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Feature #5394

closed

Anonymous Symbols, Anonymous Methods

Added by kstephens (Kurt Stephens) over 12 years ago. Updated over 11 years ago.

Status:
Rejected
Assignee:
-
Target version:
[ruby-core:39888]

Description

Proposal for Anonymous Symbols and Anonymous Methods

Anonymous Methods (AnonMeths) can be used for complex orthogonal behaviors that dispatch by receiver class without patching core or other sensitive classes in a globally visible manner.
AnonMeths are located by Anonymous Symbols (AnonSyms).
AnonSyms do not have parseable names, and can only be referenced by value, limiting namespace problems and promoting encapsulation.
AnonMeths are GCed once the AnonSym bound to them are GCed.
AnonMeths would not appear in Object#methods, thus will not confuse introspection.

Assume:


  Symbol.new() => # # an AnonSymbol than can never be parsed in ruby code.
  anon_sym = Symbol.new() # an AnonSym in a variable that can be closed-over or passed by value.

Optional Supporting Syntax:


  a.*anon_sym(args...) # equiv. to a.send(anon_sym, args...)
  class A
    def *anon_sym(args...); body...; end
  end

equiv. to:


  class A
    define_method(anon_sym) {| args... | body... }
  end

AnonSyms are not added directly to a Module's internal symbol-to-method table.
Instead, each AnonSym has an internal module-to-method table that is GCed when the AnonSym is GCed.


  rcvr.send(anon_sym, ...) 

will use anon_sym's module-to-method table to locate a method based on usual the receiver's module lookup chain.

Example Application:

Typical visitor pattern that pollutes Array and Object method namespaces:


  class Array;  def visit(visitor); each { | elem | elem.visit(visitor); } end; end
  class Object; def visit(visitor); visitor.something(self);               end; end

Functional alternative using "case ...; when ...":


  def visit(obj, visitor)
    case obj
    when Array
      obj.each { | elem | visit(elem, visitor) }
    else
      visitor.something(obj)
    end
  end

AnonMeth version:


  def visit(obj, visitor)
    sel = Symbol.new
    class Array;   def *sel(visitor); each { | elem | elem.*sel(visitor) }; end; end
    class Object;  def *sel(visitor); visitor.something(self);              end; end
    obj.*sel(visitor)
  end

Imagine that visit() needs dynamic hooks to visit different types:


  def visit(obj, visitor)
    sel = Symbol.new
    class Array;   def *sel(visitor); each { | elem | elem.*sel(visitor) }; end; end
    class Object;  def *sel(visitor); visitor.something(self);              end; end
    add_visit_methods!(sel)
    obj.*sel(visitor)
  end
  def add_visit_methods!(sel)
    class Hash;    def *sel(visitor); each { | k, v | v.*sel(visitor);      end; end
    ...
  end

The AnonSym send "rcvr.*sel(...)" dispatches, like a normal method send, directly to the appropriate AnonMeth for "*sel".
visit() can be extended dynamically by adding more AnonMeths bound to "*sel".
The functional "case ...; when..." version is difficult to extend and maintain and is likely to not perform as well as anon messages.
This is similar in style to Scheme letrecs, but is object-oriented.

This idea could be extended to Anonymous Ivars to resolve other namespacing and encapsulation issues for mixins that require state.

-- Kurt Stephens


Related issues 1 (0 open1 closed)

Related to Ruby master - Feature #4288: Allow invoking arbitrary method names with foo."something" syntaxClosedmatz (Yukihiro Matsumoto)Actions
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