Inspectors

Provides nestable inspector methods (or just inspectors) that enable assertions to be made about collections.

For example, the forAll method enables you to state that something should be true about all elements of a collection, such as that all elements should be positive:

scala> import org.scalatest._
import org.scalatest._
scala> import Assertions._
import Assertions._
scala> import Inspectors._
import Inspectors._
scala> val xs = List(1, 2, 3, 4, 5)
xs: List[Int] = List(1, 2, 3, 4, 5)
scala> forAll (xs) { x => assert(x > 0) }

Or, with matchers:

scala> import Matchers._
import Matchers._
scala> forAll (xs) { x => x should be > 0 }

To make assertions about nested collections, you can nest the inspector method invocations. For example, given the following list of lists of Int:

scala> val yss =
     |   List(
     |     List(1, 2, 3),
     |     List(1, 2, 3),
     |     List(1, 2, 3)
     |   )
yss: List[List[Int]] = List(List(1, 2, 3), List(1, 2, 3), List(1, 2, 3))

You can assert that all Int elements in all nested lists are positive by nesting two forAll method invocations, like this:

scala> forAll (yss) { ys =>
     |   forAll (ys) { y => y should be > 0 }
     | }

The full list of inspector methods are:

• forAll - succeeds if the assertion holds true for every element
• forAtLeast - succeeds if the assertion holds true for at least the specified number of elements
• forAtMost - succeeds if the assertion holds true for at most the specified number of elements
• forBetween - succeeds if the assertion holds true for between the specified minimum and maximum number of elements, inclusive
• forEvery - same as forAll, but lists all failing elements if it fails (whereas forAll just reports the first failing element)
• forExactly - succeeds if the assertion holds true for exactly the specified number of elements

The error messages produced by inspector methods are designed to make sense no matter how deeply you nest the method invocations. Here's an example of a nested inspection that fails and the resulting error message:

scala> forAll (yss) { ys =>
     |   forAll (ys) { y => y should be < 2 }
     | }
org.scalatest.exceptions.TestFailedException: forAll failed, because:
  at index 0, forAll failed, because:
    at index 1, 2 was not less than 2 (<console>:20)
  in List(1, 2, 3) (<console>:20)
in List(List(1, 2, 3), List(1, 2, 3), List(1, 2, 3))
     at org.scalatest.InspectorsHelper$.doForAll(Inspectors.scala:146)
     ...

One way the error message is designed to help you understand the error is by using indentation that mimics the indentation of the source code (optimistically assuming the source will be nicely indented). The error message above indicates the outer forAll failed because its initial List (i.e., at index 0) failed the assertion, which was that all elements of that initial List[Int] at index 0 should be less than 2. This assertion failed because index 1 of that inner list contained the value 2, which was indeed “not less than 2.” The error message for the inner list is an indented line inside the error message for the outer list. The actual contents of each list are displayed at the end in inspector error messages, also indented appropriately. The actual contents are placed at the end so that for very large collections, the contents will not drown out and make it difficult to find the messages that describe actual causes of the failure.

The forAll and forEvery methods are similar in that both succeed only if the assertion holds for all elements of the collection. They differ in that forAll will only report the first element encountered that failed the assertion, but forEvery will report all elements that fail the assertion. The tradeoff is that while forEvery gives more information, it may take longer to run because it must inspect every element of the collection. The forAll method can simply stop inspecting once it encounters the first failing element. Here's an example that shows the difference in the forAll and forEvery error messages:

scala> forAll (xs) { x => x should be < 3 }
org.scalatest.exceptions.TestFailedException: forAll failed, because:
  at index 2, 3 was not less than 3 (<console>:18)
in List(1, 2, 3, 4, 5)
     at org.scalatest.InspectorsHelper$.doForAll(Inspectors.scala:146)
     ...
scala> forEvery (xs) { x => x should be < 3 }
org.scalatest.exceptions.TestFailedException: forEvery failed, because:
  at index 2, 3 was not less than 3 (<console>:18),
  at index 3, 4 was not less than 3 (<console>:18),
  at index 4, 5 was not less than 3 (<console>:18)
in List(1, 2, 3, 4, 5)
     at org.scalatest.InspectorsHelper$.doForEvery(Inspectors.scala:226)
     ...

Note that if you're using matchers, you can alternatively use inspector shorthands for writing non-nested inspections. Here's an example:

scala> all (xs) should be > 3
org.scalatest.exceptions.TestFailedException: 'all' inspection failed, because:
  at index 0, 1 was not greater than 3
in List(1, 2, 3, 4, 5)
     at org.scalatest.InspectorsHelper$.doForAll(Inspectors.scala:146)