tags.DbTest, then you could create matching groups for FunSpecs like this:

import org.scalatest.Tag

object SlowTest extends Tag("com.mycompany.tags.SlowTest")
object DbTest extends Tag("com.mycompany.tags.DbTest")

Given these definitions, you could place FunSpec tests into groups like this:

import org.scalatest.FunSpec

class ExampleSpec extends FunSpec {

  it("should add correctly", SlowTest) {
    val sum = 1 + 1
    assert(sum === 2)
  }

  it("should subtract correctly", SlowTest, DbTest) {
    val diff = 4 - 1
    assert(diff === 3)
  }
}

This code marks both tests with the com.mycompany.tags.SlowTest tag, and test "should subtract correctly" with the com.mycompany.tags.DbTest tag.

The run method takes a Filter, whose constructor takes an optional Set[String] called tagsToInclude and a Set[String] called tagsToExclude. If tagsToInclude is None, all tests will be run except those those belonging to tags listed in the tagsToExclude Set. If tagsToInclude is defined, only tests belonging to tags mentioned in the tagsToInclude set, and not mentioned in tagsToExclude, will be run.

Shared fixtures

A test fixture is objects or other artifacts (such as files, sockets, database connections, etc.) used by tests to do their work. If a fixture is used by only one test method, then the definitions of the fixture objects can be local to the method, such as the objects assigned to sum and diff in the previous ExampleSpec examples. If multiple methods need to share an immutable fixture, one approach is to assign them to instance variables.

In some cases, however, shared mutable fixture objects may be changed by test methods such that they need to be recreated or reinitialized before each test. Shared resources such as files or database connections may also need to be created and initialized before, and cleaned up after, each test. JUnit 3 offered methods setUp and tearDown for this purpose. In ScalaTest, you can use the BeforeAndAfterEach trait, which will be described later, to implement an approach similar to JUnit's setUp and tearDown, however, this approach usually involves reassigning vars or mutating objects between tests. Before going that route, you may wish to consider some more functional approaches that avoid side effects.

Calling create-fixture methods

One approach is to write one or more create-fixture methods that return a new instance of a needed fixture object (or an holder object containing multiple needed fixture objects) each time it is called. You can then call a create-fixture method at the beginning of each test method that needs the fixture, storing the returned object or objects in local variables. Here's an example:

import org.scalatest.FunSpec
import collection.mutable.ListBuffer

class ExampleSpec extends FunSpec {

  def fixture =
    new {
      val builder = new StringBuilder("ScalaTest is ")
      val buffer = new ListBuffer[String]
    }

  describe("Testing") {

    it("should be easy") {
      val f = fixture
      f.builder.append("easy!")
      assert(f.builder.toString === "ScalaTest is easy!")
      assert(f.buffer.isEmpty)
      f.buffer += "sweet"
    }

    it("should be fun") {
      val f = fixture
      f.builder.append("fun!")
      assert(f.builder.toString === "ScalaTest is fun!")
      assert(f.buffer.isEmpty)
    }
  }
}

The “f.” in front of each use of a fixture object provides a visual indication of which objects are part of the fixture, but if you prefer, you can import the the members with “import f._” and use the names directly.

Instantiating fixture traits

A related technique is to place the fixture objects in a fixture trait and run your test code in the context of a new anonymous class instance that mixes in the fixture trait, like this:

import org.scalatest.FunSpec
import collection.mutable.ListBuffer

class ExampleSpec extends FunSpec {

  trait Fixture {
    val builder = new StringBuilder("ScalaTest is ")
    val buffer = new ListBuffer[String]
  }

  describe("Testing") {

    it("should be easy") {
      new Fixture {
        builder.append("easy!")
        assert(builder.toString === "ScalaTest is easy!")
        assert(buffer.isEmpty)
        buffer += "sweet"
      }
    }

    it("should be fun") {
      new Fixture {
        builder.append("fun!")
        assert(builder.toString === "ScalaTest is fun!")
        assert(buffer.isEmpty)
      }
    }
  }
}

Mixing in OneInstancePerTest

If every test method requires the same set of mutable fixture objects, one other approach you can take is make them simply vals and mix in trait OneInstancePerTest. If you mix in OneInstancePerTest, each test will be run in its own instance of the Suite, similar to the way JUnit tests are executed. Here's an example:

import org.scalatest.FunSpec
import org.scalatest.OneInstancePerTest
import collection.mutable.ListBuffer

class ExampleSpec extends FunSpec with OneInstancePerTest {

  val builder = new StringBuilder("ScalaTest is ")
  val buffer = new ListBuffer[String]

  describe("Testing") {

    it("should be easy") {
      builder.append("easy!")
      assert(builder.toString === "ScalaTest is easy!")
      assert(buffer.isEmpty)
      buffer += "sweet"
    }

    it("should be fun") {
      builder.append("fun!")
      assert(builder.toString === "ScalaTest is fun!")
      assert(buffer.isEmpty)
    }
  }
}

Although the create-fixture, fixture-trait, and OneInstancePerTest approaches take care of setting up a fixture before each test, they don't address the problem of cleaning up a fixture after the test completes. In this situation, you'll need to either use side effects or the loan pattern.

Mixing in BeforeAndAfter

One way to use side effects is to mix in the BeforeAndAfter trait. With this trait you can denote a bit of code to run before each test with before and/or after each test each test with after, like this:

import org.scalatest.FunSpec
import org.scalatest.BeforeAndAfter
import collection.mutable.ListBuffer

class ExampleSpec extends FunSpec with BeforeAndAfter {

  val builder = new StringBuilder
  val buffer = new ListBuffer[String]

  before {
    builder.append("ScalaTest is ")
  }

  after {
    builder.clear()
    buffer.clear()
  }

  describe("Testing") {

    it("should be easy") {
      builder.append("easy!")
      assert(builder.toString === "ScalaTest is easy!")
      assert(buffer.isEmpty)
      buffer += "sweet"
    }

    it("should be fun") {
      builder.append("fun!")
      assert(builder.toString === "ScalaTest is fun!")
      assert(buffer.isEmpty)
    }
  }
}

Overriding withFixture(NoArgTest)

An alternate way to take care of setup and cleanup via side effects is to override withFixture. Trait Suite's implementation of runTest, which is inherited by this trait, passes a no-arg test function to withFixture. It is withFixture's responsibility to invoke that test function. Suite's implementation of withFixture simply invokes the function, like this:

// Default implementation
protected def withFixture(test: NoArgTest) {
  test()
}

You can, therefore, override withFixture to perform setup before, and cleanup after, invoking the test function. If you have cleanup to perform, you should invoke the test function inside a try block and perform the cleanup in a finally clause. Here's an example:

import org.scalatest.FunSpec
import collection.mutable.ListBuffer

class ExampleSpec extends FunSpec {

  val builder = new StringBuilder
  val buffer = new ListBuffer[String]

  override def withFixture(test: NoArgTest) {
    builder.append("ScalaTest is ") // perform setup
    try {
      test() // invoke the test function
    }
    finally {
      builder.clear() // perform cleanup
      buffer.clear()
    }
  }

  describe("Testing") {

    it("should be easy") {
      builder.append("easy!")
      assert(builder.toString === "ScalaTest is easy!")
      assert(buffer.isEmpty)
      buffer += "sweet"
    }

    it("should be fun") {
      builder.append("fun!")
      assert(builder.toString === "ScalaTest is fun!")
      assert(buffer.isEmpty)
      buffer += "clear"
    }
  }
}

Note that the NoArgTest passed to withFixture, in addition to an apply method that executes the test, also includes the test name as well as the config map passed to runTest. Thus you can also use the test name and configuration objects in withFixture.

The reason you should perform cleanup in a finally clause is that withFixture is called by runTest, which expects an exception to be thrown to indicate a failed test. Thus when you invoke the test function inside withFixture, it may complete abruptly with an exception. The finally clause will ensure the fixture cleanup happens as that exception propagates back up the call stack to runTest.

Overriding withFixture(OneArgTest)

To use the loan pattern, you can extend org.scalatest.fixture.FunSpec (from the org.scalatest.fixture package) instead of FunSpec. Each test in a org.scalatest.fixture.FunSpec takes a fixture as a parameter, allowing you to pass the fixture into the test. You must indicate the type of the fixture parameter by specifying FixtureParam, and implement a withFixture method that takes a OneArgTest. This withFixture method is responsible for invoking the one-arg test function, so you can perform fixture set up before, and clean up after, invoking and passing the fixture into the test function. Here's an example:

import org.scalatest.fixture
import java.io.FileWriter
import java.io.File

class ExampleSpec extends fixture.FunSpec {

  final val tmpFile = "temp.txt"

  type FixtureParam = FileWriter

  def withFixture(test: OneArgTest) {

    val writer = new FileWriter(tmpFile) // set up the fixture
    try {
      test(writer) // "loan" the fixture to the test
    }
    finally {
      writer.close() // clean up the fixture
    }
  }

  describe("Testing") {

    it("should be easy") { writer =>
      writer.write("Hello, test!")
      writer.flush()
      assert(new File(tmpFile).length === 12)
    }

    it("should be fun") { writer =>
      writer.write("Hi, test!")
      writer.flush()
      assert(new File(tmpFile).length === 9)
    }
  }
}

For more information, see the documentation for org.scalatest.fixture.FunSpec.

Providing different fixtures to different tests

If different tests in the same FunSpec require different fixtures, you can combine the previous techniques and provide each test with just the fixture or fixtures it needs. Here's an example in which a StringBuilder and a ListBuffer are provided via fixture traits, and file writer (that requires cleanup) is provided via the loan pattern:

import java.io.FileWriter
import java.io.File
import collection.mutable.ListBuffer
import org.scalatest.FunSpec

class ExampleSpec extends FunSpec {

  final val tmpFile = "temp.txt"

  trait Builder {
    val builder = new StringBuilder("ScalaTest is ")
  }

  trait Buffer {
    val buffer = ListBuffer("ScalaTest", "is")
  }

  def withWriter(testCode: FileWriter => Any) {
    val writer = new FileWriter(tmpFile) // set up the fixture
    try {
      testCode(writer) // "loan" the fixture to the test
    }
    finally {
      writer.close() // clean up the fixture
    }
  }

  describe("Testing") {

    it("should be productive") { // This test needs the StringBuilder fixture
      new Builder {
        builder.append("productive!")
        assert(builder.toString === "ScalaTest is productive!")
      }
    }

    it("should be readable") { // This test needs the ListBuffer[String] fixture
      new Buffer {
        buffer += ("readable!")
        assert(buffer === List("ScalaTest", "is", "readable!"))
      }
    }

    it("should be friendly") { // This test needs the FileWriter fixture
      withWriter { writer =>
        writer.write("Hello, user!")
        writer.flush()
        assert(new File(tmpFile).length === 12)
    }
  }

    it("should be clear and concise") { // This test needs the StringBuilder and ListBuffer
      new Builder with Buffer {
        builder.append("clear!")
        buffer += ("concise!")
        assert(builder.toString === "ScalaTest is clear!")
        assert(buffer === List("ScalaTest", "is", "concise!"))
      }
    }

    it("should be composable") { // This test needs all three fixtures
      new Builder with Buffer {
        builder.append("clear!")
        buffer += ("concise!")
        assert(builder.toString === "ScalaTest is clear!")
        assert(buffer === List("ScalaTest", "is", "concise!"))
        withWriter { writer =>
          writer.write(builder.toString)
          writer.flush()
          assert(new File(tmpFile).length === 19)
        }
      }
    }
  }
}

In the previous example, should be productive uses only the StringBuilder fixture, so it just instantiates a new Builder, whereas should be readable uses only the ListBuffer fixture, so it just intantiates a new Buffer. should be friendly needs just the FileWriter fixture, so it invokes withWriter, which prepares and passes a FileWriter to the test (and takes care of closing it afterwords).

Two tests need multiple fixtures: should be clear and concise needs both the StringBuilder and the ListBuffer, so it instantiates a class that mixes in both fixture traits with new Builder with Buffer. should be composable needs all three fixtures, so in addition to new Builder with Buffer it also invokes withWriter, wrapping just the of the test code that needs the fixture.

Note that in this case, the loan pattern is being implemented via the withWriter method that takes a function, not by overriding org.scalatest.fixture.FunSpec's withFixture(OneArgTest) method. org.scalatest.fixture.FunSpec makes the most sense if all (or at least most) tests need the same fixture, whereas in this Suite only two tests need the FileWriter.

In the previous example, the withWriter method passed an object into the tests. Passing fixture objects into tests is generally a good idea when possible, but sometimes a side affect is unavoidable. For example, if you need to initialize a database running on a server across a network, your with-fixture method will likely have nothing to pass. In such cases, simply create a with-fixture method that takes a by-name parameter and performs setup and cleanup via side effects, like this:

def withDataInDatabase(test: => Any) {
  // initialize the database across the network
  try {
    test // "loan" the initialized database to the test
  }
  finally {
    // clean up the database
  }
}

You can then use it like:

describe("A user") {
  it("should be able to log in") {
    withDataInDatabase {
      // test user logging in scenario
    }
  }
}

Composing stackable fixture traits

In larger projects, teams often end up with several different fixtures that test classes need in different combinations, and possibly initialized (and cleaned up) in different orders. A good way to accomplish this in ScalaTest is to factor the individual fixtures into traits that can be composed using the stackable trait pattern. This can be done, for example, by placing withFixture methods in several traits, each of which call super.withFixture. Here's an example in which the StringBuilder and ListBuffer[String] fixtures used in the previous examples have been factored out into two stackable fixture traits named Builder and Buffer:

import org.scalatest.FunSpec
import org.scalatest.AbstractSuite
import collection.mutable.ListBuffer

trait Builder extends AbstractSuite { this: Suite =>

  val builder = new StringBuilder

  abstract override def withFixture(test: NoArgTest) {
    builder.append("ScalaTest is ")
    try {
      super.withFixture(test) // To be stackable, must call super.withFixture
    }
    finally {
      builder.clear()
    }
  }
}

trait Buffer extends AbstractSuite { this: Suite =>

  val buffer = new ListBuffer[String]

  abstract override def withFixture(test: NoArgTest) {
    try {
      super.withFixture(test) // To be stackable, must call super.withFixture
    }
    finally {
      buffer.clear()
    }
  }
}

class ExampleSpec extends FunSpec with Builder with Buffer {

  describe("Testing") {

    it("should be easy") {
      builder.append("easy!")
      assert(builder.toString === "ScalaTest is easy!")
      assert(buffer.isEmpty)
      buffer += "sweet"
    }

    it("should be fun") {
      builder.append("fun!")
      assert(builder.toString === "ScalaTest is fun!")
      assert(buffer.isEmpty)
      buffer += "clear"
    }
  }
}

By mixing in both the Builder and Buffer traits, ExampleSpec gets both fixtures, which will be initialized before each test and cleaned up after. The order the traits are mixed together determines the order of execution. In this case, Builder is "super" to Buffer. If you wanted Buffer to be "super" to Builder, you need only switch the order you mix them together, like this:

class Example2Spec extends FunSpec with Buffer with Builder

And if you only need one fixture you mix in only that trait:

class Example3Spec extends FunSpec with Builder

Another way to create stackable fixture traits is by extending the BeforeAndAfterEach and/or BeforeAndAfterAll traits. BeforeAndAfterEach has a beforeEach method that will be run before each test (like JUnit's setUp), and an afterEach method that will be run after (like JUnit's tearDown). Similarly, BeforeAndAfterAll has a beforeAll method that will be run before all tests, and an afterAll method that will be run after all tests. Here's what the previously shown example would look like if it were rewritten to use the BeforeAndAfterEach methods instead of withFixture:

import org.scalatest.FunSpec
import org.scalatest.BeforeAndAfterEach
import collection.mutable.ListBuffer

trait Builder extends BeforeAndAfterEach { this: Suite =>

  val builder = new StringBuilder

  override def beforeEach() {
    builder.append("ScalaTest is ")
    super.beforeEach() // To be stackable, must call super.beforeEach
  }

  override def afterEach() {
    try {
      super.afterEach() // To be stackable, must call super.afterEach
    }
    finally {
      builder.clear()
    }
  }
}

trait Buffer extends BeforeAndAfterEach { this: Suite =>

  val buffer = new ListBuffer[String]

  override def afterEach() {
    try {
      super.afterEach() // To be stackable, must call super.afterEach
    }
    finally {
      buffer.clear()
    }
  }
}

class ExampleSpec extends FunSpec with Builder with Buffer {

  describe("Testing") {

    it("should be easy") {
      builder.append("easy!")
      assert(builder.toString === "ScalaTest is easy!")
      assert(buffer.isEmpty)
      buffer += "sweet"
    }

    it("should be fun") {
      builder.append("fun!")
      assert(builder.toString === "ScalaTest is fun!")
      assert(buffer.isEmpty)
      buffer += "clear"
    }
  }
}

To get the same ordering as withFixture, place your super.beforeEach call at the end of each beforeEach method, and the super.afterEach call at the beginning of each afterEach method, as shown in the previous example. It is a good idea to invoke super.afterEach in a try block and perform cleanup in a finally clause, as shown in the previous example, because this ensures the cleanup code is performed even if super.afterAll throws an exception.

One difference to bear in mind between the before-and-after traits and the withFixture methods, is that if a withFixture method completes abruptly with an exception, it is considered a failed test. By contrast, if any of the methods on the before-and-after traits (i.e., before and after of BeforeAndAfter, beforeEach and afterEach of BeforeAndAfterEach, and beforeAll and afterAll of BeforeAndAfterAll) complete abruptly, it is considered a failed suite, which will result in a SuiteAborted event.

Shared tests

Sometimes you may want to run the same test code on different fixture objects. In other words, you may want to write tests that are "shared" by different fixture objects. To accomplish this in a FunSpec, you first place shared tests in behavior functions. These behavior functions will be invoked during the construction phase of any FunSpec that uses them, so that the tests they contain will be registered as tests in that FunSpec. For example, given this stack class:

import scala.collection.mutable.ListBuffer

class Stack[T] {

  val MAX = 10
  private val buf = new ListBuffer[T]

  def push(o: T) {
    if (!full)
      buf.prepend(o)
    else
      throw new IllegalStateException("can't push onto a full stack")
  }

  def pop(): T = {
    if (!empty)
      buf.remove(0)
    else
      throw new IllegalStateException("can't pop an empty stack")
  }

  def peek: T = {
    if (!empty)
      buf(0)
    else
      throw new IllegalStateException("can't pop an empty stack")
  }

  def full: Boolean = buf.size == MAX
  def empty: Boolean = buf.size == 0
  def size = buf.size

  override def toString = buf.mkString("Stack(", ", ", ")")
}

You may want to test the Stack class in different states: empty, full, with one item, with one item less than capacity, etc. You may find you have several tests that make sense any time the stack is non-empty. Thus you'd ideally want to run those same tests for three stack fixture objects: a full stack, a stack with a one item, and a stack with one item less than capacity. With shared tests, you can factor these tests out into a behavior function, into which you pass the stack fixture to use when running the tests. So in your FunSpec for stack, you'd invoke the behavior function three times, passing in each of the three stack fixtures so that the shared tests are run for all three fixtures. You can define a behavior function that encapsulates these shared tests inside the FunSpec that uses them. If they are shared between different FunSpecs, however, you could also define them in a separate trait that is mixed into each FunSpec that uses them.

For example, here the nonEmptyStack behavior function (in this case, a behavior method) is defined in a trait along with another method containing shared tests for non-full stacks:

trait StackBehaviors { this: FunSpec =>

  def nonEmptyStack(newStack: => Stack[Int], lastItemAdded: Int) {

    it("should be non-empty") {
      assert(!newStack.empty)
    }

    it("should return the top item on peek") {
      assert(newStack.peek === lastItemAdded)
    }

    it("should not remove the top item on peek") {
      val stack = newStack
      val size = stack.size
      assert(stack.peek === lastItemAdded)
      assert(stack.size === size)
    }

    it("should remove the top item on pop") {
      val stack = newStack
      val size = stack.size
      assert(stack.pop === lastItemAdded)
      assert(stack.size === size - 1)
    }
  }

  def nonFullStack(newStack: => Stack[Int]) {

    it("should not be full") {
      assert(!newStack.full)
    }

    it("should add to the top on push") {
      val stack = newStack
      val size = stack.size
      stack.push(7)
      assert(stack.size === size + 1)
      assert(stack.peek === 7)
    }
  }
}

Given these behavior functions, you could invoke them directly, but FunSpec offers a DSL for the purpose, which looks like this:

it should behave like nonEmptyStack(stackWithOneItem, lastValuePushed)
it should behave like nonFullStack(stackWithOneItem)

If you prefer to use an imperative style to change fixtures, for example by mixing in BeforeAndAfterEach and reassigning a stack var in beforeEach, you could write your behavior functions in the context of that var, which means you wouldn't need to pass in the stack fixture because it would be in scope already inside the behavior function. In that case, your code would look like this:

it should behave like nonEmptyStack // assuming lastValuePushed is also in scope inside nonEmptyStack
it should behave like nonFullStack

The recommended style, however, is the functional, pass-all-the-needed-values-in style. Here's an example:

class SharedTestExampleSpec extends FunSpec with StackBehaviors {

  // Stack fixture creation methods
  def emptyStack = new Stack[Int]

  def fullStack = {
    val stack = new Stack[Int]
    for (i <- 0 until stack.MAX)
      stack.push(i)
    stack
  }

  def stackWithOneItem = {
    val stack = new Stack[Int]
    stack.push(9)
    stack
  }

  def stackWithOneItemLessThanCapacity = {
    val stack = new Stack[Int]
    for (i <- 1 to 9)
      stack.push(i)
    stack
  }

  val lastValuePushed = 9

  describe("A Stack") {

    describe("(when empty)") {

      it("should be empty") {
        assert(emptyStack.empty)
      }

      it("should complain on peek") {
        intercept[IllegalStateException] {
          emptyStack.peek
        }
      }

      it("should complain on pop") {
        intercept[IllegalStateException] {
          emptyStack.pop
        }
      }
    }

    describe("(with one item)") {
      it should behave like nonEmptyStack(stackWithOneItem, lastValuePushed)
      it should behave like nonFullStack(stackWithOneItem)
    }

    describe("(with one item less than capacity)") {
      it should behave like nonEmptyStack(stackWithOneItemLessThanCapacity, lastValuePushed)
      it should behave like nonFullStack(stackWithOneItemLessThanCapacity)
    }

    describe("(full)") {

      it("should be full") {
        assert(fullStack.full)
      }

      it should behave like nonEmptyStack(fullStack, lastValuePushed)

      it("should complain on a push") {
        intercept[IllegalStateException] {
          fullStack.push(10)
        }
      }
    }
  }
}

If you load these classes into the Scala interpreter (with scalatest's JAR file on the class path), and execute it, you'll see:

scala> (new StackSpec).execute()
A Stack (when empty)
- should be empty
- should complain on peek
- should complain on pop
A Stack (with one item)
- should be non-empty
- should return the top item on peek
- should not remove the top item on peek
- should remove the top item on pop
- should not be full
- should add to the top on push
A Stack (with one item less than capacity)
- should be non-empty
- should return the top item on peek
- should not remove the top item on peek
- should remove the top item on pop
- should not be full
- should add to the top on push
A Stack (full)
- should be full
- should be non-empty
- should return the top item on peek
- should not remove the top item on peek
- should remove the top item on pop
- should complain on a push

One thing to keep in mind when using shared tests is that in ScalaTest, each test in a suite must have a unique name. If you register the same tests repeatedly in the same suite, one problem you may encounter is an exception at runtime complaining that multiple tests are being registered with the same test name. A good way to solve this problem in a FunSpec is to surround each invocation of a behavior function with a describe clause, which will prepend a string to each test name. For example, the following code in a FunSpec would register a test with the name "A Stack (when empty) should be empty":

describe("A Stack") {

  describe("(when empty)") {

    it("should be empty") {
      assert(emptyStack.empty)
    }
    // ...

If the "should be empty" test was factored out into a behavior function, it could be called repeatedly so long as each invocation of the behavior function is inside a different set of describe clauses.