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Riccardo Odone

Posted on • Originally published at odone.io on

# Testing Bank Kata in PureScript

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## 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 }
``````

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