how to ignore test utility methods when scalatest detects failures?

Question

I have this convenience method in my tests:

  def assertFormat[T: SexpFormat](start: T, expect: Sexp): Unit = {
    val sexp = start.toSexp
    assert(sexp === expect, s"${sexp.compactPrint} was not ${expect.compactPrint}")
    expect.convertTo[T] should be(start)
  }

which is basically a convenience for running an assertion pattern that I do a lot.

It's not possible to rewrite this as a Matcher because of the implicit requirement on SexpFormat[T] (although I'd be interested in hearing of ways to do this that don't require me to write the type MyFormat in foo should roundTrip[MyFormat](...))

If any tests fail inside this utility method, scalatest will flag the internals of assertFormat as being the cause of the test failure. But I really want scalatest to detect the caller of this method to be the cause of the test. How can I do that?

i.e. current output is

[info] - should support custom missing value rules *** FAILED ***
[info]   SexpNil did not equal SexpCons(SexpSymbol(:duck),SexpCons(SexpNil,SexpNil)) nil was not (:duck nil) (FormatSpec.scala:11)
[info]   org.scalatest.exceptions.TestFailedException:
[info]   at org.scalatest.Assertions$class.newAssertionFailedException(Assertions.scala:529)
[info]   at org.scalatest.FlatSpec.newAssertionFailedException(FlatSpec.scala:1691)
[info]   at org.scalatest.Assertions$AssertionsHelper.macroAssert(Assertions.scala:502)
[info]   at org.ensime.sexp.formats.FormatSpec$class.assertFormat(FormatSpec.scala:11)
[info]   at org.ensime.sexp.formats.test.FamilyFormatsSpec.assertFormat(FamilyFormatsSpec.scala:151)
[info]   at org.ensime.sexp.formats.test.FamilyFormatsSpec.roundtrip(FamilyFormatsSpec.scala:156)
[info]   at org.ensime.sexp.formats.test.FamilyFormatsSpec$$anonfun$12.apply(FamilyFormatsSpec.scala:222)
[info]   at org.ensime.sexp.formats.test.FamilyFormatsSpec$$anonfun$12.apply(FamilyFormatsSpec.scala:221)

FormatSpec.scala:11 is where my assertFormat is defined. The real failure is in FamilyFormatsSpec.scala:222 (which is calling another convenience method FamilyFormatsSpec.scala:156)

Answer 1

This is possible in ScalaTest 3.0 by taking an implicit org.scalactic.source.Position in your custom assertion. The position will then be computed (via a macro) whenever your assertFormat method is called, and that position will be picked up by the assert and matcher expression inside assertFormat. Here is how it would look:

import org.scalactic.source

def assertFormat[T: SexpFormat](start: T, expect: Sexp)(implicit pos: source.Position): Unit = {
  val sexp = start.toSexp
  assert(sexp === expect, s"${sexp.compactPrint} was not ${expect.compactPrint}")
  expect.convertTo[T] should be(start)
}

The following example illstrates it. If you have ScalaTest 3.0 on the class path, just :load the following file into the Scala REPL:

:paste

import org.scalatest._
import org.scalactic._
import Matchers._

case class Sexp(o: Any) {
  def compactPrint: String = o.toString
  def convertTo[T: SexpFormat]: Sexp = implicitly[SexpFormat[T]].convertIt(o)
  override def toString = "I'm too sexp for my shirt."
}

trait SexpFormat[T] {
  def convertIt(o: Any): Sexp = new Sexp(o)
}

implicit class Sexpify(o: Any) {
  def toSexp: Sexp = new Sexp(o)
}

implicit def universalSexpFormat[T]: SexpFormat[T] = new SexpFormat[T] {}

def assertFormat[T: SexpFormat](start: T, expect: Sexp): Unit = {
  val sexp = start.toSexp
  assert(sexp === expect, s"${sexp.compactPrint} was not ${expect.compactPrint}")
  expect.convertTo[T] should be(start)
}

import org.scalatest.exceptions.TestFailedException

val before = intercept[TestFailedException] { assertFormat(1, new Sexp) }

println(s"${before.failedCodeStackDepth} - This stack depth points to the assert call inside assertFormat")

import org.scalactic.source

def betterAssertFormat[T: SexpFormat](start: T, expect: Sexp)(implicit pos: source.Position): Unit = {
  val sexp = start.toSexp
  assert(sexp === expect, s"${sexp.compactPrint} was not ${expect.compactPrint}")
  expect.convertTo[T] should be(start)
}

val after = intercept[TestFailedException] { betterAssertFormat(1, new Sexp) }

println(s"${after.failedCodeStackDepth} - This stack depth is the betterAssertFormat call itself in your test code")

It will print:

3 - This stack depth points to the assert call inside assertFormat
4 - This stack depth is the betterAssertFormat call itself in your test code