From: "David A. Black" <dblack@...>
Date: 2008-10-29T02:36:26+09:00
Subject: Re: lambda and multiple arguments: how?!

Hi --

On Wed, 29 Oct 2008, nvp wrote:

> Hello,
>
> I've been working on porting a useful Perl module to Ruby and have
> been struggling with the differences between Perl's function
> references and Ruby's lambda, proc, etc.
>
> Basically, my Perl module has two functions that do this:
>
>  sub run_on_finish { my ($s,$code,$pid)=@_;
>    $s->{on_finish}->{$pid || 0}=$code;
>  }
>
>  sub on_finish { my ($s,$pid,@par)=@_;
>    my $code=$s->{on_finish}->{$pid} || $s->{on_finish}->{0} or return 0;
>    $code->($pid,@par);
>  };
>
> Using the above, I can do the following:
>
>  $obj->run_on_finish(sub { my($pid, $exit_code, $ident) = @_;
>  # Do something with $pid, $exit_code, $ident
>  });
>
> In Ruby, things are more difficult given that I don't fully understand
> closures yet.  I've reimplemented the methods like so:
>
>  # module
>  def run_on_finish(code, pid=0)
>    begin
>      self.do_on_finish[pid] = code
>    rescue
>      raise "error in run_on_finish\n" # Message stinks
>    end
>  end
>
>  def on_finish(pid, *params)
>    code = self.do_on_finish[pid] || self.do_on_finish[0] or return 0
>    begin
>      code.call(pid, params)
>    rescue
>      raise "lameness\n" # Message stinks
>    end
>  end
>
>  # client
>  pfm.run_on_finish(
>    lambda {
>      |pid,exit_code,ident|
>      print "LEN ", pid.length(), " -- ", "MY ARGS ", pid.join(':'), "\n"
>    }
>  )
>
> As you can see, I'm calling self.do_on_finish with the arguments
> 'pid', and an array of 'params'.
> Unfortunately Ruby only matches |pid,<second_arg>|, and this is not
> the behavior I'd expect or
> want.
>
> Outside of setting |*params| in lambda {} such that I would check/set
> my values manually, is
> there a way to provide lambda with more than two arguments?

You could call it as: code.call(pid,*params), or maybe try this
technique:

   func = lambda {|a,(b,c)| p a,b,c }
   func.call(1,[2,3])

The (b,c) notation will cause the array to be spread over those two
parameters.


David

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