How can I get the arguments passed to the caller

ruby-talk
1

Hey guys.

I have a rather weird question. I want to write a function that
programatically determines the arguments passed to its caller. Say, if i do

$stuff = []

def bar(arg)
  foo
end

def foo
  first_arg = ....
  $stuff << first_arg
end

bar(42)

# By this point, $stuff would be [42]

I know it is quite the contrived question, but I'm not interested in doing
something practical. Any leads?

2

Stefan Kanev <stefan.kanev@gmail.com> writes:

[Note: parts of this message were removed to make it a legal post.]

Hey guys.

I have a rather weird question. I want to write a function that
programatically determines the arguments passed to its caller. Say, if i do

$stuff =

def bar(arg)
  foo
end

def foo
  first_arg = ....
  $stuff << first_arg
end

bar(42)

I don't think this is possible.

# By this point, $stuff would be [42]

But to get this result you could write:

(def bar(*args)
   ($stuff = args)
end)

(bar 42)

$stuff --> [42]

···

I know it is quite the contrived question, but I'm not interested in doing
something practical. Any leads?

--
__Pascal Bourguignon__

3

I have a rather weird question. I want to write a function that
programatically determines the arguments passed to its caller. Say, if i do

$stuff =

def bar(arg)
foo
end

def foo
first_arg = ....
$stuff << first_arg
end

bar(42)

# By this point, $stuff would be [42]

I think the closest you're going to get is for the calling function to pass
its binding so that foo can read from it:

def bar(*args)
  foo(binding)
end
=> nil
def foo(env)
  puts env.eval("args.first")
end
=> nil
bar 'something'
something

I don't think you can do this transparently. You can't even refer to the
argument list within a single method (I'm thinking of the 'arguments' object
in JavaScript) without mentioning the arguments by name.

4

So, suppose I want to get evil and go dig into frames and so on. I've seen
ruby-debug do something close. Is there a part of the standard library that
allows you to introspect that way (if so, I could not find it), or a gem
maybe? And if not and I'm bend on writing C code to achieve it, where should
I start looking?

···

On Sun, Jan 25, 2009 at 9:39 PM, James Coglan <jcoglan@googlemail.com>wrote:

> I have a rather weird question. I want to write a function that
> programatically determines the arguments passed to its caller. Say, if i
do
>
> $stuff =
>
> def bar(arg)
> foo
> end
>
> def foo
> first_arg = ....
> $stuff << first_arg
> end
>
> bar(42)
>
> # By this point, $stuff would be [42]

I think the closest you're going to get is for the calling function to pass
its binding so that foo can read from it:

def bar(*args)
foo(binding)
end
=> nil
def foo(env)
puts env.eval("args.first")
end
=> nil
bar 'something'
something

I don't think you can do this transparently. You can't even refer to the
argument list within a single method (I'm thinking of the 'arguments'
object
in JavaScript) without mentioning the arguments by name.

5

James Coglan wrote:

first_arg = ....
$stuff << first_arg
end

bar(42)

# By this point, $stuff would be [42]

I think the closest you're going to get is for the calling function to
pass
its binding so that foo can read from it:

In which case, it might as well just pass its args instead :slight_smile:

There is the Binding.of_caller hack which you can find through a google
search.

Maybe you could combine this with Kernel#local_variables or
Binding#local_variables to do what you want? That's what debug.rb does

    when /^\s*l(?:ocal)?\s*$/
      var_list(eval("local_variables", binding), binding)

and just use eval to read them:

  def var_list(ary, binding)
    ary.sort!
    for v in ary
      stdout.printf " %s => %s\n", v, eval(v, binding).inspect
    end
  end

···

--
Posted via http://www.ruby-forum.com/\.

6

Depends, for methods with *args and optional arguments you have
definitely some heavy lifting to do. For all other methods however
it's easy:
   method( caller.first.sub(/.*in ./,"").sub(/.$/,"") ).arity
Probably not good enough?
cheers
Robert

···

On Sun, Jan 25, 2009 at 9:30 PM, Stefan Kanev <stefan.kanev@gmail.com> wrote:

So, suppose I want to get evil and go dig into frames and so on. I've seen
ruby-debug do something close. Is there a part of the standard library that
allows you to introspect that way (if so, I could not find it), or a gem
maybe? And if not and I'm bend on writing C code to achieve it, where should
I start looking?

7

Thanks for the advice. I would have never had thought of something as
straightforward as evaling 'local_variables' in the binding. Unfortunatelly,
I believe that this doesn't yield the arguments if they are not named.

I'm actually trying, out of curiosity, to implement a very straightforward
idea:

  %w{more chunky bacon}.each { puts it.length }

Like, have *it* to actually be the 'implicit blog argument', not unlike
perl's $_. Again, I'm doing that out of curiosity. Any ideas how I can
accomplish it without touching the interpreter's C code? :slight_smile:

···

On Mon, Jan 26, 2009 at 10:35 AM, Brian Candler <b.candler@pobox.com> wrote:

James Coglan wrote:
>> first_arg = ....
>> $stuff << first_arg
>> end
>>
>> bar(42)
>>
>> # By this point, $stuff would be [42]
>
>
>
> I think the closest you're going to get is for the calling function to
> pass
> its binding so that foo can read from it:

In which case, it might as well just pass its args instead :slight_smile:

There is the Binding.of_caller hack which you can find through a google
search.

Maybe you could combine this with Kernel#local_variables or
Binding#local_variables to do what you want? That's what debug.rb does

   when /^\s*l(?:ocal)?\s*$/
     var_list(eval("local_variables", binding), binding)

and just use eval to read them:

def var_list(ary, binding)
   ary.sort!
   for v in ary
     stdout.printf " %s => %s\n", v, eval(v, binding).inspect
   end
end
--
Posted via http://www.ruby-forum.com/\.

8

IMHO you can't because the current value is only known by method
#each. Wait, you could cook something up using thread local variables

18:41:58 tmp$ ./it.rb
4
6
5
18:42:41 tmp$ cat it.rb
#!/bin/env ruby

def it
  Thread.current[:each].last rescue nil
end

class Array
  alias _each each
  def each(&b)
    stack = Thread.current[:each] ||=
    _each do |val|
      stack.push val
      begin
        b.call
      ensure
        stack.pop
      end
    end
  end
end

%w{more chunky bacon}.each { puts it.length }
18:42:42 tmp$

But note that you have to change *all* implementations of #each which
is difficult to achieve because classes can spring into existence all
the time. Alternatively write a global each which receives the
Enumerable as argument.

Cheers

robert

···

2009/1/27 Stefan Kanev <stefan.kanev@gmail.com>:

Thanks for the advice. I would have never had thought of something as
straightforward as evaling 'local_variables' in the binding. Unfortunatelly,
I believe that this doesn't yield the arguments if they are not named.

I'm actually trying, out of curiosity, to implement a very straightforward
idea:

%w{more chunky bacon}.each { puts it.length }

Like, have *it* to actually be the 'implicit blog argument', not unlike
perl's $_. Again, I'm doing that out of curiosity. Any ideas how I can
accomplish it without touching the interpreter's C code? :slight_smile:

--
remember.guy do |as, often| as.you_can - without end

9

And I have to override Array#each. I'll also need to override #map, #select
and so on. Not a good deal :frowning:

···

On Tue, Jan 27, 2009 at 7:46 PM, Robert Klemme <shortcutter@googlemail.com>wrote:

2009/1/27 Stefan Kanev <stefan.kanev@gmail.com>:
> Thanks for the advice. I would have never had thought of something as
> straightforward as evaling 'local_variables' in the binding.
Unfortunatelly,
> I believe that this doesn't yield the arguments if they are not named.
>
> I'm actually trying, out of curiosity, to implement a very
straightforward
> idea:
>
> %w{more chunky bacon}.each { puts it.length }
>
> Like, have *it* to actually be the 'implicit blog argument', not unlike
> perl's $_. Again, I'm doing that out of curiosity. Any ideas how I can
> accomplish it without touching the interpreter's C code? :slight_smile:

IMHO you can't because the current value is only known by method
#each. Wait, you could cook something up using thread local variables

18:41:58 tmp$ ./it.rb
4
6
5
18:42:41 tmp$ cat it.rb
#!/bin/env ruby

def it
Thread.current[:each].last rescue nil
end

class Array
alias _each each
def each(&b)
   stack = Thread.current[:each] ||=
   _each do |val|
     stack.push val
     begin
       b.call
     ensure
       stack.pop
     end
   end
end
end

%w{more chunky bacon}.each { puts it.length }
18:42:42 tmp$

But note that you have to change *all* implementations of #each which
is difficult to achieve because classes can spring into existence all
the time. Alternatively write a global each which receives the
Enumerable as argument.

Cheers

robert

--
remember.guy do |as, often| as.you_can - without end