The === operation compares this Equalizer's left value (passed
to the constructor, usually via an implicit conversion) with the passed right value
for equality as determined by the expression left == right.
The === operation compares this Equalizer's left value (passed
to the constructor, usually via an implicit conversion) with the passed right value
for equality as determined by the expression left == right.
If true, === returns None. Else, === returns
a Some whose String value indicates the left and right values.
In its typical usage, the Option[String] returned by === will be passed to one of two
of trait Assertion' overloaded assert methods. If None,
which indicates the assertion succeeded, assert will return normally. But if Some is passed,
which indicates the assertion failed, assert will throw a TestFailedException whose detail
message will include the String contained inside the Some, which in turn includes the
left and right values. This TestFailedException is typically embedded in a
Report and passed to a Reporter, which can present the left and right
values to the user.
Class used via an implicit conversion to enable any two objects to be compared with
===in assertions in tests. For example:The benefit of using
assert(a === b)rather thanassert(a == b)is that aTestFailedExceptionproduced by the former will include the values ofaandbin its detail message. The implicit method that performs the conversion fromAnytoEqualizerisconvertToEqualizerin traitAssertions.In case you're not familiar with how implicit conversions work in Scala, here's a quick explanation. The
convertToEqualizermethod inAssertionsis defined as an "implicit" method that takes anAny, which means you can pass in any object, and it will convert it to anEqualizer. TheEqualizerhas===defined. Most objects don't have===defined as a method on them. Take two Strings, for example:Given this code, the Scala compiler looks for an
===method on classString, because that's the class of"hello".Stringdoesn't define===, so the compiler looks for an implicit conversion fromStringto something that does have an===method, and it finds theconvertToEqualizermethod. It then rewrites the code to this:So inside a
Suite(which mixes inAssertions,===will work on anything. The only situation in which the implicit conversion wouldn't happen is on types that have an===method already defined.The primary constructor takes one object,
left, whose type is being converted toEqualizer. Theleftvalue may be anullreference, because this is allowed by Scala's==operator.