Experiment with shouldEqv ScalaTest matcher

[non]

As @denisrosset suggested, it would be nice to have a custom matcher which uses our Eq[A] type class and which integrates with ScalaTest.

[denisrosset]

Something like this ?

I'm not (yet) doing a PR because I do not know the good strategy w.r.t. having dependencies on a particular version of scalatest from the Spire dependency.

package org.scalatest

import MatchersHelper.newTestFailedException
import spire.algebra.Eq

trait EqMatchers {
  final implicit class ShouldEqvWrapper[T](val lhs: T) {
    def shouldEqv(rhs: T)(implicit ev: Eq[T]) {
      if (!ev.eqv(lhs, rhs)) {
        val (leftee, rightee) = Suite.getObjectsForFailureMessage(lhs, rhs)
        throw newTestFailedException(FailureMessages("wasNotEqvTo", leftee, rightee))
      }
    }
  }
}

[denisrosset]

Note that EqMatchers has to live in the org.scalatest package because Suite and MatchersHelper are private.

[bvenners]

Just noticed this. There's a better way to do this. I'm teaching a class today but will try and work up an example. In short, scalatest's (scalactic's really) equality is designed so you can plug in other equality tyoeclasses such as the ones in spite or scalaz and use that for === instead of the built-in one.

[bvenners]

I whipped up a demo and put it here: https://github.com/bvenners/equality-integration-demo

[denisrosset]

It's cool that scalactic has thought of this. Still, I don't like the idea of having to silence spire === to use scalatest own operators instead.

Would it be possible to offer a base trait for should-like operators, with people extending them by implementing isEqual(x,y) ? scalatest would provide a ShouldEqBase trait with a default message and an abstract isEq method.

trait SpireEqv extends FunSuite with NonImplicitAssertions with Matchers {
  final implicit class ShouldEqvWrapper[T](val lhs: T)(implicit ev: SomeEqual[T]) extends ShouldEqBase[T] {
     override def message = "was not equivalent (in some way) to"
     def isEqual(x: T, y: T) = ev.someEqual(x, y)
  }
}

[bvenners]

I'm not sure I understand what you're suggesting. You can easily make a custom matcher that takes an Eq, which I think was your original idea. Say you call it spireEqual. You could then write:

result should spireEqual (22)

You could also use Spire's === operator already in assertions, like this:

assert(a === b)

But right now you won't get the nice good error message. What I think we could do is look at enabling a nice equality error message using any === operator, not just ScalaTest's. But this wouldn't help you using ScalaTest's Matchers for equal. I think enhancing our assert macro to handle any === operator is worth doing, if it is possible and doesn't add to compile time, but otherwise I suspect the extension points that will make sense is what I posted in the equality integration demo and custom matchers. However, I would like to make sure I understand what you're suggesting. Can you clarify?

[denisrosset]

Regrouped.