From: Bob Sidebotham <bob@...>
Date: 2004-09-24T08:39:34+09:00
Subject: Re: "no block given" with closure?

Robert Klemme wrote:

> You make it a bit too complicated, IMHO.  If you just want to invoke 
> some block on an Enumerable, then you can have that directly (the yield 
> block in the first example basically just forwards the element).  So you 
> could do this as well:

Thanks Robert (and others) for your help. I had thought that yield somehow 
dynamically found the first block up the call stack and called that. 
Evidently, that's wrong, and yield will only call a block directly 
associated with the method.

I don't think I'm making this too complicated. The example I gave was 
distilled down from some stuff in Kansas. In Kansas, you can write:

kh.select(:Customer) { |c| c.cust_id > 100 }.each do |cust|
   puts "#{cust.cust_name}"
end

The nested blocks in my original example correspond to the above. In the 
current implementation of Kansas, the database query is executed all at 
once, and returns an array of records--that array is then passed back from 
the block, above, for iteration. I wanted this to be truly iterative so I 
wrapped up the sql in a class with an each method, as below:

class KSSelection
   def initialize(&doselect)
     @doselect = doselect
   end
   def each(&userblock)
     @doselect.call(userblock)
   end
end

def select(*args)
   tables,sql,read_only = check_query_args(*args)

   ...

   KSSelection.new do |userblock|
     @dbh.select_all(sql) do |row|
       userblock.call add_object(
        tables[0].new.load(row.to_h, self, read_only))
     end
   end
end

This code works (but of course, it didn't work when I naively used yield, 
instead of userblock.call, thinking that yield was dynamically scoped). If 
there's some way to do this without the stub KSSelection class, that'd be 
really neat, but I don't think there is.

Bob

P.S. Is there an actual Ruby language reference that would answer questions 
like this?