From: William James <w_a_x_man@...>
Date: 2008-11-05T04:18:31+09:00
Subject: Re: Passing a block to a block



Brian Candler wrote:
> I notice that if you pass a block to a block, you can't use 'yield' to
> yield to that block:
>
> $ cat yield_in_block.rb
> foo = lambda { |e| puts e }
> blk = lambda { |e| yield e }
> blk.call(123, &foo)
>

blk is not a block.  It is a Proc object or closure.
(Note that it isn't a Lambda object.  Lambda is
for Lisp weenies.)

> $ ruby yield_in_block.rb
> yield_in_block.rb:2: no block given (LocalJumpError)
>         from yield_in_block.rb:3
> $ ruby19 yield_in_block.rb
> yield_in_block.rb:2:in `block in <main>': no block given (yield)
> (LocalJumpError)
>         from yield_in_block.rb:3:in `call'
>         from yield_in_block.rb:3:in `<main>'
>
> What's the reason for this? Does 'yield' only invoke the top-level block
> passed into a method?
>
> ruby1.9 is happy but only if I explicitly reference the block to yield.
>
> foo = lambda { |e| puts e }
> blk = lambda { |e,&y| y[e] }
> blk.call(123, &foo)

foo = proc{|x| puts x }
    ==>#<Proc:0x02b2ae60@(irb):1>
bar = proc{|x,prc| prc[x] }
    ==>#<Proc:0x02b24ac4@(irb):2>
bar[ 88, foo ]
88
    ==>nil