Evals

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://issues.scala-lang.org/browse/SI-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.

Test two objects for inequality.

Equivalent to x.hashCode except for boxed numeric types and null. For numerics, it returns a hash value which is consistent with value equality: if two value type instances compare as true, then ## will produce the same hash value for each of them. For null returns a hashcode where null.hashCode throws a NullPointerException.

The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).

Cast the receiver object to be of type T0.

Note that the success of a cast at runtime is modulo Scala's erasure semantics. Therefore the expression 1.asInstanceOf[String] will throw a ClassCastException at runtime, while the expression List(1).asInstanceOf[List[String]] will not. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the requested type.

Create a copy of the receiver object.

The default implementation of the clone method is platform dependent.

Tests whether the argument (that) is a reference to the receiver object (this).

The eq method implements an equivalence relation on non-null instances of AnyRef, and has three additional properties:

• It is consistent: for any non-null instances x and y of type AnyRef, multiple invocations of x.eq(y) consistently returns true or consistently returns false.
• For any non-null instance x of type AnyRef, x.eq(null) and null.eq(x) returns false.
• null.eq(null) returns true.

When overriding the equals or hashCode methods, it is important to ensure that their behavior is consistent with reference equality. Therefore, if two objects are references to each other (o1 eq o2), they should be equal to each other (o1 == o2) and they should hash to the same value (o1.hashCode == o2.hashCode).

The equality method for reference types. Default implementation delegates to eq.

See also equals in scala.Any.

Called by the garbage collector on the receiver object when there are no more references to the object.

The details of when and if the finalize method is invoked, as well as the interaction between finalize and non-local returns and exceptions, are all platform dependent.

A representation that corresponds to the dynamic class of the receiver object.

The nature of the representation is platform dependent.

The hashCode method for reference types. See hashCode in scala.Any.

Test whether the dynamic type of the receiver object is T0.

Note that the result of the test is modulo Scala's erasure semantics. Therefore the expression 1.isInstanceOf[String] will return false, while the expression List(1).isInstanceOf[List[String]] will return true. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the specified type.

Equivalent to !(this eq that).

Wakes up a single thread that is waiting on the receiver object's monitor.

Wakes up all threads that are waiting on the receiver object's monitor.

Creates a String representation of this object. The default representation is platform dependent. On the java platform it is the concatenation of the class name, "@", and the object's hashcode in hexadecimal.