al333z / anti-test

A DSL for BDD, leveraging Functional Programming techniques.


What's this?

A POC about BDD, leveraging Functional Programming techniques.


In the past we saw (and wrote) really bad code which used cucumber, with tons of global variables, null references, runtime surprises, etc..

This is our attempt to redeem ourselves.



The idea is to offer a DSL which is both lawful and readable.

  val failingScenario = for {
    x <- given("an int")(42)
    y <- and("a string")("bar")
    _ <- when("i'm grumpy")(())
    _ <- assert("i fail")(1 == 2)
  } yield ()

  val happyScenario = for {
    x <- given("an int")(42)
    y <- and("a string")("bar")
    _ <- when("i'm happy")(())
    _ <- assert("i succeed")(1 == 1)
  } yield ()

Looks like a Writer, but we need to also run effects F[_] on each step. So let's move to WriterT, and it's good. Except for failing scenarios, in which we lose the logs.

Let's introduce LoggerT, a WriterT which preserve and combine error logs. How?

  // leveraging a MonadError to combine also error logs
  def flatMap[U](f: V => LoggerT[F, L, U])(implicit
                                           monadErrorFL: MonadError[F, L],
                                           semigroupL: Semigroup[L]): LoggerT[F, L, U] =
    LoggerT {
      monadErrorFL.flatMap(run) { lv =>
 { lv2 =>
            (semigroupL.combine(lv._1, lv2._1), lv2._2)
        )(errorLog => monadErrorFL.raiseError(semigroupL.combine(lv._1, errorLog)))


Putting a MonadError constraint instead of just FlatMap, and forcing error to have type L. With a MonadError[F, L] in scope, we can combine logs with the current failure.

All the remaining parts are only about offering a nice syntax to the DSL.

Feedback required

The code is really minimal, but effective. Any feedback is more than appreaciated :)