From: Robert Klemme <bob.news@...>
Date: 2004-09-24T04:59:34+09:00
Subject: Re: "no block given" with closure?


"Bob Sidebotham" <bob@windsong.bc.ca> schrieb im Newsbeitrag 
news:rrD4d.483473$M95.426973@pd7tw1no...
> When I execute this code, I get "no block given (LocalJumpError)":
>
> class S
>   def initialize(&b1)
>     @b1 = b1
>   end
>   def each
>     @b1.call
>   end
> end
>
> s = S.new {[1,2,3].each {|i| yield i }}.each {|v| puts v}
>
> Changing the implied block/yield combination to an explicit proc object 
> works, as follows:
>
> class S
>   def initialize(&b1)
>     @b1 = b1
>   end
>   def each(&b2)
>     @b1.call(b2)
>   end
> end
>
> s = S.new { |p| [1,2,3].each { |i| p.call(i) }}.each {|v| puts v}
>
> This prints the expected 3 lines.
>
> This is with ruby 1.8.2 (2004-09-10) [i686-linux]
>
> It's quite possible that I don't understand how this is supposed to work 
> :-). Any hints would be appreciated.

Possibly.  The yield in the first example has no block that it can invoke, 
much the same as when you invoke "yield" directly:

>> yield
LocalJumpError: no block given
        from (irb):1

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:

class S
   def initialize(enum)
     @enum = enum
   end
   def each(&b)
     @enum.each &b
   end
end

s = S.new([1,2,3]).each {|v| puts v}

OTOH, if you want to use the block to generate a value which is used for the 
enumeration you could do

class S
   def initialize(&create)
     @create = create
   end
   def each(&b)
     @create.call.each &b
   end
end

s = S.new { [1,2,3] }.each {|v| puts v}

The subtle difference between these two is, that the second approach creates 
something on each invocation of S#each and iterates over it while in the 
first case all iterations run through the same instance:

>> s = S.new { [rand(10), rand(10)] }
=> #<S:0x100d43f0 @create=#<Proc:0x100d4450@(irb):41>>
>> s.each {|v| puts v}
1
6
=> [1, 6]
>> s.each {|v| puts v}
0
6
=> [0, 6]
>>
?> s.each {|v| puts v}
0
5
=> [0, 5]
>>

Hope this makes things a bit clearer.

Kind regards

    robert