Testing Bank Kata in PureScript

Intro

Last week we’ve had some fun solving the Bank Kata in PureScript. Now it’s time to add some unit tests.

In particular, we are going to test the three main functions of the kata:

deposit :: Int -> StateT (Array Transaction) Effect Unit

withdraw :: Int -> StateT (Array Transaction) Effect Unit

printStatement :: StateT (Array Transaction) Effect Unit

The Tests

Let’s start with deposit:

deposit :: Int -> StateT (Array Transaction) Effect Unit
deposit amount = do
ts <- lift nowDateTime
let t = Deposit { timestamp: ts, amount: amount }
modify\_ \ts -> ts <> [t]

Unfortunately, it uses Effect. That means, it does something impure we cannot check in a unit test.

We can fix that easily by changing the type signature into

deposit
 :: forall m. Monad m
 => Int
 -> StateT (Array Transaction) m Unit

In other words, we don’t specify the specific monad (Effect) anymore. We just say that deposit uses a monad m as a base monad for StateT.

Sadly, that does not compile. In fact, the type signature is telling a lie. In the body of the function we do ts <- lift nowDateTime. As explained in the previous post, that obliges the function to use Effect.

Luckily, this is an easy fix. Instead of using nowDateTime in deposit, we will just inject it:

deposit
 :: forall m. Monad m
 => m DateTime
 -> Int
 -> StateT (Array Transaction) m Unit

The downside of this refactoring is that we need to change the production code from deposit 500 to deposit nowDateTime 500. The upside is that we can use a unit testable monad now. Not that bad!

Here’s the test

testDeposit :: Effect Unit
testDeposit = do
 ts <- nowDateTime
 let amount = 1
 expected = Identity [Deposit {amount: amount, timestamp: ts}]
 actual = execStateT (deposit (Identity timestamp) amount) []
 assertEqual { actual: actual, expected: expected }

We wrap timestamp :: DateTime in the Identity monad so that deposit (Identity timestamp) amount has type StateT (Array Transaction) Identity Unit. That means, execStateT returns Identity (Array Transaction).

Testing withdraw follows the exact same pattern so we are not going to cover that.

Let’s move to printStatement:

printStatement :: StateT (Array Transaction) Effect Unit
printStatement = do
 s <- gets toStatement
 lift $ log s

Here the story is really similar to what we did to deposit:

printStatement :: forall m. Monad m => (String -> m Unit) -> StateT (Array Transaction) m Unit
printStatement logger = do
 s <- gets toStatement
 lift $ logger s

And the corresponding unit test:

testPrintStatementWithTransactions :: Effect Unit
testPrintStatementWithTransactions = do
 timestamp <- nowDateTime
 let d = Deposit { amount: 500, timestamp: timestamp }
 w = Withdraw { amount: 100, timestamp: timestamp }
 state = [d, w]
 expected = "expected string"
 actual = execWriter (execStateT (printStatement \s -> tell s) state)
 assertEqual { actual: actual, expected: expected }

Notice that as a base monad we use Writer. This monad gives us access to tell which allows us to append to an accumulator. That way printStatement “writes” the statement in the accumulator instead of the console.

Show me the Code

Code:

data Transaction
  = Deposit Info
  | Withdraw Info

derive instance eqTransaction :: Eq Transaction

instance showTransaction :: Show Transaction where
  show (Deposit i) = show i
  show (Withdraw i) = show i

type Info =
  { timestamp :: DateTime
   , amount    :: Int
  }

deposit :: forall m. Monad m => m DateTime -> Int -> StateT (Array Transaction) m Unit
deposit nowDateTime amount = do
  ts <- lift nowDateTime
  let t = Deposit { timestamp: ts, amount: amount }
  modify_ \ts -> ts <> [t]

withdraw :: forall m. Monad m => m DateTime -> Int -> StateT (Array Transaction) m Unit
withdraw nowDateTime amount = do
  ts <- lift nowDateTime
  let t = Withdraw { timestamp: ts, amount: amount }
  modify_ \ts -> ts <> [t]

printStatement :: forall m. Monad m => (String -> m Unit) -> StateT (Array Transaction) m Unit
printStatement logger = do
  s <- gets toStatement
  lift $ logger s

toStatement :: Array Transaction -> String
toStatement =
  fst <<< foldl fnc (Tuple "" 0)
  where
  fnc (Tuple s i) (Deposit d) =
    Tuple (s <> "\n" <> joinWith " " [ show d.timestamp, show d.amount, show $ i + d.amount]) (i + d.amount)
  fnc (Tuple s i) (Withdraw w) =
    Tuple (s <> "\n" <> joinWith " " [ show w.timestamp, "-" <> show w.amount, show $ i - w.amount]) (i - w.amount)

main :: Effect Unit
main = do
  flip evalStateT [] do
    deposit nowDateTime 500
    withdraw nowDateTime 100
    printStatement log

Tests:

main :: Effect Unit
main = do
  testDeposit
  testWithdraw
  testPrintStatementNoTransactions
  testPrintStatementWithTransactions

testDeposit :: Effect Unit
testDeposit = do
  timestamp <- nowDateTime
  let amount = 1
      expected = Identity [ Deposit { amount: amount, timestamp: timestamp } ]
      actual = execStateT (deposit (Identity timestamp) amount) [] 
  assertEqual { actual: actual, expected: expected }

testWithdraw :: Effect Unit
testWithdraw = do
  timestamp <- nowDateTime
  let amount = 1
      expected = Identity [ Withdraw { amount: amount, timestamp: timestamp } ]
      actual = execStateT (withdraw (Identity timestamp) amount) [] 
  assertEqual { actual: actual, expected: expected }

testPrintStatementNoTransactions :: Effect Unit
testPrintStatementNoTransactions = do
  let expected = ""
      actual = execWriter (evalStateT (printStatement \s -> tell s) [])
  assertEqual { actual: actual, expected: expected }

testPrintStatementWithTransactions :: Effect Unit
testPrintStatementWithTransactions = do
  timestamp <- nowDateTime
  let d = Deposit { amount: 500, timestamp: timestamp }
      w = Withdraw { amount: 100, timestamp: timestamp }
      state = [ d, w ]
      expected = "expected string"
      actual = execWriter (evalStateT (printStatement \s -> tell s) state)
  assertEqual { actual: actual, expected: expected }

Outro

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