Using EitherValues
ScalaTest's EitherValues
trait provides an implicit conversion that adds left.value
and right.value
methods
to Either
, which will return the selected value of the Either
if defined,
or throw TestFailedException
if not.
This construct allows you to express in one statement that an Either
should be left or right
and that its value should meet some expectation. Here's are some examples:
either1.right.value should be > 9
either2.left.value should be ("Muchas problemas")
Or, using assertions instead of matcher expressions:
assert(either1.right.value > 9)
assert(either2.left.value === "Muchas problemas")
Were you to simply invoke right.get
or left.get
on the Either
,
if the Either
wasn't defined as expected (e.g., it was a Left
when you expected a Right
), it
would throw a NoSuchElementException
:
val either: Either[String, Int] = Left("Muchas problemas")
either.right.get should be > 9
The NoSuchElementException
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 EitherValues
, to get
a stack depth exception you would need to make two statements, like this:
val either: Either[String, Int] = Left("Muchas problemas")
either should be ('right)
either.right.get should be > 9
The EitherValues
trait allows you to state that more concisely:
val either: Either[String, Int] = Left("Muchas problemas")
either.right.value should be > 9
Next, we can look at another goodie. Let's learn about using PartialFunctionValues.