Write for debug

In my last session of the GDCR, during the Red phase I ended up with this test:

spec :: Spec
spec =
  describe "SpaceX" $ do
    describe "computeLaunchCost" $ do
      it "Reused rocket should be launch only" $
        computeLaunchCost ReusedRocket `shouldBe` launchCost
      it "First launched rocket should be launch and build" $
        computeLaunchCost FirstLaunchedRocket `shouldBe` (launchCost <> buildCost)

And thie result:

  test/Spec.hs:17:47:
  1) SpaceX.computeLaunchCost First launched rocket should be launch and build
       expected: Cost {getCost = 2873037}
        but got: Cost {getCost = 336552}

At this time, I lack adding the build cost, which is normal.

But what if I came back several days/weeks later on it?

I would have to:

1. Figure out how the computation is done
2. Compute the diffrence
3. Find out that this number is the build cost
4. Check that the specs are confirming it

However, if I change my design so:

newtype Cost = Cost {getCost :: Int}
  deriving stock (Eq, Ord, Show)

become

data Cost = Cost
 { builds :: Int,
   launches :: Int
 }
 deriving stock (Eq, Ord, Show)

then failure is obvious:

  1) SpaceX.computeLaunchCost First launched rocket should be launch and build
       expected: Cost {builds = 1, launches = 1}
        but got: Cost {builds = 0, launches = 1}