Feature #5394
closedAnonymous Symbols, Anonymous Methods
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
Updated by Anonymous about 13 years ago
On Oct 3, 2011, at 3:11 PM, Kurt Stephens wrote:
Optional Supporting Syntax:
a.*anon_sym(args...) # equiv. to a.send(anon_sym, args...) class A def *anon_sym(args...); body...; end end
I'm not sure about the anonymous symbols but you might want to take a look at this discussion:
http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-core/34550
regarding using strings/expressions to invoke methods without the use of send().
Gary Wright
Updated by kstephens (Kurt Stephens) about 13 years ago
Updated by mame (Yusuke Endoh) about 13 years ago
Hello,
To all:
This proposal aims to address pollution and collision problem of name
space, as well as refinement. But the approach is different.
Though the explanation is not so catchy :-), language designers may
want to check it out.
To Kurt:
I recommend you to clarify the advantages and disadvantages between
your proposal and the traditional ones (like refinement and classbox).
I have some points. Because class statement changes the scope of local
variables, the following code will not work:
anon_sym = Symbol.new()
class A
def *anon_sym(args...); body...; end
end
And, the syntax "a.*anon_sym" collides against the call to method *
x, y = 3, 2
p(x.*y) #=> 6
The letter is not essential problem, but the former is not negligible.
#4288 aims to provide chemistry with other languages and the discussion
focuses on "syntax game", so it is better to distinguish between them.
--
Yusuke Endoh mame@tsg.ne.jp
Updated by nobu (Nobuyoshi Nakada) about 13 years ago
Hi,
(11/10/04 4:11), Kurt Stephens wrote:
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.
Is this double dispatching system your true goal? If so, anonymous
symbol doesn't seem the only way nor the best way, for me.
--
Nobu Nakada
Updated by mame (Yusuke Endoh) over 12 years ago
- Status changed from Open to Feedback
Updated by alexeymuranov (Alexey Muranov) over 12 years ago
I just thought myself about anonymous methods and wanted to post it here with "Joke" status. I thought about a possibility to change at runtime the method name, or switch two method bodies, and not to require that every method have a name. With every object there would be associated a Set
of methods, some named, some not, some having multiple names.
Updated by mame (Yusuke Endoh) almost 12 years ago
- Target version set to 2.6
Updated by matz (Yukihiro Matsumoto) almost 12 years ago
- Status changed from Feedback to Rejected
I refuse to add new syntax described in the proposal.
And I think refinement will do the task the OP intended by anon symbols.
So I mark this 'rejected'. Feel free to reopen it.
But I expect you to persuade me.
Matz.