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UPDATE: Please note: Even if Rubyistas claim it, yield / block constructs are not coroutines

Intro

The following is a standard example in Ruby for the combination of code-blocks {... } and yield-statement:

#!/usr/bin/ruby def test puts "You are in the method" yield 1 puts "You are again back to the method" yield 2 end test {|a| puts "You are in the block #{a}"}

Output

/usr/bin/ruby -w /home/lanx/perl/ruby/yield.rb You are in the method You are in the block 1 You are again back to the method You are in the block 2
Goal
sub test { say "You are in the method"; yield 1; say "You are again back to the method"; yield 2; } test b{ say "You are in the block $_[0]" };

should produce the same results

Approach

Just to prove my strong opinion that Ruby is semantically just a Perl dialect I tried to simulate it with some syntactic sugar:

use strict; use warnings; use feature qw/say/; =pod Block sub is basically syntactic sugar for taking an anonymous sub {} and returning it. Blessing it with "Block" facilitates syntax checks. =cut sub b(&) { my $c_block=shift; bless $c_block, "Block"; return $c_block; } =pod "yield" uses a little know trick to access the @_ of the caller. It checks the last argument of the caller, if its a ref blessed to "Block" and calls it with the own arguments. Note: Ruby allows mixing normal arguments and blocks, as long as the block is in the last position. When using & prototypes in Perl like in C<test(&&)> only the first block can be passed w/o leading sub. =cut sub yield { package DB; my ($package, $filename, $line, $subroutine)=caller(1); no warnings; my $c_block = $DB::args[-1]; package main; return $c_block->(@_) if ref $c_block eq "Block"; die "$subroutine called w/o block in $filename line $line"; } =pod simulating the ruby test-code =cut sub test { say "You are in the method"; yield 1; say "You are again back to the method"; yield 2; } test b{ say "You are in the block $_[0]" };

Output

/usr/bin/perl -w /home/lanx/perl/ruby/yield.pl You are in the method You are in the block 1 You are again back to the method You are in the block 2

Comments

Of course all of this comes with a performance penalty.

But it's worth noting that Ruby itself is considered much slower than Perl5.

An additional implementation in XS or as a new feature pragma would solve performance issues in a compatible way, w/o breaking compatibility

Main differences remaining to the original code are:

  • Ruby tries to avoid curlies with do/end pairs: block/end pairs could be inserted in Perl but I'm not sure if it's worth the trouble.
  • No semicolons in Ruby, this could be (somehow) done with a source filter ... but again, worth the trouble?
  • argument signatures are lengthy in Perl, something like  my ($a,$b)=@_; takes >5 letters more than just |a,b|.

Curious for more comments...

Cheers Rolf

( addicted to the Perl Programming Language)


In reply to RFC: Simulating Ruby's "yield" and "blocks" in Perl by LanX

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