Scala Compiler2.13.5 < Back
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# Packages

-  package root

  The Scala compiler and reflection APIs.

  The Scala compiler and reflection APIs.

  Definition Classes
    root
-  package scala
  Definition Classes
    root
-  package reflect
  Definition Classes
    scala
-  package reify
  Definition Classes
    reflect
-  package phases
  Definition Classes
    reify
- Calculate
- Metalevels
- Reify
- Reshape

t

scala.reflect.reify.phases

# Metalevels

###

#### trait Metalevels extends AnyRef

Self Type
  Reifier
Source
  Metalevels.scala

Linear Supertypes
AnyRef, Any
Known Subclasses
ApplyReifier, Reifier, UnapplyReifier, Phases, Reifier
Type Hierarchy

Ordering

1. Alphabetic
2. By Inheritance

Inherited

1. Metalevels
2. AnyRef
3. Any

Implicitly

1. by any2stringadd
2. by StringFormat
3. by Ensuring
4. by ArrowAssoc

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Visibility

1. Public
2. Protected

### Value Members

1.  final def !=(arg0: Any): Boolean
   Definition Classes
     AnyRef → Any
2.  final def ##: Int
   Definition Classes
     AnyRef → Any
3.  def +(other: String): String
   Implicit
     This member is added by an implicit conversion from Metalevels toany2stringadd[Metalevels] performed by method any2stringadd in scala.Predef.
   Definition Classes
     any2stringadd
4.  def ->[B](y: B): (Metalevels, B)
   Implicit
     This member is added by an implicit conversion from Metalevels toArrowAssoc[Metalevels] performed by method ArrowAssoc in scala.Predef.
   Definition Classes
     ArrowAssoc
   Annotations
     @inline()
5.  final def ==(arg0: Any): Boolean
   Definition Classes
     AnyRef → Any
6.  final def asInstanceOf[T0]: T0
   Definition Classes
     Any
7.  def clone(): AnyRef
   Attributes
     protected[lang]
   Definition Classes
     AnyRef
   Annotations
     @throws(classOf[java.lang.CloneNotSupportedException]) @native()
8.  def ensuring(cond: (Metalevels) => Boolean, msg: => Any): Metalevels
   Implicit
     This member is added by an implicit conversion from Metalevels toEnsuring[Metalevels] performed by method Ensuring in scala.Predef.
   Definition Classes
     Ensuring
9.  def ensuring(cond: (Metalevels) => Boolean): Metalevels
   Implicit
     This member is added by an implicit conversion from Metalevels toEnsuring[Metalevels] performed by method Ensuring in scala.Predef.
   Definition Classes
     Ensuring
10.  def ensuring(cond: Boolean, msg: => Any): Metalevels
    Implicit
      This member is added by an implicit conversion from Metalevels toEnsuring[Metalevels] performed by method Ensuring in scala.Predef.
    Definition Classes
      Ensuring
11.  def ensuring(cond: Boolean): Metalevels
    Implicit
      This member is added by an implicit conversion from Metalevels toEnsuring[Metalevels] performed by method Ensuring in scala.Predef.
    Definition Classes
      Ensuring
12.  final def eq(arg0: AnyRef): Boolean
    Definition Classes
      AnyRef
13.  def equals(arg0: AnyRef): Boolean
    Definition Classes
      AnyRef → Any
14.  def finalize(): Unit
    Attributes
      protected[lang]
    Definition Classes
      AnyRef
    Annotations
      @throws(classOf[java.lang.Throwable])
15.  def formatted(fmtstr: String): String
    Implicit
      This member is added by an implicit conversion from Metalevels toStringFormat[Metalevels] performed by method StringFormat in scala.Predef.
    Definition Classes
      StringFormat
    Annotations
      @inline()
16.  final def getClass(): Class[_ <: AnyRef]
    Definition Classes
      AnyRef → Any
    Annotations
      @native()
17.  def hashCode(): Int
    Definition Classes
      AnyRef → Any
    Annotations
      @native()
18.  final def isInstanceOf[T0]: Boolean
    Definition Classes
      Any
19.  val metalevels: tools.nsc.Global.AstTransformer { ... /* 4 definitions in type refinement */ }

    Makes sense of cross-stage bindings.

    Makes sense of cross-stage bindings.

    Analysis of cross-stage bindings becomes convenient if we introduce the notion of metalevels. Metalevel of a tree is a number that gets incremented every time you reify something and gets decremented when you splice something. Metalevel of a symbol is equal to the metalevel of its definition.

    Example 1. Consider the following snippet:

    reify { val x = 2 // metalevel of symbol x is 1, because it's declared inside reify val y = reify{x} // metalevel of symbol y is 1, because it's declared inside reify // metalevel of Ident(x) is 2, because it's inside two reifies y.splice // metalevel of Ident(y) is 0, because it's inside a designator of a splice }

    Cross-stage bindings are introduced when symbol.metalevel != curr_metalevel. Both bindings introduced in Example 1 are cross-stage.

    Depending on what side of the inequality is greater, the following situations might occur:

    1) symbol.metalevel < curr_metalevel. In this case reifier will generate a free variable that captures both the name of the symbol (to be compiled successfully) and its value (to be run successfully). For example, x in Example 1 will be reified as follows: Ident(newFreeVar("x", IntTpe, x))

    2) symbol.metalevel > curr_metalevel. This leads to a metalevel breach that violates intuitive perception of splicing. As defined in macro spec, splicing takes a tree and inserts it into another tree - as simple as that. However, how exactly do we do that in the case of y.splice? In this very scenario we can use dataflow analysis and inline it, but what if y were a var, and what if it were calculated randomly at runtime?

    This question has a genuinely simple answer. Sure, we cannot resolve such splices statically (i.e. during macro expansion of `reify`), but now we have runtime toolboxes, so noone stops us from picking up that reified tree and evaluating it at runtime (in fact, this is something that `Expr.splice` does transparently).

    This is akin to early vs late binding dilemma. The prior is faster, plus, the latter (implemented with reflection) might not work because of visibility issues or might be not available on all platforms. But the latter still has its uses, so I'm allowing metalevel breaches, but introducing the -Xlog-runtime-evals to log them.

    upd. We no longer do that. In case of a runaway `splice` inside a `reify`, one will get a static error. Why? Unfortunately, the cute idea of transparently converting between static and dynamic splices has failed. 1) Runtime eval that services dynamic splices requires scala-compiler.jar, which might not be on library classpath 2) Runtime eval incurs a severe performance penalty, so it'd better to be explicit about it

    As we can see, the only problem is the fact that lhs'es of `splice` can be code blocks that can capture variables from the outside. Code inside the lhs of an `splice` is not reified, while the code from the enclosing reify is.

    Hence some bindings become cross-stage, which is not bad per se (in fact, some cross-stage bindings have sane semantics, as in the example above). However this affects freevars, since they are delicate inter-dimensional beings that refer to both current and next planes of existence. When splicing tears the fabric of the reality apart, some freevars have to go single-dimensional to retain their sanity.

    Example 2. Consider the following snippet:

    reify { val x = 2 reify{x}.splice }

    Since the result of the inner reify is wrapped in a splice, it won't be reified together with the other parts of the outer reify, but will be inserted into that result verbatim.

    The inner reify produces an Expr[Int] that wraps Ident(freeVar("x", IntTpe, x)). However the freevar the reification points to will vanish when the compiler processes the outer reify. That's why we need to replace that freevar with a regular symbol that will point to reified x.

    Example 3. Consider the following fragment:

    reify { val x = 2 val y = reify{x} y.splice }

    In this case the inner reify doesn't appear next to splice, so it will be reified together with x. This means that no special processing is needed here.

    Example 4. Consider the following fragment:

    reify { val x = 2 { val y = 2 val z = reify{reify{x + y}} z.splice }.splice }

    The reasoning from Example 2 still holds here - we do need to inline the freevar that refers to x. However, we must not touch anything inside the splice'd block, because it's not getting reified.

20.  final def ne(arg0: AnyRef): Boolean
    Definition Classes
      AnyRef
21.  final def notify(): Unit
    Definition Classes
      AnyRef
    Annotations
      @native()
22.  final def notifyAll(): Unit
    Definition Classes
      AnyRef
    Annotations
      @native()
23.  final def synchronized[T0](arg0: => T0): T0
    Definition Classes
      AnyRef
24.  def toString(): String
    Definition Classes
      AnyRef → Any
25.  final def wait(): Unit
    Definition Classes
      AnyRef
    Annotations
      @throws(classOf[java.lang.InterruptedException])
26.  final def wait(arg0: Long, arg1: Int): Unit
    Definition Classes
      AnyRef
    Annotations
      @throws(classOf[java.lang.InterruptedException])
27.  final def wait(arg0: Long): Unit
    Definition Classes
      AnyRef
    Annotations
      @throws(classOf[java.lang.InterruptedException]) @native()

### Deprecated Value Members

1.  def →[B](y: B): (Metalevels, B)
   Implicit
     This member is added by an implicit conversion from Metalevels toArrowAssoc[Metalevels] performed by method ArrowAssoc in scala.Predef.
   Definition Classes
     ArrowAssoc
   Annotations
     @deprecated
   Deprecated

     (Since version 2.13.0) Use `->` instead. If you still wish to display it as one character, consider using a font with programming ligatures such as Fira Code.

### Inherited from AnyRef

### Inherited from Any

### Inherited by implicit conversion any2stringadd fromMetalevels to any2stringadd[Metalevels]

### Inherited by implicit conversion StringFormat fromMetalevels to StringFormat[Metalevels]

### Inherited by implicit conversion Ensuring fromMetalevels to Ensuring[Metalevels]

### Inherited by implicit conversion ArrowAssoc fromMetalevels to ArrowAssoc[Metalevels]

### Ungrouped