From: James Coglan <jcoglan@...>
Date: 2009-03-20T19:15:39+09:00
Subject: Re: Passing a named function instead of a code block?

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2009/3/20 7stud -- <bbxx789_05ss@yahoo.com>

> James Coglan wrote:
> > 2009/3/20 7stud -- <bbxx789_05ss@yahoo.com>
> >
> >>
> >> (ArgumentError)
> >>        from r1test.rb:8
> >>
> >>
> >> Why does ruby make you use the tortured syntax:
> >>
> >> &method(:square1)
> >>
> >> for a method vs. the easier syntax for a Proc object?
> >
> >
> >
> > square2 is a variable name (ie. something you've made an assignment to),
> > it's just a reference to the lambda object. However, square1 is a method
> > and
> > Ruby allows calling methods without parens, so 'square1' is actually
> > interpreted as a method call to square1 with no arguments. Therefore, to
> > grab a method as an object without calling it, we need to use
> > method(:square1).
>
> Ok.  But there is a certain amount of hypocrisy in that explanation
> Look here:
>
> &square1
> :square1
>
> In the first expression there is a method call, and in the second there
> isn't.  Yet, you could describe both those lines as: a method name
> 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.

--
James Coglan
http://github.com/jcoglan

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