Merge remote-tracking branch 'ingo/topic/reflection-docs' into topic/reflection

VladUreche · VladUreche · commit 5617e4653aa0 · 2012-10-10T14:10:40.000+02:00
diff --git a/src/reflect/scala/reflect/api/Symbols.scala b/src/reflect/scala/reflect/api/Symbols.scala
@@ -3,205 +3,19 @@ package api
 
 /** This trait defines symbols and operations on them.
  *
- *  See the [[docs.scala-lang.org/overviews/reflection/overview.html Reflection Guide]] for a description of symbols 
- *  and infomation on getting started with Scala reflection API.
+ *  Symbols are used to establish bindings between a name and the entity it refers to, such as a class or a method.
+ *  Anything you define and can give a name to in Scala has a symbol. 
  *
- *  === Symbols from a compile-time perspective ===
+ *  Certain types of tree nodes, such as [[scala.reflect.api.Trees#Ident Ident]] (references to identifiers) and 
+ *  [[scala.reflect.api.Trees#Select Select]] (references to members) expose method [[Trees.SymTreeApi.symbol]] 
+ *  to obtain the symbol that represents their declaration.
  *
- *  Anything you define in Scala has a symbol. If you give something a name, then it has a symbol associated with it. 
- *  If you didn't give it a name, but you could have, then it has a symbol.
+ *  See the [[docs.scala-lang.org/overviews/reflection/overview.html Reflection Guide]] for more details and 
+ *  intricacies about symbols.
  *
- *  Symbols are used by the Scala compiler to establish bindings. When typechecking a Scala program,
- *  the compiler populates [[scala.reflect.api.Trees#RefTrees ref trees]], such as [[scala.reflect.api.Trees#Ident Ident]]
- *  (references to identifiers) and [[scala.reflect.api.Trees#Select Select]] (references to members)
- *  with symbols that represent the declarations being referred to. Populating means setting the `symbol`
- *  field to a non-empty value.
- *
- *  Here's an example of how trees look after the `typer` phase of the Scala compiler (this phase performs the typechecking).
- *  {{{
- *  >cat Test.scala
- *  def foo[T: TypeTag](x: Any) = x.asInstanceOf[T]
- *
- *  >scalac -Xprint:typer -uniqid Test.scala
- *  [[syntax trees at end of typer]]// Scala source: Test.scala
- *  def foo#8339
- *    [T#8340 >: Nothing#4658 <: Any#4657]
- *    (x#9529: Any#4657)
- *    (implicit evidence$1#9530: TypeTag#7861[T#8341])
- *    : T#8340 =
- *  x#9529.asInstanceOf#6023[T#8341];
- *  }}}
- *
- *  Shortly put, we write a small snippet and then compile it with scalac, asking the compiler to dump the trees
- *  after the typer phase, printing unique ids of the symbols assigned to trees (if any).
- *
- *  The resulting printout shows that identifiers have been linked to corresponding definitions.
- *  For example, on the one hand, the `ValDef("x", ...)`, which represents the parameter of the method `foo`,
- *  defines a method symbol with `id=9529`. On the other hand, the `Ident("x")` in the body of the method
- *  got its `symbol` field set to the same symbol, which establishes the binding.
- *
- *  In the light of this discussion, it might come as a surprise that the definition of the type parameter `T`
- *  has a symbol with `id=8340`, whereas references to this type parameter all have a symbol with `id=8341`.
- *  This is the only exception from the general principe outlined above. It happens because the Scala compiler
- *  skolemizes type parameters (creates new symbols very similar to the original ones) before entering scopes
- *  that define these parameters. This is an advanced feature of Scala, and the knowledge of it is needed only
- *  when writing complex macros, but symbols in the macro universe [[scala.reflect.macros.Universe]] have the
- *  `deskolemize` method, which goes back from skolems to the originating type parameters.
- *
- *  === Symbols from a runtime perspective ===
- *
- *  From the point of view of a runtime reflection framework, symbols are akin to [[java.lang.reflect.Member]] from Java
- *  and [[System.Reflection.MemberInfo]] from .NET. But not only they represent members - they also represent
- *  classes, objects and even packages.
- *
- *  Also similarly to the base classes in the reflection facilities of JVM and .NET, Scala symbols have subclasses
- *  that describe particular flavors of definitions. [[scala.reflect.api.Symbols#TermSymbol]] models term definitions
- *  (such as lazy and eager vals, vars and parameters of methods). Its subclasses are [[scala.reflect.api.Symbols#MethodSymbol]]
- *  and [[scala.reflect.api.Symbols#ModuleSymbol]] (representing "modules", which in Scala compiler speak mean "objects").
- *  [[scala.reflect.api.Symbols#TypeSymbol]] along with its subclass [[scala.reflect.api.Symbols#ClassSymbol]]
- *  describes type definitions in Scala (type aliases, type members, type parameters, classes and traits).
- *
- *  Most reflection APIs that return symbols return non-specific [[scala.reflect.api.Symbols#Symbol]], because upon failure
- *  they don't raise exceptions, but rather produce `NoSymbol`, a special singleton, which is a null object for symbols.
- *  Therefore to use such APIs one has to first check whether a callee returned a valid symbol and, if yes, then perform
- *  a cast using one of the `asTerm`, `asMethod`, `asModule`, `asType` or `asClass` methods. This is arguably inconvenient
- *  and might be improved in the future.
- *
- *  Unlike [[scala.reflect.api.Trees trees]] and [[scala.reflect.api.Types types]], symbols should not be created directly.
- *  Instead one should load the symbols from the global symbol table maintained by the compiler.
- *  To get a symbol that corresponds to a top-level class or object, one can use the `staticClass` and `staticModule`
- *  methods of [[scala.reflect.api.Mirror]]. To get a symbol that corresponds to a certain member, there are `members`
- *  and `declarations` methods of [[scala.reflect.api.Types#Type]], which brings the discussion to the next point: type signatures.
- *
- *  Each symbol has a type signature, which describes its type and is available via the `typeSignature` method
- *  on [[scala.reflect.api.Symbols#Symbol]]. Classes have signatures of the [[scala.reflect.api.Types#ClassInfoType]] type,
- *  which knows the list of its members and declarations. Modules per se don't have interesting signatures. To access members
- *  of modules, one first has to obtain a module class (using the `moduleClass` method) and then inspect its signature.
- *  Members have type signatures of their own: method signatures feature information about parameters and result types,
- *  type member signatures store upper and lower bounds and so on.
- *
- *  One thing to know about type signatures is that `typeSignature` method always returns signatures in the most generic
- *  way possible, even if the underlying symbol is obtained from an instantiation of a generic type. For example, signature
- *  of the method `def map[B](f: (A) ⇒ B): List[B]`, which refers to the type parameter `A` of the declaring class `List[A]`,
- *  will always feature `A`, regardless of whether `map` is loaded from the `List[_]` or from `List[Int]`. To get a signature
- *  with type parameters appropriately instantiated, one should use `typeSignatureIn`.
- *
- *  Symbols are at the heart of the reflection API. Along with the type signatures, which are arguably the most important
- *  use of reflection, they provide comprehensive information about the underlying definitions. This includes various
- *  `isXXX` test methods such as `isPublic` or `isFinal`, `params` and `returnType` methods for method symbols,
- *  `baseClasses` for class symbols and so on. Be prepared - some of these methods don't make sense on the ultimate
- *   base class Symbol, so they are declared in subclasses.
- *
- *  === Exploring symbols ===
- *
- *  In this example we'll try to get a hold on a symbol that represents the `map` method of `List`,
- *  and then do something interesting with it.
- *
- *  First of all, to obtain a symbol, one needs to load its enclosing top-level class or module.
- *  There are two ways of doing that. The first one is getting a symbol by its name using a mirror
- *  (refer to [[scala.reflect.api.package the reflection overview]] for information about mirrors).
- *  Another one is getting a type with [[scaa.reflect.api.Types#typeOf]] and using its `typeSymbol` method.
- *  The second approach is preferable, because it's typesafe, but sometimes it's unavailable.
- *
- *  {{{
- *  scala> import scala.reflect.runtime.universe._
- *  import scala.reflect.runtime.universe._
- *
- *  scala> val cm = runtimeMirror(getClass.getClassLoader)
- *  cm: reflect.runtime.universe.Mirror = JavaMirror with ...
- *
- *  scala> val list = cm.staticClass("scala.List")
- *  list: reflect.runtime.universe.ClassSymbol = class List
- *
- *  scala> val list = typeOf[List[_]].typeSymbol
- *  list: reflect.runtime.universe.Symbol = class List
- *  }}}
- *
- *  Now when the enclosing class is obtained, there's a straight path to getting its member
- *  using `typeSignature` and `member` methods discussed above:
- *
- *  {{{
- *  scala> val map = list.typeSignature.member("map": TermName).asMethod
- *  map: reflect.runtime.universe.MethodSymbol = method map
- *
- *  scala> map.typeSignature
- *  res0: reflect.runtime.universe.Type = [B, That](f: A => B)(implicit bf:
- *  scala.collection.generic.CanBuildFrom[Repr,B,That])That
- *
- *  scala> map.typeSignatureIn(typeOf[List[Int]])
- *  res1: reflect.runtime.universe.Type = [B, That](f: Int => B)(implicit bf:
- *  scala.collection.generic.CanBuildFrom[List[Int],B,That])That
- *
- *  scala> map.params
- *  res2: List[List[reflect.runtime.universe.Symbol]] = List(List(value f), List(value bf))
- *
- *  scala> val filter = map.params(0)(0)
- *  filter: reflect.runtime.universe.Symbol = value f
- *
- *  scala> filter.name
- *  res3: reflect.runtime.universe.Name = f
- *
- *  scala> filter.typeSignature
- *  res4: reflect.runtime.universe.Type = A => B
- *  }}}
- *
- *  === Gotcha #1: Overloaded methods ===
- *
- *  Be careful though, because overloaded methods are represented as instances of TermSymbol
- *  with multiple `alternatives` that have to be resolved manually. For example, a lookup
- *  for a member named `mkString` will produce not a MethodSymbol, but a TermSymbol:
- *
- *  {{{
- *  scala> list.typeSignature.member("mkString": TermName)
- *  res1: reflect.runtime.universe.Symbol = value mkString
- *
- *  scala> val mkString = list.typeSignature.member("mkString": TermName).asTerm
- *  mkString: reflect.runtime.universe.TermSymbol = value mkString
- *
- *  scala> mkString.isMethod
- *  res0: Boolean = false
- *
- *  scala> mkString.alternatives
- *  res1: List[reflect.runtime.universe.Symbol] = List(method mkString, method mkString, method mkString)
- *
- *  scala> mkString.alternatives foreach println
- *  method mkString
- *  method mkString
- *  method mkString
- *
- *  scala> mkString.alternatives foreach (alt => println(alt.typeSignature))
- *  => String
- *  (sep: String)String
- *  (start: String, sep: String, end: String)String
- *  }}}
- *
- *  Once one has a symbol, that symbol can be used for reflective invocations. For example,
- *  having a TermSymbol corresponding to a field it's possible to get or set a value of that field.
- *  Having a MethodSymbol makes it possible to invoke the corresponding methods. ClassSymbols
- *  can be instantiated. ModuleSymbols can provide corresponding singleton instances. This is described
- *  in detail on [[scala.reflect.api.package the reflection overview page]].
- *
- *  === Gotcha #2: Module classes ===
- *
- *  Internally the Scala compiler represents objects with two symbols: a module symbol and a module class symbol.
- *  The former is a term symbol, used everywhere a module is referenced (e.g. in singleton types or in expressions),
- *  while the latter is a type symbol, which carries the type signature (i.e. the member list) of the module.
- *  This implementation detail can be easily seen by compiling a trivial snippet of code. Invoking the Scala
- *  compiler on `object C` will generate C$.class. That's exactly the module class.
- *
- *  Note that module classes are different from companion classes. Say, for `case class C`, the compiler
- *  will generate three symbols: `type C`, `term C` and (another one) `type C`, where the first type `C`
- *  represents the class `C` (which contains auto-generated `copy`, `productPrefix`, `productArity` etc) and
- *  the second type `C` represents the signature of object `C` (which contains auto-generated factory,
- *  extractor etc). There won't be any name clashes, because the module class isn't added to the symbol table
- *  directly and is only available through `<module>.moduleClass`. For the sake of completeness, it is possible
- *  to go back from a module class to a module via `<module class>.module`.
- *
- *  Separation between modules and module classes is something that we might eliminate in the future, but for now
- *  this obscure implementation detail has to be taken into account when working with reflection. On the one hand,
- *  it is necessary to go to a module class to get a list of members for an object. On the other hand, it is
- *  necessary to go from a module class back to a module to get a singleton instance of an object. The latter
- *  scenario is described at Stack Overflow: [[http://stackoverflow.com/questions/12128783 How can I get the actual object referred to by Scala 2.10 reflection?]].
+ *  @define SYM_ACCESSORS Class [[Symbol]] defines `isXXX` test methods such as `isPublic` or `isFinal`, `params` and 
+ *  `returnType` methods for method symbols, `baseClasses` for class symbols and so on. Some of these methods don't 
+ *  make sense for certain subclasses of `Symbol` and return `NoSymbol`, `Nil` or other empty values. 
  */
 trait Symbols { self: Universe =>
 
@@ -278,6 +92,8 @@ trait Symbols { self: Universe =>
 
   /** The API of symbols.
    *  The main source of information about symbols is the [[scala.reflect.api.Symbols]] page.
+   * 
+   *  $SYM_ACCESSORS
    */
   trait SymbolApi { this: Symbol =>
 
@@ -430,8 +246,12 @@ trait Symbols { self: Universe =>
     def typeSignatureIn(site: Type): Type
 
     /** The type signature of this symbol.
-     *  Note if the symbol is a member of a class, one almost always is interested
-     *  in `typeSignatureIn` with a site type instead.
+     *
+     *  This method always returns signatures in the most generic way possible, even if the underlying symbol is obtained from an 
+     *  instantiation of a generic type. For example, signature
+     *  of the method `def map[B](f: (A) ⇒ B): List[B]`, which refers to the type parameter `A` of the declaring class `List[A]`,
+     *  will always feature `A`, regardless of whether `map` is loaded from the `List[_]` or from `List[Int]`. To get a signature
+     *  with type parameters appropriately instantiated, one should use `typeSignatureIn`.
      */
     def typeSignature: Type
 
@@ -579,6 +399,8 @@ trait Symbols { self: Universe =>
 
   /** The API of term symbols.
    *  The main source of information about symbols is the [[scala.reflect.api.Symbols]] page.
+   *
+   *  $SYM_ACCESSORS
    */
   trait TermSymbolApi extends SymbolApi { this: TermSymbol =>
     /** Term symbols have their names of type `TermName`.
@@ -662,6 +484,8 @@ trait Symbols { self: Universe =>
 
   /** The API of type symbols.
    *  The main source of information about symbols is the [[scala.reflect.api.Symbols]] page.
+   *
+   *  $SYM_ACCESSORS
    */
   trait TypeSymbolApi extends SymbolApi { this: TypeSymbol =>
     /** Type symbols have their names of type `TypeName`.
@@ -729,6 +553,8 @@ trait Symbols { self: Universe =>
 
   /** The API of method symbols.
    *  The main source of information about symbols is the [[scala.reflect.api.Symbols]] page.
+   *
+   *  $SYM_ACCESSORS
    */
   trait MethodSymbolApi extends TermSymbolApi { this: MethodSymbol =>
     final override def isMethod = true
@@ -765,6 +591,8 @@ trait Symbols { self: Universe =>
 
   /** The API of module symbols.
    *  The main source of information about symbols is the [[scala.reflect.api.Symbols]] page.
+   *
+   *  $SYM_ACCESSORS
    */
   trait ModuleSymbolApi extends TermSymbolApi { this: ModuleSymbol =>
     /** The class implicitly associated with the object definition.
@@ -780,6 +608,8 @@ trait Symbols { self: Universe =>
 
   /** The API of class symbols.
    *  The main source of information about symbols is the [[scala.reflect.api.Symbols]] page.
+   *
+   *  $SYM_ACCESSORS
    */
   trait ClassSymbolApi extends TypeSymbolApi { this: ClassSymbol =>
     final override def isClass = true
@@ -848,6 +678,8 @@ trait Symbols { self: Universe =>
 
   /** The API of free term symbols.
    *  The main source of information about symbols is the [[scala.reflect.api.Symbols]] page.
+   *
+   *  $SYM_ACCESSORS
    */
   trait FreeTermSymbolApi extends TermSymbolApi { this: FreeTermSymbol =>
     final override def isFreeTerm = true
@@ -862,6 +694,8 @@ trait Symbols { self: Universe =>
 
   /** The API of free type symbols.
    *  The main source of information about symbols is the [[scala.reflect.api.Symbols]] page.
+   *
+   *  $SYM_ACCESSORS
    */
   trait FreeTypeSymbolApi extends TypeSymbolApi { this: FreeTypeSymbol =>
     final override def isFreeType = true
