scalalandio / pulp

Scala library for guiceless dependency injection

GitHub

Pulp

https://travis-ci.org/scalalandio/pulp Maven Central License

Too much fructose is not good for your health, so you should remove Guice from your diet. This small experimental project allows you to have your fruity projects with no Guice.

Getting started

Library is available for Scala 2.11, 2.12 and Scala.js 0.6. Add it with:

libraryDependencies += "io.scalaland" %% "pulp" % pulpVersion
addCompilerPlugin("org.scalamacros" % "paradise" % "2.1.0" cross CrossVersion.full)

or if you cross-build with Scala.js:

libraryDependencies += "io.scalaland" %%% "pulp" % pulpVersion
addCompilerPlugin("org.scalamacros" % "paradise" % "2.1.0" cross CrossVersion.full)

Latest version can be checked on Maven and is displayed on the badge above.

Ammonite users can try it out with:

import $ivy.`io.scalaland:pulp_2.12:0.0.4`, io.scalaland.pulp._
interp.load.plugin.ivy("org.scalamacros" % "paradise_2.12.4" % "2.1.0")

With Ammonite 1.1.0 you can try out this showoff code!

Usage

See DOCS for specific cases or read further for understanding the general idea.

Motivation

I wanted to avoid runtime reflection based dependency injection in my program while still avoiding the need to pass everything manually. Existing ways of doing DI in Scala that I knew of were:

  • manual dependency injection
  • usage of runtime reflection like Guice or one used by Spring Framework
  • semi-manual DI via something like MacWire
  • usage of Scala's built-in implicits

All of above have some pros and cons thought it's mostly up to programmers' taste to decide which trade off they like better (though they would often defend their own choice as the only reasonable).

I wanted to go with implicits, but that generating a bit of a boilerplate:

class A
class B (implicit a: A)
class C (implicit b: B)
class D (implicit b: B, c: C)

implicit val a: A = new A
implicit val b: B = new B
implicit val c: C = new C

Additionally we pollute the scope with tons of manually written implicits, including these passed by constructor.

Instead we could move them to companion objects and wrap in a dedicated type to ensure they won't accidentally mix with other implicits:

trait Provider[T] { def get(): T }
object Provider { def get[T: Provider]: T = implicitly[Provider[T]].get() }

class A
object A { implicit def provide: Provider[A] = () => new A }
class B (a: A)
object B { implicit def provide(implicit a: Provider[A]): Provider[B] = () => new B(a.get()) }
class C (b: B)
object B { implicit def provide(implicit b: Provider[B]): Provider[C] = () => new C(b.get()) }
class D (b: B, c: C)
object D { implicit def provide(implicit b: Provider[B], c: Provider[C]): Provider[D] = () => new D(b.get(), c.get()) }

Provider.get[D]

However, as we can see it brings a lot of boilerplate to the table. But what if we generated all of that code? E.g. with macro annotations:

@Wired class A
@Wired class B (a: A)
@Wired class C (b: B)
@Wired class D (b: B, c: C)

Provider.get[D]

That's basically what Pulp does.

Features

  • both monomorphic and polymorphic classes
  • existing companion objects will be extended and missing generated
  • class might have or have not dependencies passed via constructor
  • type-class derivation via import io.scalaland.pulp.semiauto._

Limitations

Pulp uses implicits for passing objects around. It means that Provider[T] must be in scope of initialization for each dependency required by our class. We might pass it manually, write implicit by hand or take from companion object - remember however that only classes annotated with @Wired will have implicit Providers generated.

Additionally whether something will have one or more instances is not guaranteed for @Wired - if one need to ensure that there will be only one Provider or that each Provider of some type will always return new instance one should use @Singleton or @Factory.

Last but not least such implementation of Providers is invariant - if we have trait A and @Wired class AImpl extends A it will not be resolved for A unless we explicitly provide

implicit val a = Provider.upcast[AImpl, A]

or (if implementation is accessible to interface's scope):

@ImplementedAs[AImpl] class A

@Wired class AImpl extends A