haskell-dependency-injection-example.hs

addTwoNumbers a b = add a b

add = (+)

tests = do
  let addTwoNumbers =  assemble defs "addTwoNumbers"
  addTwoNumbers 3 4 `shouldBe` 7 

  let addTwoNumbers =  assemble defs{add=(*)} "addTwoNumbers"
  addTwoNumbers 3 4 `shouldBe` 12



time = getCurrentTime

yesterday = fmap (addTime ((-1) * 60 * 60 * 24)) time 

timeRangeSinceYesterday = (,) <$> yesterday <*> time

tests2 = do
  let trsy =  assemble defs "timeRangeSinceYesterday"
  trsy >>= (`shouldBe` ({- non-deterministic -})) 

  let trsy =  assemble defs{time=return $ parseTime "2010-02-02 13:00"} "timeRangeSinceYesterday"
  trsy >>= (`shouldBe` (arseTime "2010-02-01 13:00", arseTime "2010-02-02 13:00")) 
  -- always deterministic, mocked a time function that is 2 levels deep in the
  -- call stack without having to pass it through its parents explicitly
  --
  -- This benefit becomes more and more apparent the more dependencies we want 
  -- to be mockable, and the deeper the call hierarchy is.