The document discusses dependency injection and different approaches to implement it in Scala code. It presents a trade service that depends on a trade repository interface. It shows how to inject a Redis implementation of the repository by partially applying the service functions. It also discusses using the Reader monad to delay injection of dependencies like the trade object. Finally, it proposes a typeclass based approach to dependency injection.

1. Dependency Injection Beyond the Cake Pattern

2. Debasish Ghosh @debasishg on twitter Code @ http://github.com/debasishg Blog @ Ruminations of a Programmer http:// debasishg.blogspot.com

4. import TradeModel._ // a Repository abstraction trait TradeRepository { def fetch(refNo: String ): Trade def update(trade: Trade ): Trade }

5. // a Trading service trait TradeService { // fetches a trade based on the reference no val fetchTrade: TradeRepository => String => Trade = {repo => refNo => repo.fetch(refNo)} // updates a trade with the given values val updateTrade: TradeRepository => Trade => Trade = {repo => trade => repo.update(trade)} }

6. // a Trading service trait TradeService { // fetches a trade based on the reference no val fetchTrade: TradeRepository => String => Trade = {repo => refNo => repo.fetch(refNo)} // updates a trade with the given values val updateTrade: TradeRepository => Trade => Trade = {repo => trade => repo.update(trade)} } Repository is still abstract

7. suppose we would like to use a Redis based Repository .. class RedisTradeRepository extends TradeRepository { def fetch(refNo: String ): Trade = //.. Redis based implementation def update(trade: Trade ): Trade = //.. Redis based implementation } need to indicate that to the service class ..

8. define partial application of the service methods using the Redis based repository implementation in a separate module .. object TradeServiceWithRedisRepo extends TradeService { // partially applied functions val fetchTrade_c = fetchTrade( new RedisTradeRepository ) val updateTrade_c = updateTrade( new RedisTradeRepository ) }

9. define partial application of the service methods using the Redis based repository implementation in a separate module .. object TradeServiceWithRedisRepo extends TradeService { // partially applied functions val fetchTrade_c = fetchTrade(new RedisTradeRepository ) val updateTrade_c = updateTrade(new RedisTradeRepository ) } Concrete implementation injected

10. val fetchTrade: TradeRepository => String => Trade val fetchTrade_c: String => Trade import TradeServiceWithRedisRepo._ val t = fetchTrade_c("ref-123") by using the appropriate module, we can switch Repository implementations ..

11. instead of currying individual functions, we can curry a composed function .. // warning: needs scalaz! val withTrade = for { t <- fetchTrade n <- updateTrade } yield (t map n) val withTrade_c = withTrade( new RedisTradeRepository ) Monadic binding Of functions

13. // enrichment of trade // implementation follows problem domain model val enrich = for { // get the tax/fee ids for a trade taxFeeIds <- forTrade // calculate tax fee values taxFeeValues <- taxFeeCalculate // enrich trade with net amount netAmount <- enrichTradeWith } yield ((taxFeeIds map taxFeeValues) map netAmount)

14. // get the list of tax/fees for this trade val forTrade: Trade => Option [ List [ TaxFeeId ]] = {trade => // .. implementation } // all tax/fees for a specific trade val taxFeeCalculate: Trade => List [ TaxFeeId ] => List [( TaxFeeId , BigDecimal )] = {t => tids => //.. implementation } val enrichTradeWith: Trade => List [( TaxFeeId , BigDecimal )] => BigDecimal = {trade => taxes => //.. implementation }

15. val enrich = for { // get the tax/fee ids for a trade taxFeeIds <- forTrade // calculate tax fee values taxFeeValues <- taxFeeCalculate // enrich trade with net amount netAmount <- enrichTradeWith } yield ((taxFeeIds map taxFeeValues) map netAmount) (TradeModel.Trade) => Option[BigDecimal] :type enrich

17. trait TradeService { def fetchTrade(refNo: String )( implicit repo: TradeRepository ) = repo.fetch(refNo) def updateTrade(trade: Trade )( implicit repo: TradeRepository ) = repo.update(trade) } object TradeService extends TradeService typeclass based dependency injection ..

18. implicit object RedisTradeRepository extends TradeRepository { def fetch(refNo: String ): Trade = //.. Redis based implementation def update(trade: Trade ): Trade = //.. Redis based implementation } typeclass instance for Redis based repository ..

19. import TradeService ._ import Repositories.RedisTradeRepository def run = { updateTrade(fetchTrade(&quot;r-123&quot;)) //.. }