Allow setting the specs2's ScalaCheck seed one sbt's command line

SimpleCheck.scala

package com.simpleenergy

import java.util.Random
import org.specs2.ScalaCheck
import org.specs2.execute.{AsResult, Failure, Result}
import org.specs2.main.CommandLineArguments
import org.specs2.matcher.Parameters
import org.specs2.mutable.Specification
import org.specs2.specification.{Example, ExampleFactory}
import scalaz.syntax.monoid._
import scalaz.syntax.std.string._

/**
  * Allow setting specs2's ScalaCheck seed on sbt's command line
  * 
  * When running tests in sbt, you can now do this:
  * 
  *     > test-only *.ExampleTest -- seed 1407603641933
  * 
  * When a test fails or errors, the seed will be printed:
  * 
  *     [info] double addition should
  *     [info] x be associative
  *     [error]    A counter-example is [-8.988465674311579E307, 8.988465674311579E307, 1.0] (after 37 tries)
  *     [error]     (Prop.scala:623)
  *     [error]    ; Failed with seed of 1407604405985 (ExampleTest.scala:6)
  **/
trait SimpleCheck extends Specification with ScalaCheck with CommandLineArguments {
  lazy val seed =
    arguments.commandLine.value("seed").flatMap(_.parseLong.toOption).getOrElse {
      System.currentTimeMillis()
    }

  implicit lazy val simpleParams =
    Parameters(rng = new Random(seed))

  override def exampleFactory: ExampleFactory = new MutableExampleFactory {
    override def newExample[T: AsResult](description: String, t: => T): Example = {
      import Result.ResultFailureMonoid
      val result = AsResult(t)
      val updated =
        if (result.isFailure || result.isError)
          result |+| Failure(s"Failed with seed of ${seed}")
        else
          result

      super.newExample(description, updated)
    }
  }
}

Hi Brian,

You probably you can trim it down to just overriding one implicit:

  val outer = this

  override implicit def propAsResult[P <: Prop](implicit p: Parameters): AsResult[P] = {
    new AsResult[P] {
      def asResult(prop: =>P): execute.Result = 
        outer.propAsResult(p.copy(rng = new Random(seed)))
          .asResult(prop).mapMessage(_+" with seed "+seed)
    }
  }