我表面上看有一个简单的宏问题,我一直在努力奋战了好几个小时,没有运气。也许有更多经验的人可以提供帮助。
我有以下宏:
import scala.language.experimental.macros
import scala.reflect.macros.blackbox.Context
object MacroObject {
def run(s: String): Unit =
macro runImpl
def runImpl(c: Context)(s: c.Tree): c.Tree = {
import c.universe._
println(s) // <-- I need the macro to know the value of s at compile time
q"()"
}
}
问题是这样的:我想让宏知道传递给它的值s
-不是s
的AST,而是s
本身。具体来说,我希望它具有以下行为:
def runTheMacro(str: String): Unit = MacroObject.run(str)
final val HardCodedString1 = "Hello, world!"
runTheMacro(HardCodedString1) // the macro should print "Hello, world!"
// to the console during macro expansion
final val HardCodedString2 = "So long!"
runTheMacro(HardCodedString2) // the macro should print "So long!"
// to the console during macro expansion
保证,将唯一传递给runTheMacro
的字符串是硬编码的常量值(即在编译时已知)。
这有可能吗?怎么做到的?
-
编辑:还有以下限制:
c.Tree
s,而不是c.Expr[_]
s(旧版代码;不能更改该部分)toolbox
:import scala.reflect.runtime.currentMirror
import scala.tools.reflect.ToolBox
private val toolbox = currentMirror.mkToolBox()
/** Evaluate the given code fragment at compile time. */
private def eval[A](code: String): A = {
import scala.reflect.runtime.{universe => u}
val uTree: u.Tree = toolbox.parse(code)
toolbox.eval(uTree).asInstanceOf[A]
}
答案 0 :(得分:0)
您的eval
是运行时反射的eval
,编译时宏的eval
将是c.eval
。
"Hello, world!"
in
final val HardCodedString1 = "Hello, world!"
runTheMacro(HardCodedString1)
是HardCodedString1
的运行时值。
您不能在编译时访问运行时值。
在编译时,字符串HardCodedString1
的树对val
树的右侧一无所知。
Scala: what can code in Context.eval reference?
如果您确实需要在程序树中使用运行时值,则必须将其编译推迟到运行时
import scala.reflect.runtime.currentMirror
import scala.reflect.runtime.universe._
import scala.tools.reflect.ToolBox
object MacroObject {
val toolbox = currentMirror.mkToolBox()
def run(s: String): Unit = {
toolbox.eval(q"""
println($s)
()
""")
}
}
runTheMacro(HardCodedString1)//Hello, world!
runTheMacro(HardCodedString2)//So long!
或者,在编译时,您可以以某种方式找到封闭类的树,并在其中寻找val
树,并在其右边找到
def runImpl(c: blackbox.Context)(s: c.Tree): c.Tree = {
import c.universe._
var rhs: Tree = null
val traverser = new Traverser {
override def traverse(tree: Tree): Unit = {
tree match {
case q"$mods val $tname: $tpt = $expr" if tname == TermName("HardCodedString1") =>
rhs = expr
case _ => ()
}
super.traverse(tree)
}
}
traverser.traverse(c.enclosingClass) // deprecated
val rhsStr =
if (rhs != null) c.eval[String](c.Expr(c.untypecheck(rhs.duplicate)))
else c.abort(c.enclosingPosition, "no val HardCodedString1 defined")
println(rhsStr)
q"()"
}
runTheMacro(HardCodedString1)//Warning:scalac: Hello, world!
或针对所有此类变量
def runImpl(c: blackbox.Context)(s: c.Tree): c.Tree = {
import c.universe._
val sEvaluated =
try {
c.eval[String](c.Expr(c.untypecheck(s.duplicate)))
} catch {
case e: IllegalArgumentException if e.getMessage.startsWith("Could not find proxy") =>
s match {
case q"$sName" =>
var rhs: Tree = null
val traverser = new Traverser {
override def traverse(tree: Tree): Unit = {
tree match {
case q"$mods val $tname: $tpt = $expr" if tname == sName =>
rhs = expr
case _ => ()
}
super.traverse(tree)
}
}
traverser.traverse(c.enclosingClass)
if (rhs != null) c.eval[String](c.Expr(c.untypecheck(rhs.duplicate)))
else c.abort(c.enclosingPosition, s"no val $sName defined")
case _ => c.abort(c.enclosingPosition, s"unsupported tree $s")
}
}
println(sEvaluated)
q"()"
}
MacroObject.run(HardCodedString1) //Warning:scalac: Hello, world!
MacroObject.run(HardCodedString2) //Warning:scalac: So long!
runTheMacro
在这种情况下将不起作用:Error: no val str defined
。
要使其正常工作,您也可以使其成为一个宏
def runTheMacro(str: String): Unit = macro runTheMacroImpl
def runTheMacroImpl(c: blackbox.Context)(str: c.Tree): c.Tree = {
import c.universe._
q"MacroObject.run($str)"
}
runTheMacro(HardCodedString1) //Warning:scalac: Hello, world!
runTheMacro(HardCodedString2) //Warning:scalac: So long!