Gryff

bank-kata

Implement a bank account that can deposit, withdraw and print a statement to the console.

Follow the Tell, don't ask principle. All methods should be void.

void Deposit(int)
void Withdraw(int)
void PrintStatement()

Gryff

Develop an API that moves a Mars Rover around on a grid.

Rules

• You are given the initial starting point (0,0,N) of a rover.
• 0,0 are X,Y co-ordinates on a grid of (10,10).
• N is the direction it is facing (i.e. N,S,E,W).
• L and R allow the rover to rotate left and right.
• M allows the rover to move one point in the current direction.
• The rover receives a char array of commands e.g. RMMLM and returns the finishing point after the moves e.g. 2,1,N

Gryff

-- we'll need an instance of `Writer String` for `MonadCurrentDateTime` for our test
instance MonadCurrentDateTime (Writer String) where
  currentDateTime = pure firstOfJan2019

testMyBank :: StateT [Transaction] (Writer String) ()
testMyBank = do
  deposit 200
  withdraw 100
  deposit 3000
  printStatement

Gryff

deposit amount = do
  now <- lift currentDateTime
  modify $ \transactions -> transactions ++ [Deposit amount now]

Gryff

instance MonadCurrentDateTime IO where
  currentDateTime = getCurrentTime -- this comes from Data.Time

-- in our test file
instance MonadCurrentDateTime Identity where
  currentDateTime = pure firstOfJan2019

Gryff

deposit :: (Monad m, MonadCurrentDateTime m) => Int -> StateT [Transaction] m ()

withdraw :: (Monad m, MonadCurrentDateTime m) => Int -> StateT [Transaction] m ()

Gryff

class MonadCurrentDateTime m where
  currentDateTime :: m UTCTime

Gryff

firstOfJan2019 = UTCTime (fromGregorian 2019 01 01) (secondsToDiffTime 0)

-- ...
deposit amount = modify $ \transactions -> transactions ++ [Deposit amount firstOfJan2019]

-- same for `withdraw`, and in test functions

Gryff

import Data.Time

data Transaction = Deposit Int UTCTime | Withdrawal Int UTCTime

Gryff

-- our Transaction type
data Transaction = Deposit Int | Withdrawal Int

-- our bank functions
deposit :: Monad m => Int -> StateT [Transaction] m ()
deposit amount = modify $ \transactions -> transactions ++ [Deposit amount]

withdraw :: Monad m => Int -> StateT [Transaction] m ()
withdraw amount = modify $ \transactions -> transactions ++ [Withdrawal amount]