Scala cake pattern and multiproject

Question

In common-project I have this:

trait DBProvider trait DBTableNamesProvider trait DefaultDBProvider extends DBProvider trait DefaultTableNames extends DBTableNamesProvider trait MyService extends DBProvider with DBTableNamesProvider object MyService { def apply() = new MyService with DefaultDBProvider with DefaultTableNames {} }

In projectA which has a reference to common-project as a jar I wish to construct MyService

projectA (has dependency on common-project):

object MyOtherApp { trait MyOtherTableName extends DBTableNamesProvider val MyCustomService = MyService() with MyOtherTableName // will not compile how to reuse the module's MyService() with another implementation of one of the traits? }

The above will not compile I cannot just call MyService() construction and override some of the dependencies.

The above is what I wish to do, I wish to override from a different project the factory construction of MyService() apply with my own implementation of MyProjectATableNames is that possible in scala? if not what is the recommended way without code repetition?

Dima · Accepted Answer · 2017-12-05 18:49:35Z

 val MyCustomService = new MyService() with MyOtherTableName

should work

If you want to also inherit from the DefaultDBProvider and DefaultTableNames, you would have to either list them explicitly as well:

val MyCustomService = new MyService() with MyOtherTableName with DefaultDBProvider with DefaultTableNames

or create an intermediate trait in the common library:

trait DefaultService extends MyService with DefaultDBProvider with DefaultTableNames

ntviet18 · Accepted Answer · 2017-12-05 16:16:01Z

You cannot override constructed type like that because MyService() is an object not a type anymore. So to reuse your code you might create a class like

class ConfiguredMyService extends DefaultDBProvider with DefaultTableNames

in the first app you can declare

object MyService { def apply() = new ConfiguredMyService }

and in the second

val MyCustomService = new ConfiguredMyService with MyOtherTableName

Note: Nowadays, Cake pattern is considered an anti-pattern, so I recommend you checkout Dependency Injection.

MyService is a trait , and you certainly CAN extend it anonymously like that. Also, there is nothing wrong with the cake pattern, except, maybe, for the opinions of a couple of bloggers who happen to like Play.

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