From: Brian McCallister <brianm@...>
Date: 2005-03-24T06:10:30+09:00
Subject: Block Passing and &foo Parameters

My understanding of the &parameter syntax is that it just wraps a 
passed block in a proc:

irb(main):003:0> def one(&func)
irb(main):004:1>   func.call
irb(main):005:1>   end
=> nil
irb(main):006:0> one { puts 1 }
1
=> nil
irb(main):007:0>

What would break if we had the &param parameter actually act as a 
declared parameter which must still be at the end:

irb(main):008:0> one(lambda {puts 1})
ArgumentError: wrong number of arguments (1 for 0)
         from (irb):8:in `one'
         from (irb):8
         from :0
irb(main):009:0>

You can approximate this now with a default for the argument:

irb(main):011:0> def two(func = lambda { yield })
irb(main):012:1>   func.call
irb(main):013:1>   end
=> nil
irb(main):014:0> two { puts 1}
1
=> nil
irb(main):015:0> two(lambda { puts 2} )
2
=> nil
irb(main):016:0>

The advantage to something like this is if you have one function you 
want to pass to a number of things, such as lists:

irb(main):016:0> a = [1,2,3]
=> [1, 2, 3]
irb(main):017:0> b = [4,5,6]
=> [4, 5, 6]
irb(main):018:0> func = lambda {|i| puts i}
=> #<Proc:0x0034b1b8@(irb):18>
irb(main):019:0> a.each func
ArgumentError: wrong number of arguments (1 for 0)
         from (irb):19:in `each'
         from (irb):19
         from :0
irb(main):020:0> b.each func
ArgumentError: wrong number of arguments (1 for 0)
         from (irb):20:in `each'
         from (irb):20
         from :0
irb(main):021:0>

You can add this now via:

irb(main):031:0> class Array
irb(main):032:1>   alias old_each each
irb(main):033:1>   def each(func = lambda {|i| yield i})
irb(main):034:2>     old_each {|i| func.call i}
irb(main):035:2>     end
irb(main):036:1>   end
=> nil
irb(main):037:0> [1,2,3].each func
1
2
3
=> [1, 2, 3]
irb(main):038:0>

What would break if a special case allowed for passing of a Proc in the 
place of &param in place of passing in a block?

-Brian