Feature #5394
closed
Anonymous 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
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