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trait Evals extends AnyRef

EXPERIMENTAL

A slice of the Scala macros context that provides a facility to evaluate trees.

Self Type
blackbox.Context
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  1. abstract def eval[T](expr: blackbox.Context.Expr[T]): T

    Takes a typed wrapper for a tree of type T and evaluates it to a value of type T.

    Takes a typed wrapper for a tree of type T and evaluates it to a value of type T.

    Can be used to perform compile-time computations on macro arguments to the extent permitted by the shape of the arguments.

    Known issues: because of https://github.com/scala/bug/issues/5748 trees being evaluated first need to undergo untypecheck. Resetting symbols and types mutates the tree in place, therefore the conventional approach is to duplicate the tree first.

    scala> def impl(c: Context)(x: c.Expr[String]) = {
         | val x1 = c.Expr[String](c.untypecheck(x.tree.duplicate))
         | println(s"compile-time value is: ${c.eval(x1)}")
         | x
         | }
    impl: (c: Context)(x: c.Expr[String])c.Expr[String]
    
    scala> def test(x: String) = macro impl
    test: (x: String)String
    
    scala> test("x")
    compile-time value is: x
    res0: String = x
    
    scala> test("x" + "y")
    compile-time value is: xy
    res1: String = xy
    
    scala> val x = "x"
    x: String = x
    
    scala> test(x + "y")
    compile-time value is: xy
    res2: String = xy
    
    scala> { val x = "x"; test(x + "y") }
    error: exception during macro expansion:
    scala.tools.reflect.ToolBoxError: reflective compilation failed

    Note that in the last case evaluation has failed, because the argument of a macro refers to a runtime value x, which is unknown at compile time.

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