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Creating a Specs2 matcher in a modular way
type TF = A => Double
(f: TF) must computeSameResultsAs(g: TF,tolerance: Double, tests: Set[A])

(f: TF) must computeSameResultsAs(g: TF)
               .withTolerance(tolerance)
               .onValues(tests: Set[A])

def computeSameResultsAs[A](ref: A => Double, tolerance: Double, args: Set[A]): Matcher[A => Double] =
  args.map(beCloseOnArg(ref, tolerance, _)).reduce(_ and _)

def beCloseOnArg[A](ref: A => Double, tolerance: Double, arg: A): Matcher[A => Double] =
  closeTo(ref(arg), tolerance) ^^ ((_: A => Double).apply(arg))

def beCloseOnArg[A](ref: A => Double, tolerance: Double, arg: A): Matcher[A => Double] =
  closeTo(ref(arg), tolerance) ^^ ((_: A => Double).apply(arg) aka "result on argument " + arg)

def computeSameResultsAs[A](g: A => Double,
                            tolerance: Double = 0.0,
                            values: Seq[A] = Seq()) = TFMatcher(g, tolerance, values)

case class TFMatcher[A](g: A => Double,
                        tolerance: Double = 0.0,
                        values: Seq[A] = Seq()) extends Matcher[A => Double] {

  def apply[S <: A => Double](f: Expectable[S]) = {
    // see definition below
  }

  def withTolerance(t: Double) = TFMatcher(g, t, values)
  def onValues(tests: A*) = TFMatcher(g, tolerance, tests)
}

val f = (i: Int) => i.toDouble
val g = (i: Int) => i.toDouble + 0.1

"f must be close to another similar function with a tolerance" in {
  f must computeSameResultsAs[Int](g).withTolerance(0.5).onValues(1, 2, 3)
}

def apply[S <: A => Double](f: Expectable[S]) = {
  val res = ((v: A) => beCloseTo(g(v) +/- tolerance).apply(theValue(f.value(v)))).forall(values)

  val message = "f is "+(if (res.isSuccess) "" else "not ")+
                "close to g with a tolerance of "+tolerance+" "+
                "on values "+values.mkString(",")+": "+res.message
   result(res.isSuccess, message, message, f)
 }

((v: A) => beCloseTo(g(v) +/- tolerance).apply(theValue(f.value(v)))).forall(values)

val f = (i: Int) => i.toDouble
val g = (i: Int) => i.toDouble + 1.0

"f must be close to another similar function with a tolerance" in {
  f must computeSameResultsAs[Int](g, tolerance = 0.5, values = Seq(1, 2, 3))
}

def computeSameResultsAs[A](ref: A => Double, tolerance: Double, values: Seq[A]): Matcher[A => Double] = (f: A => Double) => {
  verifyFunction((a: A) => (beCloseTo(ref(a) +/- tolerance)).apply(theValue(f(a)))).forall(values)
}

In the sequence '1, 2, 3', the 1st element is failing: 1.0 is not close to 2.0 +/- 0.5

implicit def functionResultToMatcher[T](f: T => MatchResult[_]): Matcher[T] = (t: T) => {
  val result = f(t)
  (result.isSuccess, result.message)
}

1.0 is not close to 2.0 +/- 0.5; 2.0 is not close to 3.0 +/- 0.5; 3.0 is not close to 4.0 +/- 0.5

def computeSameResultsAs[A](ref: A => Double, tolerance: Double, values: Seq[A]): Matcher[A => Double] = (f: A => Double) => {
  ((a: A) => beCloseTo(ref(a) +/- tolerance) ^^ f).forall(values)
}

def computeSameResultsAs[A](ref: A => Double, tolerance: Double, values: Seq[A]): Matcher[A => Double] = (f: A => Double) => {
  ((a: A) => beCloseTo(ref(a) +/- tolerance) ^^ ((a1: A) => f(a) aka "the value")).forall(values)
}

In the sequence '1, 2, 3', the 1st element is failing: the value '1.0' is not close to 2.0 +/- 0.5