Generating a class from string and instantiating it in Scala 2.10

by

W.r.t compilation toolboxes can only run expressions = return values, but not resulting classes or files/byte arrays with compilation results.

However it’s still possible to achieve what you want, since in Scala it’s so easy to go from type level to value level using implicit values:

Edit. In 2.10.0-RC1 some methods of ToolBox have been renamed. parseExpr is now just parse, and runExpr is now called eval.

scala> import scala.reflect.runtime._ // requires scala-reflect.jar
                                      // in REPL it's implicitly added 
                                      // to the classpath
                                      // but in your programs
                                      // you need to do this on your own
import scala.reflect.runtime

scala> val cm = universe.runtimeMirror(getClass.getClassLoader)
cm @ 41d0fe80: reflect.runtime.universe.Mirror = JavaMirror with scala.tools.nsc.interpreter.IMain$TranslatingClassLoader...

scala> import scala.tools.reflect.ToolBox // requires scala-compiler.jar
                                          // in REPL it's implicitly added 
                                          // to the classpath
                                          // but in your programs
                                          // you need to do this on your own
import scala.tools.reflect.ToolBox

scala> val tb = cm.mkToolBox()
tb: scala.tools.reflect.ToolBox[reflect.runtime.universe.type] = scala.tools.reflect.ToolBoxFactory$ToolBoxImpl@3a962da5

scala> tb.runExpr(tb.parseExpr("class C; scala.reflect.classTag[C].runtimeClass"))
res2: Any = class __wrapper$1$f9d572ca0d884bca9333e251c64e980d$C$1

Update #1. If you don’t need a java.lang.Class and just need to instantiate the compiled class, you can write new C directly in the string submitted to runExpr.

Update #2. It is also possible to have runExpr use custom mapping from variable names to runtime values. For example:

scala> val build = scala.reflect.runtime.universe.build
build: reflect.runtime.universe.BuildApi = scala.reflect.internal.BuildUtils$BuildImpl@50d5afff

scala> val x = build.setTypeSignature(build.newFreeTerm("x", 2), typeOf[Int])
x: reflect.runtime.universe.FreeTermSymbol = free term x

scala> tb.runExpr(Apply(Select(Ident(x), newTermName("$plus")), List(Literal(Constant(2)))))
res0: Any = 4

In this example I create a free term that has a value of 2 (the value doesn’t have to be a primitive – it can be your custom object) and bind an identifier to it. This value is then used as-is in the code that is compiled and run by a toolbox.

The example uses manual AST assembly, but it’s possible to write a function that parses a string, finds out unbound identifiers, looks up values for them in some mapping and then creates corresponding free terms. There’s no such function in Scala 2.10.0 though.

Leave a Comment

tech