By-Need (aka Lazy) arguments in Scala
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Adds support for call-by-need parameters

This is an attempt (read as experiment) to add by-need arguments in Scala. It is implemented using scalameta macro annotations.


import com.tudorzgureanu.lazyargs._

def lazyFoo(cond: Boolean)(@Lazy bar: String) = if (cond) bar + bar else ""


You will need to use Scala 2.11.8+ or 2.12.x.

  • Add the bintray repo resolver for this project:
resolvers += Resolver.bintrayIvyRepo("tudorzgureanu", "generic")
  • Add the dependency to this project:
libraryDependencies += "com.tudorzgureanu" %% "scala-lazy-arguments" % "0.1.0"
  • Add scalameta paradise compiler plugin and scalameta dependency:
addCompilerPlugin("org.scalameta" % "paradise" % "3.0.0-M8" cross CrossVersion.full)

libraryDependencies += "org.scalameta" %% "scalameta" % "1.8.0" % Provided

You can find an example of a build.sbt file with all the required dependencies bellow:

scalaVersion := "2.12.2"

resolvers += Resolver.bintrayIvyRepo("tudorzgureanu", "generic")

addCompilerPlugin("org.scalameta" % "paradise" % "3.0.0-M8" cross CrossVersion.full)

libraryDependencies ++= Seq(
  "com.tudorzgureanu" %% "scala-lazy-arguments" % "0.1.0",
  "org.scalameta" %% "scalameta" % "1.8.0" % Provided

Why call-by-need and why call-by-name is not enough?

Scala provides support for by-need arguments (def foo(bar: => String)) but they are evaluated on every occurence in the code. Call-by-need is a memoised version of call-by-name (i.e. it is evaluated only once when it's used for the first time).

Implementation details

The current implementation relies on local lazy vals so keep in mind when using it.

Future plans

I will try to find time to follow the progress of scalameta and apply any improvements to this project. If you have any ideas feel free to open an issue.

Also, I plan to add a naive lazy implementation for the code that doesn't involve any multi-threading (which is most of the time). This will replace the local lazy vals with a simple (non thread-safe) cache.