This project shows how Scalactic's Equality can be integrated with other equality mechanisms such as the Eq typeclass from Spire or the Equal typeclass from Scalaz, and how that can then be used in ScalaTest when testing projects that use those libraries. It was motivated by this issue in Spire: [typelevel/spire#294]
First, the Haskell-inspired typeclass approach to equality in Spire and Scalaz has some problems with how Scala applies implicit conversions. Here's an example in Scalaz:
-------------
> console
[info] Starting scala interpreter...
[info]
Welcome to Scala version 2.10.4 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_65).
Type in expressions to have them evaluated.
Type :help for more information.
scala> import scalaz._
import scalaz._
scala> import Scalaz._
import Scalaz._
scala> 1L === 1 // An implicit Long => Int conversion is applied
res0: Boolean = true
scala> 1 === 1L // Asymmetrically, this doesn't compile
<console>:14: error: could not find implicit value for parameter F0: scalaz.Equal[Any]
1 === 1L
^
scala> 1 === () // This fails to compile, as you'd wish
<console>:14: error: not enough arguments for method ===: (other: Int)Boolean.
Unspecified value parameter other.
1 === ()
^
scala> () === 1 // Asymmetrically, this compiles, and moreover yields true!
<console>:14: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
() === 1
^
res3: Boolean = true
scala>
scala> case class Box[T](o: T)
defined class Box
scala>
scala> Box(1) === Box(1) // This fails to compile as planned, because no Equal[Box[Int]] exists
<console>:16: error: could not find implicit value for parameter F0: scalaz.Equal[Box[Int]]
Box(1) === Box(1)
^
scala>
scala> implicit def boxEqual[T] =
| new Equal[Box[T]] {
| def equal(a: Box[T], b: Box[T]): Boolean = a == b
| }
boxEqual: [T]=> scalaz.Equal[Box[T]]
scala>
scala> Box(1) === Box(1) // Now it works
res5: Boolean = true
scala>
scala> Box(1) === 1 // This correctly not compile, as the types don't match
<console>:17: error: could not find implicit value for parameter F0: scalaz.Equal[Any]
Box(1) === 1
^
scala> 1 === Box(1) // And the compile error is symmetric
<console>:17: error: could not find implicit value for parameter F0: scalaz.Equal[Object]
1 === Box(1)
^
scala>
scala> import scala.language.implicitConversions
import scala.language.implicitConversions
scala> implicit def widenIntToBox(i: Int): Box[Int] = Box(i)
widenIntToBox: (i: Int)Box[Int]
scala>
scala> Box(1) === 1 // But define an Int => Box[Int] implicit widening conversion
res8: Boolean = true
scala> 1 === Box(1) // And you get that asymmetry again
<console>:19: error: could not find implicit value for parameter F0: scalaz.Equal[Object]
1 === Box(1)
^
scala> :q
[success] Total time: 19 s, completed Aug 6, 2014 8:35:57 PMSpire's Eq typeclass exhibits very similar behavior:
> console
[info] Starting scala interpreter...
[info]
Welcome to Scala version 2.10.4 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_65).
Type in expressions to have them evaluated.
Type :help for more information.
scala> import spire.algebra._
import spire.algebra._
scala> import spire.math._
import spire.math._
scala> import spire.implicits._
import spire.implicits._
scala> 1L === 1
res0: Boolean = true
scala> 1 === 1L // Spire's Eq has a similar problem to Scalaz's Equal
<console>:17: error: diverging implicit expansion for type spire.algebra.Eq[Any]
starting with method SeqOrder in trait SeqInstances2
1 === 1L
^
scala> 1 === ()
<console>:17: error: macros application do not support named and/or default arguments
1 === ()
^
scala> () === 1 // But at least both of these comparisons with Unit fail to compile in Spire
<console>:17: error: could not find implicit value for evidence parameter of type spire.algebra.Eq[AnyVal]
() === 1
^
scala>
scala> case class Box[T](o: T)
defined class Box
scala>
scala> Box(1) === Box(1) // As before, this is by design as no Eq[Box[Int]] as yet exists
<console>:19: error: could not find implicit value for evidence parameter of type spire.algebra.Eq[Box[Int]]
Box(1) === Box(1)
^
scala>
scala> implicit def boxEq[T] =
| new Eq[Box[T]] {
| def eqv(a: Box[T], b: Box[T]): Boolean = a == b
| }
boxEq: [T]=> spire.algebra.Eq[Box[T]]
scala>
scala> Box(1) === Box(1) // Now this works
res5: Boolean = true
scala>
scala> Box(1) === 1 // A slightly scary compiler error message, but...
<console>:20: error: diverging implicit expansion for type spire.algebra.Eq[Any]
starting with method SeqOrder in trait SeqInstances2
Box(1) === 1
^
scala> 1 === Box(1) // The good thing is both of these fail to compile symmetrically
<console>:20: error: could not find implicit value for evidence parameter of type spire.algebra.Eq[Object]
1 === Box(1)
^
scala>
scala> import scala.language.implicitConversions
import scala.language.implicitConversions
scala> implicit def widenIntToBox(i: Int): Box[Int] = Box(i)
widenIntToBox: (i: Int)Box[Int]
scala>
scala> Box(1) === 1 // But define the widening conversion, and the asymmetry shows up again
res8: Boolean = true
scala> 1 === Box(1)
<console>:22: error: could not find implicit value for evidence parameter of type spire.algebra.Eq[Object]
1 === Box(1)
^
scala>
scala> val half = r"1/2"
half: spire.math.Rational = 1/2
scala>
scala> half === 0.5 // Note that it happens with Spire's own implicit widening conversions
res10: Boolean = true
scala> 0.5 === half // Such as Double => Rational.
<console>:23: error: could not find implicit value for evidence parameter of type spire.algebra.Eq[Object]
0.5 === half
^
scala> :q
[success] Total time: 22 s, completed Aug 6, 2014 8:36:20 PMBy contrast, Scalactic's equality exhibits symmetry in whether or not two types compile:
> console
[info] Starting scala interpreter...
[info]
Welcome to Scala version 2.10.4 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_65).
Type in expressions to have them evaluated.
Type :help for more information.
scala> import org.scalactic._
import org.scalactic._
scala> import ConversionCheckedTripleEquals._
import ConversionCheckedTripleEquals._
scala> 1L === 1
res0: Boolean = true
scala> 1 === 1L // Both compile
res1: Boolean = true
scala> 1 === ()
<console>:14: error: types Int and Unit do not adhere to the type constraint selected for the === and !== operators; the missing implicit parameter is of type org.scalactic.Constraint[Int,Unit]
1 === ()
^
scala> () === 1 // Neither compile
<console>:14: error: types Unit and Int do not adhere to the type constraint selected for the === and !== operators; the missing implicit parameter is of type org.scalactic.Constraint[Unit,Int]
() === 1
^
scala>
scala> case class Box[T](o: T)
defined class Box
scala>
scala> Box(1) === Box(1) // This compiles because Scalactic provides a default Equivalence[T] typeclass
res4: Boolean = true
scala>
scala> Box(1) === 1
<console>:16: error: types Box[Int] and Int do not adhere to the type constraint selected for the === and !== operators; the missing implicit parameter is of type org.scalactic.Constraint[Box[Int],Int]
Box(1) === 1
^
scala> 1 === Box(1) // These symmetrically don't compile in all three libraries
<console>:16: error: types Int and Box[Int] do not adhere to the type constraint selected for the === and !== operators; the missing implicit parameter is of type org.scalactic.Constraint[Int,Box[Int]]
1 === Box(1)
^
scala>
scala> import scala.language.implicitConversions
import scala.language.implicitConversions
scala> implicit def widenIntToBox(i: Int): Box[Int] = Box(i)
widenIntToBox: (i: Int)Box[Int]
scala>
scala> Box(1) === 1
res7: Boolean = true
scala> 1 === Box(1) // But in Scalactic, this compiles in either arrangement
res8: Boolean = true
scala> :q
[success] Total time: 15 s, completed Aug 6, 2014 8:36:39 PMNext I'll demo how you can integrate Scalactic with other libraries equality mechanisms. This makes it nicer to test projects that use libraries like Scalaz or Spire with ScalaTest, but also solves the asymmetry problems. You'll find equalitydemo.SpireEquality object in the src/main/scala directory of this project.
> console
[info] Starting scala interpreter...
[info]
Welcome to Scala version 2.10.4 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_65).
Type in expressions to have them evaluated.
Type :help for more information.
scala> import spire.algebra._
import spire.algebra._
scala> import spire.math._
import spire.math._
scala> import spire.implicits.{eqOps => _, _} // Must turn off Spires === operator
import spire.implicits.{eqOps=>_, _}
scala> import equalitydemo.SpireEquality._
import equalitydemo.SpireEquality._
scala> 1L === 1
res0: Boolean = true
scala> 1 === 1L // This compiles symmetrically
res1: Boolean = true
scala> 1 === ()
<console>:20: error: types Int and Unit do not adhere to the type constraint selected for the === and !== operators; the missing implicit parameter is of type org.scalactic.Constraint[Int,Unit]
1 === ()
^
scala> () === 1 // This fails to compile symmetrically
<console>:20: error: types Unit and Int do not adhere to the type constraint selected for the === and !== operators; the missing implicit parameter is of type org.scalactic.Constraint[Unit,Int]
() === 1
^
scala>
scala> case class Box[T](o: T)
defined class Box
scala>
scala> Box(1) === Box(1) // This does not compile because no Eq[Box[Int]] exists
<console>:22: error: types Box[Int] and Box[Int] do not adhere to the type constraint selected for the === and !== operators; the missing implicit parameter is of type org.scalactic.Constraint[Box[Int],Box[Int]]
Box(1) === Box(1)
^
scala>
scala> implicit def boxEq[T] =
| new Eq[Box[T]] {
| def eqv(a: Box[T], b: Box[T]): Boolean = a == b
| }
boxEq: [T]=> spire.algebra.Eq[Box[T]]
scala>
scala> Box(1) === Box(1) // Now it works
res5: Boolean = true
scala>
scala> Box(1) === 1
<console>:23: error: types Box[Int] and Int do not adhere to the type constraint selected for the === and !== operators; the missing implicit parameter is of type org.scalactic.Constraint[Box[Int],Int]
Box(1) === 1
^
scala> 1 === Box(1) // This is as before
<console>:23: error: types Int and Box[Int] do not adhere to the type constraint selected for the === and !== operators; the missing implicit parameter is of type org.scalactic.Constraint[Int,Box[Int]]
1 === Box(1)
^
scala>
scala> import scala.language.implicitConversions
import scala.language.implicitConversions
scala> implicit def widenIntToBox(i: Int): Box[Int] = Box(i)
widenIntToBox: (i: Int)Box[Int]
scala>
scala> Box(1) === 1 // But now, this is symmetric.
res8: Boolean = true
scala> 1 === Box(1) // I.e., by integrating Scalactic with Spire, you get compile symmetry with Spire's Eq
res9: Boolean = true
scala>
scala> val half = r"1/2"
half: spire.math.Rational = 1/2
scala>
scala> half === 0.5
res10: Boolean = true
scala> 0.5 === half // Here it is again. Compile symmetry with Spire's Eq.
res11: Boolean = true
scala> :q
[success] Total time: 18 s, completed Aug 6, 2014 8:37:00 PMAnd the same thing goes for Scalaz:
> console
[info] Starting scala interpreter...
[info]
Welcome to Scala version 2.10.4 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_65).
Type in expressions to have them evaluated.
Type :help for more information.
scala> import scalaz._
import scalaz._
scala> import Scalaz.{ToEqualOps => _, _} // Must turn off Scalaz's === operator
import Scalaz.{ToEqualOps=>_, _}
scala> import equalitydemo.ScalazEquality._
import equalitydemo.ScalazEquality._
scala> 1L === 1
res0: Boolean = true
scala> 1 === 1L // This is fixed, symmetric.
res1: Boolean = true
scala> 1 === ()
<console>:17: error: types Int and Unit do not adhere to the type constraint selected for the === and !== operators; the missing implicit parameter is of type org.scalactic.Constraint[Int,Unit]
1 === ()
^
scala> () === 1 // This is fixed, symmetric and, especially, doesn't result in true!
<console>:17: error: types Unit and Int do not adhere to the type constraint selected for the === and !== operators; the missing implicit parameter is of type org.scalactic.Constraint[Unit,Int]
() === 1
^
scala>
scala> case class Box[T](o: T)
defined class Box
scala>
scala> Box(1) === Box(1) // Doesn't compile because no Equal[Box[Int]]
<console>:19: error: types Box[Int] and Box[Int] do not adhere to the type constraint selected for the === and !== operators; the missing implicit parameter is of type org.scalactic.Constraint[Box[Int],Box[Int]]
Box(1) === Box(1)
^
scala>
scala> implicit def boxEqual[T] =
| new Equal[Box[T]] {
| def equal(a: Box[T], b: Box[T]): Boolean = a == b
| }
boxEqual: [T]=> scalaz.Equal[Box[T]]
scala>
scala> Box(1) === Box(1) // Now it works
res5: Boolean = true
scala>
scala> Box(1) === 1 // These still don't work, as desired
<console>:20: error: types Box[Int] and Int do not adhere to the type constraint selected for the === and !== operators; the missing implicit parameter is of type org.scalactic.Constraint[Box[Int],Int]
Box(1) === 1
^
scala> 1 === Box(1)
<console>:20: error: types Int and Box[Int] do not adhere to the type constraint selected for the === and !== operators; the missing implicit parameter is of type org.scalactic.Constraint[Int,Box[Int]]
1 === Box(1)
^
scala>
scala> import scala.language.implicitConversions
import scala.language.implicitConversions
scala> implicit def widenIntToBox(i: Int): Box[Int] = Box(i)
widenIntToBox: (i: Int)Box[Int]
scala>
scala> Box(1) === 1 // But now you have symmetry here too
res8: Boolean = true
scala> 1 === Box(1)
res9: Boolean = true
scala> :q
[success] Total time: 19 s, completed Aug 6, 2014 8:37:29 PMThat said, the original motivation for this demo was to show how to enhance ScalaTest to make it nicer to test Spire. The same technique works for Scalaz. What I think might make sense is to create scalatest-plus-spire and a scalatest-plus-scalaz projects if people are interested. Essentially you can use assertions or matchers that use the Eq or Equal typeclasses instead of Scalactic's built-in Equivalence typeclass if that helps you test code that uses Scalaz and/or Spire. You can mix and match them, because importing one hides the other. Here's the same demo, but this time using ScalaTest's assertions:
> test:console
[info] Starting scala interpreter...
[info]
Welcome to Scala version 2.10.4 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_65).
Type in expressions to have them evaluated.
Type :help for more information.
scala> import scalaz._
import scalaz._
scala> import Scalaz.{ToEqualOps => _, _}
import Scalaz.{ToEqualOps=>_, _}
scala> import equalitydemo.ScalazAssertions._
import equalitydemo.ScalazAssertions._
scala> assert(1L === 1)
scala> assert(1 === 1L)
scala> assert(1 === ())
<console>:17: error: types Int and Unit do not adhere to the type constraint selected for the === and !== operators; the missing implicit parameter is of type org.scalactic.Constraint[Int,Unit]
assert(1 === ())
^
scala> assert(() === 1)
<console>:17: error: types Unit and Int do not adhere to the type constraint selected for the === and !== operators; the missing implicit parameter is of type org.scalactic.Constraint[Unit,Int]
assert(() === 1)
^
scala>
scala> case class Box[T](o: T)
defined class Box
scala>
scala> assert(Box(1) === Box(1))
<console>:19: error: types Box[Int] and Box[Int] do not adhere to the type constraint selected for the === and !== operators; the missing implicit parameter is of type org.scalactic.Constraint[Box[Int],Box[Int]]
assert(Box(1) === Box(1))
^
scala>
scala> implicit def boxEqual[T] =
| new Equal[Box[T]] {
| def equal(a: Box[T], b: Box[T]): Boolean = a == b
| }
boxEqual: [T]=> scalaz.Equal[Box[T]]
scala>
scala> assert(Box(1) === Box(1))
scala>
scala> assert(Box(1) === 1)
<console>:20: error: types Box[Int] and Int do not adhere to the type constraint selected for the === and !== operators; the missing implicit parameter is of type org.scalactic.Constraint[Box[Int],Int]
assert(Box(1) === 1)
^
scala> assert(1 === Box(1))
<console>:20: error: types Int and Box[Int] do not adhere to the type constraint selected for the === and !== operators; the missing implicit parameter is of type org.scalactic.Constraint[Int,Box[Int]]
assert(1 === Box(1))
^
scala>
scala> import scala.language.implicitConversions
import scala.language.implicitConversions
scala> implicit def widenIntToBox(i: Int): Box[Int] = Box(i)
widenIntToBox: (i: Int)Box[Int]
scala>
scala> assert(Box(1) === 1)
scala> assert(1 === Box(1))
scala> assert(2 === Box(1)) // Note that you still get nice error messages from ScalaTest's assert macro:
org.scalatest.exceptions.TestFailedException: 2 did not equal Box(1)
at org.scalatest.Assertions$class.newAssertionFailedException(Assertions.scala:500)
...
scala> :q
[success] Total time: 12 s, completed Aug 6, 2014 8:37:42 PMAnd here's the same example with Spire, but using matchers:
> test:console
[info] Starting scala interpreter...
[info]
Welcome to Scala version 2.10.4 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_65).
Type in expressions to have them evaluated.
Type :help for more information.
scala> import spire.algebra._
import spire.algebra._
scala> import spire.math._
import spire.math._
scala> import spire.implicits.{eqOps => _, _}
import spire.implicits.{eqOps=>_, _}
scala> import equalitydemo.SpireMatchers._
import equalitydemo.SpireMatchers._
scala> 1L must === (1)
scala> 1 must === (1L)
scala>
scala> case class Box[T](o: T)
defined class Box
scala>
scala> Box(1) must === (Box(1))
<console>:22: error: types Box[Int] and Box[Int] do not adhere to the type constraint selected for the === and !== operators; the missing implicit parameter is of type org.scalactic.Constraint[Box[Int],Box[Int]]
Box(1) must === (Box(1))
^
scala>
scala> implicit def boxEq[T] =
| new Eq[Box[T]] {
| def eqv(a: Box[T], b: Box[T]): Boolean = a == b
| }
boxEq: [T]=> spire.algebra.Eq[Box[T]]
scala>
scala> Box(1) must === (Box(1))
scala>
scala> Box(1) must === (1)
<console>:23: error: types Box[Int] and Int do not adhere to the type constraint selected for the === and !== operators; the missing implicit parameter is of type org.scalactic.Constraint[Box[Int],Int]
Box(1) must === (1)
^
scala> 1 must === (Box(1))
<console>:23: error: types Int and Box[Int] do not adhere to the type constraint selected for the === and !== operators; the missing implicit parameter is of type org.scalactic.Constraint[Int,Box[Int]]
1 must === (Box(1))
^
scala>
scala> import scala.language.implicitConversions
import scala.language.implicitConversions
scala> implicit def widenIntToBox(i: Int): Box[Int] = Box(i)
widenIntToBox: (i: Int)Box[Int]
scala>
scala> Box(1) must === (1)
scala> 1 must === (Box(1))
scala>
scala> val half = r"1/2"
half: spire.math.Rational = 1/2
scala>
scala> half must === (0.5)
scala> 0.5 must === (half)
scala> 2 must === (Box(1)) // Again, you still get ScalaTest's nice error messages:
org.scalatest.exceptions.TestFailedException: 2 did not equal Box(1)
at org.scalatest.MatchersHelper$.newTestFailedException(MatchersHelper.scala:160)
...
Below is what I copied and pasted into the REPL that produced the above output, in case you want to try it. Just clone the project and type sbt.
> console
import scalaz._
import Scalaz._
1L === 1
1 === 1L
1 === ()
() === 1
case class Box[T](o: T)
Box(1) === Box(1)
implicit def boxEqual[T] =
new Equal[Box[T]] {
def equal(a: Box[T], b: Box[T]): Boolean = a == b
}
Box(1) === Box(1)
Box(1) === 1
1 === Box(1)
import scala.language.implicitConversions
implicit def widenIntToBox(i: Int): Box[Int] = Box(i)
Box(1) === 1
1 === Box(1)
--------------------------------------------
> console
import spire.algebra._
import spire.math._
import spire.implicits._
1L === 1
1 === 1L
1 === ()
() === 1
case class Box[T](o: T)
Box(1) === Box(1)
implicit def boxEq[T] =
new Eq[Box[T]] {
def eqv(a: Box[T], b: Box[T]): Boolean = a == b
}
Box(1) === Box(1)
Box(1) === 1
1 === Box(1)
import scala.language.implicitConversions
implicit def widenIntToBox(i: Int): Box[Int] = Box(i)
Box(1) === 1
1 === Box(1)
val half = r"1/2"
half === 0.5
0.5 === half
--------------------------------------------
> console
import org.scalactic._
import ConversionCheckedTripleEquals._
1L === 1
1 === 1L
1 === ()
() === 1
case class Box[T](o: T)
Box(1) === Box(1)
Box(1) === 1
1 === Box(1)
import scala.language.implicitConversions
implicit def widenIntToBox(i: Int): Box[Int] = Box(i)
Box(1) === 1
1 === Box(1)
--------------------------------------------
> console
import spire.algebra._
import spire.math._
import spire.implicits.{eqOps => _, _}
import equalitydemo.SpireEquality._
1L === 1
1 === 1L
1 === ()
() === 1
case class Box[T](o: T)
Box(1) === Box(1)
implicit def boxEq[T] =
new Eq[Box[T]] {
def eqv(a: Box[T], b: Box[T]): Boolean = a == b
}
Box(1) === Box(1)
Box(1) === 1
1 === Box(1)
import scala.language.implicitConversions
implicit def widenIntToBox(i: Int): Box[Int] = Box(i)
Box(1) === 1
1 === Box(1)
val half = r"1/2"
half === 0.5
0.5 === half
--------------------------------------------
console>
import scalaz._
import Scalaz.{ToEqualOps => _, _}
import equalitydemo.ScalazEquality._
1L === 1
1 === 1L
1 === ()
() === 1
case class Box[T](o: T)
Box(1) === Box(1)
implicit def boxEqual[T] =
new Equal[Box[T]] {
def equal(a: Box[T], b: Box[T]): Boolean = a == b
}
Box(1) === Box(1)
Box(1) === 1
1 === Box(1)
import scala.language.implicitConversions
implicit def widenIntToBox(i: Int): Box[Int] = Box(i)
Box(1) === 1
1 === Box(1)
--------------------------------------------
> test:console
import scalaz._
import Scalaz.{ToEqualOps => _, _}
import equalitydemo.ScalazAssertions._
assert(1L === 1)
assert(1 === 1L)
assert(1 === ())
assert(() === 1)
case class Box[T](o: T)
assert(Box(1) === Box(1))
implicit def boxEqual[T] =
new Equal[Box[T]] {
def equal(a: Box[T], b: Box[T]): Boolean = a == b
}
assert(Box(1) === Box(1))
assert(Box(1) === 1)
assert(1 === Box(1))
import scala.language.implicitConversions
implicit def widenIntToBox(i: Int): Box[Int] = Box(i)
assert(Box(1) === 1)
assert(1 === Box(1))
assert(2 === Box(1))
--------------------------------------------
> test:console
import spire.algebra._
import spire.math._
import spire.implicits.{eqOps => _, _}
import equalitydemo.SpireMatchers._
1L must === (1)
1 must === (1L)
case class Box[T](o: T)
Box(1) must === (Box(1))
implicit def boxEq[T] =
new Eq[Box[T]] {
def eqv(a: Box[T], b: Box[T]): Boolean = a == b
}
Box(1) must === (Box(1))
Box(1) must === (1)
1 must === (Box(1))
import scala.language.implicitConversions
implicit def widenIntToBox(i: Int): Box[Int] = Box(i)
Box(1) must === (1)
1 must === (Box(1))
val half = r"1/2"
half must === (0.5)
0.5 must === (half)
2 must === (half)