From: Rick DeNatale <rick.denatale@...>
Date: 2006-09-13T01:39:57+09:00
Subject: Re: Struggling with Blocks

On 9/11/06, Rick DeNatale <rick.denatale@gmail.com> wrote:

>
> I have to admit that I was struggling a bit when I made the analogy to
> an anonymous method.  The key is that begin pushes a new stack frame
> onto the call stack.
>
> You are right about the scoping of local variables.

Having spent a bit more time perusing the 1.8 source code, my analogy
was a little off.

What really happens is that the rescue and ensure statements
effectively push a tag onto a stack which comprises local variables on
the C call stack, and then do a setjmp before executing the code being
protected, the tag contains the jmp_buf for setjmp, along with other
state needed for exception handling. Raising the exception does a
longjmp to the jmp_buf in the tag on the top of the stack, passing a
state which indicates that an exception is being raised.

The tag stack is also used for other purposes such as implementing
break, next, redo, and retry in loops.

Now I guess I could say that this "kind of" like an internal call, but
it's stretching things a bit further than I'd like

The break...end statement really just serves as a syntactic marker to
delimit where a series of statements being protected by
rescue/else/ensure starts, and where the last rescue/else/ensure ends.
If a begin..end sequence doesn't contain any rescue/ensure clauses, it
has no effect.

def..end also can mark those protected statements, although it also
serves the purpose of delimiting a method.

Now there might be a reason why the do..end, or {..} of a block can't
have the same feature, but I'm scratching my head to figure out why.
You CAN have a block which contains a single statement with a rescue
modifier:

    lambda do
         raise "help!" rescue "gotcha"
    end

is legal as is the semantically equivalent:

   lambda {raise "help!" rescue "gotcha"}

but

   lambda do
           raise "help!"
           rescue
                 "gotcha"
   end

is not.

Perhaps someone else (Matz?) can enlighten me/us on why

-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/