ScalaTest makes three assertions available by default in any style trait. You can use:

  • assert for general assertions;
  • assertResult to differentiate expected from actual values;
  • intercept to ensure a bit of code throws an expected exception.

In any Scala program, you can write assertions by invoking assert and passing in a Boolean expression:

val left = 2
val right = 1
assert(left == right)

If the passed expression is true, assert will return normally. If false, Scala's assert will complete abruptly with an AssertionError. This behavior is provided by the assert method defined in object Predef, whose members are implicitly imported into every Scala source file.

ScalaTest provides a domain specific language (DSL) for expressing assertions in tests using the word should. ScalaTest matchers provides five different ways to check equality, each designed to address a different need. They are:

result should equal(3) // can customize equality
result should ===(3) // can customize equality and enforce type constraints
result should be(3) // cannot customize equality, so fastest to compile
result shouldEqual 3 // can customize equality, no parentheses required
result shouldBe 3       // cannot customize equality, so fastest to compile, no parentheses required

Come on, your turn: true and false values can be compared with should matchers:

true should be(res0)

Booleans in asserts can test equality:

val v1 = 4
v1 shouldEqual res0

Sometimes we expect you to fill in the values:

assert(res0 == 1 + 1)