From: Christopher Dicely <cmdicely@...>
Date: 2009-03-22T17:16:25+09:00
Subject: Re: Passing a named function instead of a code block?

On 3/21/09, 7stud -- <bbxx789_05ss@yahoo.com> wrote:
> James Coglan wrote:
>> 2009/3/20 7stud -- <bbxx789_05ss@yahoo.com>
>>
>>> >> &method(:square1)
>>> > grab a method as an object without calling it, we need to use
>>> preceded by some symbol.
>> Yes, it probably looks that way. To see the difference it helps to know
>> how
>> Ruby is parsed. :square1 is an atomic unit representing the symbol whose
>> name is 'square1'. ":" is not an operator, it is part of the syntax for
>> symbols. However, "&" is an operator responsible for casting between
>> procs
>> and blocks. The expression '&square1' should be read '&( square1 )',
>> that is
>> we call square1 and cast the result of that using '&'. The same applies
>> to
>> 'method'. 'method' is a method that takes a symbol/string and returns
>> the
>> Method object with that name in the current scope. 'method(square1)'
>> would
>> be interpreted as a call to square1, passing the result to 'method'.
>>
>> So, '&square1' throws an error because you're calling a method with
>> insufficient arguments. '&:square1' would try to cast a symbol to a
>> proc,
>> which if you're using ActiveSupport would return the block { |object|
>> object.square1 }.
>>
>> '&square2' is fine as square2 is just a variable referring to a proc.
>> Likewise, '&method(:square1)' is fine because method(:square1) is a
>> Method
>> object, which can be cast to a block.
>
> Thanks for the explanation.  You said that (&) casts between block and
> Procs.  But (&) also appears to be casting a Method to a block in this
> line:
>
> puts c.collect(&method(:fib))

Actually, its casting a Method to a Proc (by calling to_proc on it)
and then treating the proc as a block. To see that's what is
happening, try this:

class Integer
  def to_proc
    Proc.new { self }
  end
end

(1..10).map(&1)

> Can (&) cast a block to a Method?

No. & in a method call always treats a the given proc as a block given
to the method while & in a method definition always makes a block
given to the method as a Proc. Its probably best not to think of this
as "casting" except insofar as the first one "casts" to a proc first
by calling to_proc; blocks aren't objects, and if you do this:

def consume(object=nil)
  if block_given? then yield else object end
end

p = lambda { puts "foo!" }
consume(&p)

You don't get the results you' d get if an object of the
(non-existent) class "Block" was passed to consume, you get the
results you would get if consume was called with a block. So thinking
of & as "casting" an argument to a different class in the method call
is misleading.