Re: The Block Returns

[Mark A. Biggar]

Austin Hastings wrote:


-----Original Message-----
From: Luke Palmer [mailto:[EMAIL PROTECTED]
Sent: Thursday, October 02, 2003 10:23 PM
To: Jeff Clites
Cc: [EMAIL PROTECTED]; [EMAIL PROTECTED]
Subject: Re: The Block Returns
Jeff Clites writes:

Speaking to the practical side, I have written code that has to 
disentangle
itself from the failure of a complex startup sequence. I'd love to be 
able
to build a dynamic exit sequence. (In fact, being able to do <C>&block 
.=
{ more_stuff(); };</C> is way up on my list...)
I've wanted to do that sort of thing before, but it seems simpler 
(conceptually and practically) to build up an array of cleanup 
subs/blocks to execute in sequence, rather than to have a .= for 
blocks. (Another reason it's handy to keep them separate is in cases in 
which each needs to return some information--maybe a status which 
determines whether to proceed, etc.)
But this is already supported, in its most powerful form:

   wrap &block: { call; other_stuff() }


Hmm, no.

That does a call, which presumes a return, which burns up who-knows-how-many mips. Given that the compiler is being forced to remember the code in case someone overloads the semicolon operator, or whatever, I don't think it's unreasonable to catenate the .source values of various blocks, and that seems a reasonable behavior for <C>infix:.=(Block, Block) {...}</C>.

Especially since the other way turns into:

  macro atexit(Block $b) {
    get_the_current_sub().eval("my &block = {};")
      unless defined &block;
    wrap &block: { call; $b(); };
  }
Which makes two calls per additional whosit.

Frankly, I think I'd rather see:

  macro atexit($code) is parsed(/{ <Perl6.line>* }/) {
    get_the_current_sub().eval("my $block;")
      unless defined $block;
    $block .= $code;
  }
  macro return($retval) {
    eval($block) if defined $block;
    leave Routine, $retval;
  }
But that imposes <C>eval()</C> pretty frequently. Better to provide some lower-level hackish way to agglutinate Blocks.


Isn't this one of the prime examples of why CPS is being use, it allows
for Tail Recursion Optimization.  With TRO all your worries about
overhead do to the wrap go away.
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