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Classes as modules of partially-applied functions
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| //Functional programming is all about: | |
| //. Pure, total and deterministic functions | |
| //. ADTs (Algebraic Data Types) | |
| //. Typeclasses | |
| //However, in hybrid languages like Scala, OOP classes can still play a role! | |
| //--------------------------------------------------------- | |
| //Given: | |
| //A simple domain class | |
| case class Frunfles(id: Long, value: String) | |
| //A package object acting as a module containing stand-alone functions | |
| //for CRUD operations on Frunfles | |
| package app.frunfles | |
| package object crud { | |
| def findAllFrunfles[F[_]: Monad](db: DatabaseConnection): F[List[Frunfles]] = ??? | |
| def findFrunflesById[F[_]: Monad](db: DatabaseConnection, id: Long): F[Option[Frunfles]] = ??? | |
| def findFrunflesByValue[F[_]: Monad](db: DatabaseConnection, value: String): F[Option[Frunfles]] = ??? | |
| def insertFrunfles[F[_]: Monad](db: DatabaseConnection, f: Frunfles): F[Unit] = ??? | |
| def updateFrunfles[F[_]: Monad](db: DatabaseConnection, f: Frunfles): F[Unit] = ??? | |
| def deleteFrunfles[F[_]: Monad](db: DatabaseConnection, id: Long): F[Unit] = ??? | |
| } | |
| //These functions are quite repetitive; all of then require `[F[_]: Monad]` and `db: DatabaseConnection`, | |
| //and this is a tame example! Techniques like Tagless Final or Free Monads can make function signatures | |
| //have many context bounds (like `: Monad` in the example) or implicit parameters. | |
| //--------------------------------------------------------- | |
| //Classes to the rescue! | |
| package app.frunfles.crud | |
| final class FrunflesRepository[F[_]: Monad](db: DatabaseConnection) { | |
| def findAllFrunfles(): F[List[Frunfles]] = ??? | |
| def findFrunflesById(id: Long): F[Option[Frunfles]] = ??? | |
| def findFrunflesByValue(value: String): F[Option[Frunfles]] = ??? | |
| def insertFrunfles(f: Frunfles): F[Unit] = ??? | |
| def updateFrunfles(f: Frunfles): F[Unit] = ??? | |
| def deleteFrunfles(id: Long): F[Unit] = ??? | |
| } | |
| //The class can group all the common parts in its declaration and constructor! | |
| //Its like in Math when we factor by taking a common term => ab + ac = a(b + c) | |
| //Making the `DatabaseConnection` available to all functions and removing the parameter | |
| //is similar to apply only the `db` parameter to produce new functions. | |
| //That's why the class is a "module of partially-applied functions"! | |
| //Conclusion: you can start you code with stand-alone functions and | |
| //"factor" them with a class when necessary. |
Breaking news! Scala 3 context functions offer an alternative approach to classes and Reader: https://gist.github.com/programaker/51c31e8c1ca3d4715b872d610c1a636c
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Another discussion on the topic: https://twitter.com/alexelcu/status/1359034466572861441
A more succinct definition:
Classes are closures with names