org.scalatest.matchers.Matcher

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

Note: We are planning on adding some new matchers to ScalaTest in a future release, and would like your feedback. Please let us know if you have felt the need for a matcher ScalaTest doesn't yet provide, whether or not you wrote a custom matcher for it. Please email your feedback to bill AT artima.com.

If none of the built-in matcher syntax satisfy a particular need you have, you can create custom Matchers 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 anapply 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 Matchers, 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).

Other ways to create matchers

There are other ways to create new matchers besides defining one as shown above. For example, you would normally check to ensure an option is defined like this:

Some("hi") should be ('defined)

If you wanted to get rid of the tick mark, you could simply define defined like this:

val defined = 'defined

Now you can check that an option is defined without the tick mark:

Some("hi") should be (defined)

Perhaps after using that for a while, you realize you're tired of typing the parentheses. You could get rid of them with another one-liner:

val beDefined = be (defined)

Now you can check that an option is defined without the tick mark or the parentheses:

Some("hi") should beDefined

You can also use ScalaTest matchers' logical operators to combine existing matchers into new ones, like this:

val beWithinTolerance = be >= 0 and be <= 10

Now you could check that a number is within the tolerance (in this case, between 0 and 10, inclusive), like this:

num should beWithinTolerance

When defining a full blown matcher, one shorthand is to use one of the factory methods in Matcher's companion object. For example, instead of writing this:

val beOdd =
  new Matcher[Int] {
    def apply(left: Int) =
      MatchResult(
        left % 2 == 1,
        left + " was not odd",
        left + " was odd"
      )
  }

You could alternately write this:

val beOdd =
  Matcher { (left: Int) =>
    MatchResult(
      left % 2 == 1,
      left + " was not odd",
      left + " was odd"
    )
  }

Either way you define the beOdd matcher, you could use it like this:

3 should beOdd
4 should not (beOdd)

You can also compose matchers. If for some odd reason, you wanted a Matcher[String] that checked whether a string, when converted to an Int, was odd, you could make one by composing beOdd with a function that converts a string to an Int, like this:

val beOddAsInt = beOdd compose { (s: String) => s.toInt }

Now you have a Matcher[String] whose apply method first invokes the converter function to convert the passed string to an Int, then passes the resulting Int to beOdd. Thus, you could usebeOddAsInt like this:

"3" should beOddAsInt
"4" should not (beOddAsInt)

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 shouldmethod that takes a Matcher[Orange]. If the static type of the matcher being passed to should isMatcher[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 aMatcher[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, aMatcher[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 Fruits. The reason it will compile is that given that Matcher is contravariant in its type parameter, T, aMatcher[Fruit] is indeed a subtype of Matcher[Orange].

go to: companion

linear super types: (T) ⇒ MatchResult, AnyRef, Any

self type: Matcher[T]

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Matcher
Function1
AnyRef
Any

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Public
All

Impl.

Concrete
Abstract

Value Members

def != (arg0: AnyRef) : Boolean

attributes: final
definition classes: AnyRef
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
def ## () : Int

attributes: final
definition classes: AnyRef → Any
def $asInstanceOf [T0] () : T0

attributes: final
definition classes: AnyRef
def $isInstanceOf [T0] () : Boolean

attributes: final
definition classes: AnyRef
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
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
def andThen [A] (g: (MatchResult) ⇒ A) : (T) ⇒ A

definition classes: Function1
def apply (left: T) : 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.
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 applyon 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
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 expression1.asInstanceOf[String] will throw a ClassCastException at runtime, while the expressionList(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
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
def compose [U] (g: (U) ⇒ T) : Matcher[U]

Compose this matcher with the passed function, returning a new matcher.
Compose this matcher with the passed function, returning a new matcher.
This method overrides compose on Function1 to return a more specific function type of Matcher. For example, given a beOdd matcher defined like this:
```
val beOdd =
  new Matcher[Int] {
    def apply(left: Int) =
      MatchResult(
        left % 2 == 1,
        left + " was not odd",
        left + " was odd"
      )
  }
```
You could use beOdd like this:
```
3 should beOdd
4 should not (beOdd)
```
If for some odd reason, you wanted a Matcher[String] that checked whether a string, when converted to an Int, was odd, you could make one by composing beOdd with a function that converts a string to an Int, like this:
```
val beOddAsInt = beOdd compose { (s: String) => s.toInt }
```
Now you have a Matcher[String] whose apply method first invokes the converter function to convert the passed string to an Int, then passes the resulting Int to beOdd. Thus, you could usebeOddAsInt like this:
```
"3" should beOddAsInt
"4" should not (beOddAsInt)
```
definition classes: Matcher → Function1
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
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
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 finalizeand non-local returns and exceptions, are all platform dependent.
attributes: protected
definition classes: AnyRef
def getClass () : java.lang.Class[_]

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
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
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 expression1.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
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
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
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
def synchronized [T0] (arg0: T0) : T0

attributes: final
definition classes: AnyRef
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
def wait () : Unit

attributes: final
definition classes: AnyRef
def wait (arg0: Long, arg1: Int) : Unit

attributes: final
definition classes: AnyRef
def wait (arg0: Long) : Unit

attributes: final
definition classes: AnyRef

Matcher

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

Creating custom matchers

Other ways to create matchers

Matcher's variance

Value Members

def != (arg0: AnyRef) : Boolean

def != (arg0: Any) : Boolean

def ## () : Int

def $asInstanceOf [T0] () : T0

def $isInstanceOf [T0] () : Boolean

def == (arg0: AnyRef) : Boolean

def == (arg0: Any) : Boolean

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

def apply (left: T) : MatchResult

def asInstanceOf [T0] : T0

def clone () : AnyRef

def compose [U] (g: (U) ⇒ T) : Matcher[U]

def eq (arg0: AnyRef) : Boolean

def equals (arg0: Any) : Boolean

def finalize () : Unit

def getClass () : java.lang.Class[_]

def hashCode () : Int

def isInstanceOf [T0] : Boolean

def ne (arg0: AnyRef) : Boolean

def notify () : Unit

def notifyAll () : Unit

def synchronized [T0] (arg0: T0) : T0

def toString () : String

def wait () : Unit

def wait (arg0: Long, arg1: Int) : Unit

def wait (arg0: Long) : Unit

Inherited from (T) ⇒ MatchResult

Inherited from AnyRef

Inherited from Any