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Dependency Injection in Scala - Beyond the Cake Pattern

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Discusses how to do dependency injection in Scala using functional programming.

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  • At slide n17 we have an 'implicit repo' at each method, why? Isnt that possible to leave only one implicit right inside TradeService trait body?

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Dependency Injection in Scala - Beyond the Cake Pattern

  1. 1. Dependency Injection Beyond the Cake Pattern
  2. 2. Debasish Ghosh @debasishg on twitter Code @ http://github.com/debasishg Blog @ Ruminations of a Programmer http:// debasishg.blogspot.com
  3. 3. Open Source Footprints <ul><li>Committer in Akka ( http:// akka.io ) </li></ul><ul><li>Owner/Founder of : </li></ul><ul><ul><li>sjson (JSON serialization library for Scala objects – http://github.com/debasishg/sjson ) </li></ul></ul><ul><ul><li>scala-redis (Redis client for Scala – http://github.com/debasishg/scala-redis ) </li></ul></ul><ul><ul><li>scouchdb (Scala driver for Couchdb – http:// github.com/debasishg/scouchdb ) </li></ul></ul>
  4. 4. import TradeModel._ // a Repository abstraction trait TradeRepository { def fetch(refNo: String ): Trade def update(trade: Trade ): Trade }
  5. 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. 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. 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. 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. 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. 10. val fetchTrade: TradeRepository => String => Trade val fetchTrade_c: String => Trade import  TradeServiceWithRedisRepo._ val  t = fetchTrade_c(&quot;ref-123&quot;) by using the appropriate module, we can switch Repository implementations ..
  11. 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
  12. 12. domain rule .. <ul><li>enrichment of trade is done in steps: </li></ul><ul><li>get the tax/fee ids for a trade </li></ul><ul><li>calculate tax/fee values </li></ul><ul><li>enrich trade with net cash amount </li></ul>
  13. 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. 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. 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
  16. 16. The Reader Monad <ul><li>Note how the Trade is being delayed in injection </li></ul><ul><li>How we don’t have to repeat the Trade argument in each invocation of the functions </li></ul><ul><li>This is an implementation of the Reader monad – Trade is only being read to compute all calculations </li></ul><ul><li>Partial Application is the key </li></ul>
  17. 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. 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. 19. import TradeService ._ import Repositories.RedisTradeRepository def run = { updateTrade(fetchTrade(&quot;r-123&quot;)) //.. }

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