Using PartialFunctionValues

ScalaTest's PartialFunctionValues trait provides an implicit conversion that adds a valueAt method to PartialFunction, which will return the value (result) of the function applied to the argument passed to valueAt, or throw TestFailedException if the partial function is not defined at the argument.

This construct allows you to express in one statement that a partial function should be defined for a particular input, and that its result value should meet some expectation. Here's an example:

pf.valueAt("IV") should equal (4)

Or, using an assertion instead of a matcher expression:

assert(pf.valueAt("IV") === 4)

Were you to simply invoke apply on the PartialFunction, passing in an input value, if the partial function wasn't defined at that input, it would throw some exception, but likely not one that provides a stack depth:

// Note: a Map[K, V] is a PartialFunction[K, V]
val pf: PartialFunction[String, Int] = Map("I" -> 1, "II" -> 2, "III" -> 3, "IV" -> 4)

pf("V") should equal (5) // pf("V") throws NoSuchElementException

The NoSuchElementException thrown in this situation would cause the test to fail, but without providing a stack depth pointing to the failing line of test code. This stack depth, provided by TestFailedException (and a few other ScalaTest exceptions), makes it quicker for users to navigate to the cause of the failure. Without PartialFunctionValues, to get a stack depth exception you would need to make two statements, like this:

val pf: PartialFunction[String, Int] = Map("I" -> 1, "II" -> 2, "III" -> 3, "IV" -> 4)

pf.isDefinedAt("V") should be (true) // throws TestFailedException
pf("V") should equal (5)

The PartialFunctionValues trait allows you to state that more concisely:

val pf: PartialFunction[String, Int] = Map("I" -> 1, "II" -> 2, "III" -> 3, "IV" -> 4)

pf.valueAt("V") should equal (5) // pf.valueAt("V") throws TestFailedException

Next, learn about using PrivateMethodTester.