From: Elliot Temple <curi@...>
Date: 2006-06-16T10:40:48+09:00
Subject: Symbol#to_proc Problem

http://blog.hasmanythrough.com/articles/read/8

this link suggests a way to hack Symbol to allow

[1, 2, 3].map(&:to_s)   # => ["1", "2", "3"]

my question is how to change the hack to work correctly for other  
methods. details of the problem are below:

it doesn't work for reversing arrays:

curi-g5:~ curi$ ruby -v
ruby 1.8.4 (2005-12-24) [powerpc-darwin8.5.0]

irb(main):017:0> class Symbol
irb(main):018:1>   def to_proc
irb(main):019:2>     Proc.new { |obj, *args| obj.send(self, *args) }
irb(main):020:2>   end
irb(main):021:1> end
=> nil
irb(main):022:0> [[1,2,3], [4,5,6], [7,8,9]].map {|i| i.reverse}
=> [[3, 2, 1], [6, 5, 4], [9, 8, 7]]

irb(main):023:0> [[1,2,3], [4,5,6], [7,8,9]].map &:reverse
NoMethodError: undefined method `reverse' for 1:Fixnum
         from (irb):19:in `send'
         from (irb):19:in `to_proc'
         from (irb):23
         from (null):0


the problem is that *args is capturing part of the array. here's the  
same thing happening without the Symbol hack involved:

irb(main):024:0> [[1,2,3], [4,5,6], [7,8,9]].map {|i, *args| i.reverse}
NoMethodError: undefined method `reverse' for 1:Fixnum
         from (irb):24
         from (irb):24
         from (null):0


so how should the Symbol hack be redone to work properly?

just removing *args is no good. here is an example that needs *args:

irb(main):027:0> class Fixnum
irb(main):028:1> def foo x
irb(main):029:2> self+x
irb(main):030:2> end
irb(main):031:1> end
=> nil
irb(main):032:0> { 1=>2, 3=>4}.map &:foo
=> [3, 7]


so to illustrate what happens without *args: it solves one problem  
while creating another:

irb(main):039:0> class Symbol
irb(main):040:1>   def to_proc
irb(main):041:2>     Proc.new { |obj| obj.send(self) }
irb(main):042:2>   end
irb(main):043:1> end
=> nil
irb(main):044:0> [[1,2,3], [4,5,6], [7,8,9]].map &:reverse  # works now!
=> [[3, 2, 1], [6, 5, 4], [9, 8, 7]]

irb(main):045:0> { 1=>2, 3=>4}.map &:foo  # breaks now!
NoMethodError: undefined method `foo' for [1, 2]:Array
         from (irb):41:in `send'
         from (irb):41:in `to_proc'
         from (irb):45:in `map'
         from (irb):45
         from :0


-- Elliot Temple
http://www.curi.us/blog/