org.scalatest.matchers

# Matcher

#### trait Matcher[-T] extends (T) ⇒ MatchResult

Trait extended by objects that can match a value of the specified type. The value to match is passed to the matcher's `apply` method. The result is a `MatchResult`. A matcher is, therefore, a function from the specified type, `T`, to a `MatchResult`.

#### Creating custom matchers

If none of the built-in matcher syntax satisfy a particular need you have, you can create custom `Matcher`s that allow you to place your own syntax directly after `should` or `must`. For example, class `java.io.File` has a method `exists`, which indicates whether a file of a certain path and name exists. Because the `exists` method takes no parameters and returns `Boolean`, you can call it using `be` with a symbol or `BePropertyMatcher`, yielding assertions like:

    file should be ('exists)  // using a symbol
    file should be (inExistance)   // using a BePropertyMatcher

Although these expressions will achieve your goal of throwing a `TestFailedException` if the file does not exist, they don't produce the most readable code because the English is either incorrect or awkward. In this case, you might want to create a custom `Matcher[java.io.File]`named `exist`, which you could then use to write expressions like:

    // using a plain-old Matcher
    file should exist
    file should not (exist)
    file should (exist and have ('name ("temp.txt")))

One good way to organize custom matchers is to place them inside one or more traits that you can then mix into the suites or specs that need them. Here's an example:

    trait CustomMatchers {  class FileExistsMatcher extends Matcher[java.io.File] {    def apply(left: java.io.File) = {      val fileOrDir = if (left.isFile) "file" else "directory"      val failureMessageSuffix =
            fileOrDir + " named " + left.getName + " did not exist"      val negatedFailureMessageSuffix =
            fileOrDir + " named " + left.getName + " existed"      MatchResult(
            left.exists,
            "The " + failureMessageSuffix,
            "The " + negatedFailureMessageSuffix,
            "the " + failureMessageSuffix,
            "the " + negatedFailureMessageSuffix
          )
        }
      }  val exist = new FileExistsMatcher
    }// Make them easy to import with:
    // import CustomMatchers._
    object CustomMatchers extends CustomMatchers

Note: the `CustomMatchers` companion object exists to make it easy to bring the matchers defined in this trait into scope via importing, instead of mixing in the trait. The ability to import them is useful, for example, when you want to use the matchers defined in a trait in the Scala interpreter console.

This trait contains one matcher class, `FileExistsMatcher`, and a `val` named `exist` that refers to an instance of `FileExistsMatcher`. Because the class extends `Matcher[java.io.File]`, the compiler will only allow it be used to match against instances of `java.io.File`. A matcher must declare an`apply` method that takes the type decared in `Matcher`'s type parameter, in this case `java.io.File`. The apply method will return a `MatchResult` whose `matches` field will indicate whether the match succeeded. The `failureMessage` field will provide a programmer-friendly error message indicating, in the event of a match failure, what caused the match to fail.

The `FileExistsMatcher` matcher in this example determines success by calling `exists` on the passed `java.io.File`. It does this in the first argument passed to the `MatchResult` factory method:

            left.exists,

In other words, if the file exists, this matcher matches. The next argument to `MatchResult`'s factory method produces the failure message string:

            "The " + failureMessageSuffix,

If the passed `java.io.File` is a file (not a directory) and has the name `temp.txt`, for example, the failure message would be:

    The file named temp.txt did not exist

For more information on the fields in a `MatchResult`, including the subsequent three fields that follow the failure message, please see the documentation for `MatchResult`.

Given the `CustomMatchers` trait as defined above, you can use the `exist` syntax in any suite or spec in which you mix in the trait:

    class ExampleSpec extends Spec with ShouldMatchers with CustomMatchers {  describe("A temp file") {    it("should be created and deleted") {      val tempFile = java.io.File.createTempFile("delete", "me")      try {
            // At this point the temp file should exist
            tempFile should exist
          }
          finally {
            tempFile.delete()
          }      // At this point it should not exist
          tempFile should not (exist)
        }
      }
    }

Note that when you use custom `Matcher`s, you will need to put parentheses around the custom matcher when if follows `not`, as shown in the last assertion above: `tempFile should not (exist)`.

#### Matcher's variance

`Matcher` is contravariant in its type parameter, `T`, to make its use more flexible. As an example, consider the hierarchy:

    class Fruit
    class Orange extends Fruit
    class ValenciaOrange extends Orange

Given an orange:

    val orange = Orange

The expression "`orange should`" will, via an implicit conversion in `ShouldMatchers`, result in an object that has a `should`method that takes a `Matcher[Orange]`. If the static type of the matcher being passed to `should` is`Matcher[Valencia]` it shouldn't (and won't) compile. The reason it shouldn't compile is that the left value is an `Orange`, but not necessarily a `Valencia`, and a`Matcher[Valencia]` only knows how to match against a `Valencia`. The reason it won't compile is given that `Matcher` is contravariant in its type parameter, `T`, a`Matcher[Valencia]` is not a subtype of `Matcher[Orange]`.

By contrast, if the static type of the matcher being passed to `should` is `Matcher[Fruit]`, it should (and will) compile. The reason it should compile is that given the left value is an `Orange`, it is also a `Fruit`, and a `Matcher[Fruit]` knows how to match against `Fruit`s. The reason it will compile is that given that `Matcher` is contravariant in its type parameter, `T`, a`Matcher[Fruit]` is indeed a subtype of `Matcher[Orange]`.

authors:

1. Bill Venners

Inherited

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1. Function1
2. AnyRef
3. Any

Visibility

1. Public
2. All

### Value Members

1. #### def !=(arg0: AnyRef): Boolean

   attributes: final
   definition classes: AnyRef

2. #### def !=(arg0: Any): Boolean

   `o != arg0` is the same as `!(o == (arg0))`.

   `o != arg0` is the same as `!(o == (arg0))`.

   arg0

     the object to compare against this object for dis-equality.

   returns

     `false` if the receiver object is equivalent to the argument; `true` otherwise.

   attributes: final
   definition classes: Any

3. #### def ##(): Int

   attributes: final
   definition classes: AnyRef → Any

4. #### def $asInstanceOf[T0](): T0

   attributes: final
   definition classes: AnyRef

5. #### def $isInstanceOf[T0](): Boolean

   attributes: final
   definition classes: AnyRef

6. #### def ==(arg0: AnyRef): Boolean

   `o == arg0` is the same as `if (o eq null) arg0 eq null else o.equals(arg0)`.

   `o == arg0` is the same as `if (o eq null) arg0 eq null else o.equals(arg0)`.

   arg0

     the object to compare against this object for equality.

   returns

     `true` if the receiver object is equivalent to the argument; `false` otherwise.

   attributes: final
   definition classes: AnyRef

7. #### def ==(arg0: Any): Boolean

   `o == arg0` is the same as `o.equals(arg0)`.

   `o == arg0` is the same as `o.equals(arg0)`.

   arg0

     the object to compare against this object for equality.

   returns

     `true` if the receiver object is equivalent to the argument; `false` otherwise.

   attributes: final
   definition classes: Any

8. #### def andThen[A](g: (MatchResult) ⇒ A): (T) ⇒ A

   definition classes: Function1

9. #### def apply(left: T): MatchResult

   Check to see if the specified object, `left`, matches, and report the result in the returned `MatchResult`.

   Check to see if the specified object, `left`, matches, and report the result in the returned `MatchResult`. The parameter is named `left`, because it is usually the value to the left of a `should` or `must` invocation. For example, in:

       list should equal (List(1, 2, 3))

   The `equal (List(1, 2, 3))` expression results in a matcher that holds a reference to the right value, `List(1, 2, 3)`. The `should` method invokes `apply`on this matcher, passing in `list`, which is therefore the "`left`" value. The matcher will compare the `list` (the `left` value) with `List(1, 2, 3)` (the right value), and report the result in the returned `MatchResult`.

   left

     the value against which to match

   returns

     the `MatchResult` that represents the result of the match

   attributes: abstract
   definition classes: Matcher → Function1

10. #### def asInstanceOf[T0]: T0

    This method is used to cast the receiver object to be of type `T0`.

    This method is used to cast the receiver object to be of type `T0`.

    Note that the success of a cast at runtime is modulo Scala's erasure semantics. Therefore the expression`1.asInstanceOf[String]` will throw a `ClassCastException` at runtime, while the expression`List(1).asInstanceOf[List[String]]` will not. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the requested typed.

    returns

      the receiver object.

    attributes: final
    definition classes: Any

11. #### def clone(): AnyRef

    This method creates and returns a copy of the receiver object.

    This method creates and returns a copy of the receiver object.

    The default implementation of the `clone` method is platform dependent.

    returns

      a copy of the receiver object.

    attributes: protected
    definition classes: AnyRef

12. #### def compose[A](g: (A) ⇒ T): (A) ⇒ MatchResult

    definition classes: Function1

13. #### def eq(arg0: AnyRef): Boolean

    This method is used to test whether the argument (`arg0`) is a reference to the receiver object (`this`).

    This method is used to test whether the argument (`arg0`) is a reference to the receiver object (`this`).

    The `eq` method implements an [http://en.wikipedia.org/wiki/Equivalence_relation equivalence relation] on non-null instances of `AnyRef`: * It is reflexive: for any non-null instance `x` of type `AnyRef`, `x.eq(x)` returns `true`. * It is symmetric: for any non-null instances `x` and `y` of type `AnyRef`, `x.eq(y)` returns `true` if and only if `y.eq(x)` returns `true`. * It is transitive: for any non-null instances `x`, `y`, and `z` of type `AnyRef` if `x.eq(y)` returns `true` and `y.eq(z)` returns `true`, then `x.eq(z)` returns `true`.

    Additionally, the `eq` method has three other properties. * It is consistent: for any non-null instances `x` and `y` of type `AnyRef`, multiple invocations of `x.eq(y)` consistently returns `true` or consistently returns `false`. * For any non-null instance `x` of type `AnyRef`, `x.eq(null)` and `null.eq(x)` returns `false`. * `null.eq(null)` returns `true`.

    When overriding the `equals` or `hashCode` methods, it is important to ensure that their behavior is consistent with reference equality. Therefore, if two objects are references to each other (`o1 eq o2`), they should be equal to each other (`o1 == o2`) and they should hash to the same value (`o1.hashCode == o2.hashCode`).

    arg0

      the object to compare against this object for reference equality.

    returns

      `true` if the argument is a reference to the receiver object; `false` otherwise.

    attributes: final
    definition classes: AnyRef

14. #### def equals(arg0: Any): Boolean

    This method is used to compare the receiver object (`this`) with the argument object (`arg0`) for equivalence.

    This method is used to compare the receiver object (`this`) with the argument object (`arg0`) for equivalence.

    The default implementations of this method is an [http://en.wikipedia.org/wiki/Equivalence_relation equivalence relation]: * It is reflexive: for any instance `x` of type `Any`, `x.equals(x)` should return `true`. * It is symmetric: for any instances `x` and `y` of type `Any`, `x.equals(y)` should return `true` if and only if `y.equals(x)` returns `true`. * It is transitive: for any instances `x`, `y`, and `z` of type `AnyRef` if `x.equals(y)` returns `true` and `y.equals(z)` returns `true`, then `x.equals(z)` should return `true`.

    If you override this method, you should verify that your implementation remains an equivalence relation. Additionally, when overriding this method it is often necessary to override `hashCode` to ensure that objects that are "equal" (`o1.equals(o2)` returns `true`) hash to the same scala.Int (`o1.hashCode.equals(o2.hashCode)`).

    arg0

      the object to compare against this object for equality.

    returns

      `true` if the receiver object is equivalent to the argument; `false` otherwise.

    definition classes: AnyRef → Any

15. #### def finalize(): Unit

    This method is called by the garbage collector on the receiver object when garbage collection determines that there are no more references to the object.

    This method is called by the garbage collector on the receiver object when garbage collection determines that there are no more references to the object.

    The details of when and if the `finalize` method are invoked, as well as the interaction between `finalize`and non-local returns and exceptions, are all platform dependent.

    attributes: protected
    definition classes: AnyRef

16. #### def getClass(): java.lang.Class[_ <: java.lang.Object]

    Returns a representation that corresponds to the dynamic class of the receiver object.

    Returns a representation that corresponds to the dynamic class of the receiver object.

    The nature of the representation is platform dependent.

    returns

      a representation that corresponds to the dynamic class of the receiver object.

    attributes: final
    definition classes: AnyRef

17. #### def hashCode(): Int

    Returns a hash code value for the object.

    Returns a hash code value for the object.

    The default hashing algorithm is platform dependent.

    Note that it is allowed for two objects to have identical hash codes (`o1.hashCode.equals(o2.hashCode)`) yet not be equal (`o1.equals(o2)` returns `false`). A degenerate implementation could always return `0`. However, it is required that if two objects are equal (`o1.equals(o2)` returns `true`) that they have identical hash codes (`o1.hashCode.equals(o2.hashCode)`). Therefore, when overriding this method, be sure to verify that the behavior is consistent with the `equals` method.

    returns

      the hash code value for the object.

    definition classes: AnyRef → Any

18. #### def isInstanceOf[T0]: Boolean

    This method is used to test whether the dynamic type of the receiver object is `T0`.

    This method is used to test whether the dynamic type of the receiver object is `T0`.

    Note that the test result of the test is modulo Scala's erasure semantics. Therefore the expression`1.isInstanceOf[String]` will return `false`, while the expression `List(1).isInstanceOf[List[String]]` will return `true`. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the requested typed.

    returns

      `true` if the receiver object is an instance of erasure of type `T0`; `false` otherwise.

    attributes: final
    definition classes: Any

19. #### def ne(arg0: AnyRef): Boolean

    `o.ne(arg0)` is the same as `!(o.eq(arg0))`.

    `o.ne(arg0)` is the same as `!(o.eq(arg0))`.

    arg0

      the object to compare against this object for reference dis-equality.

    returns

      `false` if the argument is not a reference to the receiver object; `true` otherwise.

    attributes: final
    definition classes: AnyRef

20. #### def notify(): Unit

    Wakes up a single thread that is waiting on the receiver object's monitor.

    Wakes up a single thread that is waiting on the receiver object's monitor.

    attributes: final
    definition classes: AnyRef

21. #### def notifyAll(): Unit

    Wakes up all threads that are waiting on the receiver object's monitor.

    Wakes up all threads that are waiting on the receiver object's monitor.

    attributes: final
    definition classes: AnyRef

22. #### def synchronized[T0](arg0: T0): T0

    attributes: final
    definition classes: AnyRef

23. #### def toString(): String

    Returns a string representation of the object.

    Returns a string representation of the object.

    The default representation is platform dependent.

    returns

      a string representation of the object.

    definition classes: Function1 → AnyRef → Any

24. #### def wait(): Unit

    attributes: final
    definition classes: AnyRef

25. #### def wait(arg0: Long, arg1: Int): Unit

    attributes: final
    definition classes: AnyRef

26. #### def wait(arg0: Long): Unit

    attributes: final
    definition classes: AnyRef