Creating custom matchers

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 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 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).

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 Fruits. 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].