Posted on Apr 14, 2019 • Originally published at odone.me on Mar 18, 2019

Testing Bank Kata in PureScript

You can keep reading here or jump to my blog to get the full experience, including the wonderful pink, blue and white palette.

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 }