Scala 条件编译

Question

我正在编写一个 Scala 程序，我希望它能与一个大库的两个版本一起工作。

这个大型库的版本 2 非常轻微地更改了 API(只有一个类构造函数签名有一个额外的参数)。

// Lib v1
class APIClass(a: String, b:Integer){
...
}

// Lib v2
class APIClass(a: String, b: Integer, c: String){
...
}


// And my code extends APIClass.. And I have no #IFDEF

class MyClass() extends APIClass("x", 1){ //  <--  would be APIClass("x", 1, "y") in library v2
  ...
}

我真的不想分支我的代码。因为那时我需要维护两个分支，明天需要维护 3,4,.. 分支以进行微小的 API 更改:(

理想情况下，我们会在 Scala 中有一个简单的预处理器，但这个想法很久以前就被 Scala 社区拒绝了。

我真的无法理解的一件事是:可以 Scalameta在这种情况下帮助模拟预处理器？ IE。有条件地解析两个源文件以 - 比如说 - 编译时已知的环境变量？

如果不是，您将如何解决这个现实生活中的问题？

Answer 1

1. 如果在 javac 或 scalac 之前运行 cpp，C++ 预处理器可以与 Java/Scala 一起使用(还有 Manifold )。

---

2. 如果你真的想在 Scala 中进行条件编译，你可以使用 macro annotation (在编译时展开)

macros/src/main/scala/extendsAPIClass.scala

import scala.annotation.{StaticAnnotation, compileTimeOnly}
import scala.language.experimental.macros
import scala.reflect.macros.blackbox

@compileTimeOnly("enable macro paradise")
class extendsAPIClass extends StaticAnnotation {
  def macroTransform(annottees: Any*): Any = macro ExtendsAPIClassMacro.impl
}

object ExtendsAPIClassMacro {
  def impl(c: blackbox.Context)(annottees: c.Tree*): c.Tree = {
    import c.universe._
    annottees match {
      case q"$mods class $tpname[..$tparams] $ctorMods(...$paramss) extends { ..$earlydefns } with ..$parents { $self => ..$stats }" :: tail => 
        def updateParents(parents: Seq[Tree], args: Seq[Tree]) = 
          q"""${tq"APIClass"}(..$args)""" +: parents.filter { case tq"scala.AnyRef" => false; case _ => true }

        val parents1 = sys.env.get("LIB_VERSION") match {
          case Some("1") => updateParents(parents, Seq(q""" "x" """, q"1"))
          case Some("2") => updateParents(parents, Seq(q""" "x" """, q"1", q""" "y" """))
          case None      => parents
        }

        q"""
          $mods class $tpname[..$tparams] $ctorMods(...$paramss) extends { ..$earlydefns } with ..$parents1 { $self => ..$stats }
          ..$tail
        """
    }
  }
}

core/src/main/scala/MyClass.scala(如果 LIB_VERSION=2)

@extendsAPIClass
class MyClass

//Warning:scalac: {
//  class MyClass extends APIClass("x", 1, "y") {
//    def <init>() = {
//      super.<init>();
//      ()
//    }
//  };
//  ()
//}

build.sbt

ThisBuild / name := "macrosdemo"

lazy val commonSettings = Seq(
  scalaVersion := "2.13.2",
  organization := "com.example",
  version := "1.0.0",
  scalacOptions ++= Seq(
    "-Ymacro-debug-lite",
    "-Ymacro-annotations",
  ),
)

lazy val macros: Project = (project in file("macros")).settings(
  commonSettings,
  libraryDependencies ++= Seq(
    scalaOrganization.value % "scala-reflect" % scalaVersion.value,
  )
)

lazy val core: Project = (project in file("core")).aggregate(macros).dependsOn(macros).settings(
  commonSettings,
  )
)

---

3. 或者您可以使用 Scalameta用于代码生成(在编译之前的时间)

build.sbt

ThisBuild / name := "scalametacodegendemo"

lazy val commonSettings = Seq(
  scalaVersion := "2.13.2",
  organization := "com.example",
  version := "1.0.0",
)

lazy val common = project
  .settings(
    commonSettings,
  )

lazy val in = project
  .dependsOn(common)
  .settings(
    commonSettings,
  )

lazy val out = project
  .dependsOn(common)
  .settings(
    sourceGenerators in Compile += Def.task {
      Generator.gen(
        inputDir = sourceDirectory.in(in, Compile).value,
        outputDir = sourceManaged.in(Compile).value
      )
    }.taskValue,
    commonSettings,
  )

project/build.sbt

libraryDependencies += "org.scalameta" %% "scalameta" % "4.3.10"

项目/Generator.scala

import sbt._

object Generator {
  def gen(inputDir: File, outputDir: File): Seq[File] = {
    val finder: PathFinder = inputDir ** "*.scala"

    for(inputFile <- finder.get) yield {
      val inputStr = IO.read(inputFile)
      val outputFile = outputDir / inputFile.toURI.toString.stripPrefix(inputDir.toURI.toString)
      val outputStr = Transformer.transform(inputStr)
      IO.write(outputFile, outputStr)
      outputFile
    }
  }
}

项目/Transformer.scala

import scala.meta._

object Transformer {
  def transform(input: String): String = {
    val (v1on, v2on) = sys.env.get("LIB_VERSION") match {
      case Some("1") => (true, false)
      case Some("2") => (false, true)
      case None      => (false, false)
    }
    var v1 = false
    var v2 = false
    input.tokenize.get.filter(_.text match {
      case "// Lib v1" =>
        v1 = true
        false
      case "// End Lib v1" =>
        v1 = false
        false
      case "// Lib v2" =>
        v2 = true
        false
      case "// End Lib v2" =>
        v2 = false
        false
      case _ => (v1on && v1) || (v2on && v2) || (!v1 && !v2)
    }).mkString("")
  }
}

common/src/main/scala/com/api/APIClass.scala

package com.api

class APIClass(a: String, b: Integer, c: String)

在/src/main/scala/com/example/MyClass.scala

package com.example

import com.api.APIClass

// Lib v1
class MyClass extends APIClass("x", 1)
// End Lib v1

// Lib v2
class MyClass extends APIClass("x", 1, "y")
// End Lib v2

out/target/scala-2.13/src_managed/main/scala/com/example/MyClass.scala

(如果 LIB_VERSION=2，则 sbt out/compile 之后)

package com.example

import com.api.APIClass

class MyClass extends APIClass("x", 1, "y")

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