From: Mark Hubbart <discord@...>
Date: 2004-05-02T06:47:41+09:00
Subject: Re: Ruby Newbie (ooh, that rhymes!) - When do I use do/end,  when {}?


On May 1, 2004, at 2:06 PM, Jean-Hugues ROBERT wrote:

> At 05:54 02/05/2004 +0900, you wrote:
>> I've read the User Guide and other docs on the net, but I'm a little
>> unsure as to whether I can ALWAYS substitute the DO/END with curly
>> brackets for a block?  For example, I've only ever seen referenced to
>> hash definitions using curlies.  What's the rule here?  Is it purely
>> down to preference?
>
> Welcome.
> You can always substitute the DO/END with curly brackets for a block.
> However, in some cases, you may need some extra parenthesis.
> This is due to {} having a different precedence than do/end and
> correlated impact on associativity.

yes, in ambiguous situations, it can require parent either way. For 
example, if you write

   array.inject num.to_f {|a,b| a * b }

Ruby assumes you want the block to be passed to the num.to_f method 
call. This is probably not what you want, though. To solve that one, 
put parens around the num.to_f call:

   array.inject( num.to_f ) {|a,b| a * b }

That will work properly. likewise, there can be similar problems with 
do...end:

   obj.foo array.map do |item|
     p item
     item * 2
   end

This snippet will hand the block to obj.foo, which is probably not what 
you want. to solve it, you could do this:

   obj.foo( array.map do |item|
     p item
     item * 2
   end )

... but that's ugly. you would probably want to just break down and do 
it in two steps:

   a = array.map do |item|
     p item
     item * 2
   end
   obj.foo a

or convert it to a one liner:

   obj.foo array.map{|item| p item; item * 2 }

cheers,
--Mark