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comp.lang.ruby

define_method vs eval "def ..."

Tomasz Wegrzanowski

8/3/2006 1:46:00 PM

Hello,

Just wondering, with eval "def ..." I can do:
eval "def #{tag}(*args, &blk) xml(:#{tag}, *args, &blk) end"

Of course it would be nicer to do:
define_method(tag) {|*args,&blk|, xml(tag, *args, &blk) }
But that throws a parse error.

Is there some way to define method that takes a block argument
without calling eval ?

--
Tomasz Wegrzanowski [ http://t-a-w.blo... ]
4 Answers

Logan Capaldo

8/3/2006 1:53:00 PM

0


On Aug 3, 2006, at 9:45 AM, Tomasz Wegrzanowski wrote:

> Hello,
>
> Just wondering, with eval "def ..." I can do:
> eval "def #{tag}(*args, &blk) xml(:#{tag}, *args, &blk) end"
>
> Of course it would be nicer to do:
> define_method(tag) {|*args,&blk|, xml(tag, *args, &blk) }
> But that throws a parse error.
>
> Is there some way to define method that takes a block argument
> without calling eval ?
>
Ruby 1.9 lets blocks take blocks like define_method(:meth) { |*args,
&blk| ... }

Currently theres no way to do it in 1.8 AFAIK.

> --
> Tomasz Wegrzanowski [ http://t-a-w.blo... ]
>

Nobuyoshi Nakada

8/3/2006 1:54:00 PM

0

Hi,

At Thu, 3 Aug 2006 22:45:40 +0900,
Tomasz Wegrzanowski wrote in [ruby-talk:205997]:
> Is there some way to define method that takes a block argument
> without calling eval ?

Using 1.9.

--
Nobu Nakada

Tammo Freese

8/3/2006 11:21:00 PM

0


Robert Dober wrote:
[...]
> and it seems to make it into Ruby2 since June
> http://pub.cozmixng.org/~t...
> cmd=view;name=Ruby2.0BlockParameterAndMultipleValueAssignment
> which I consider extremly good news :)


I am suprised that the blocks will work in such a way.
How do these play together with the new define_method ?

Examples from
http://pub.cozmixng.org/~t...
cmd=view;name=Ruby2.0BlockParameterAndMultipleValueAssignment
:

- Proc.new{|x| x}.call(0,1) returns 0.
- Proc.new{|x,y| [x,y]}.call(0) returns [0,nil].

So missing arguments seem to be set to nil, and additional arguments
seem to be ignored. What would happen for this code fragment:

proc_one = Proc.new{|x| x}
proc_two = Proc.new{|x,y| [x,y]}

define_method(:meth_one) &proc_one
define_method(:meth_two) &proc_two

p proc_one.call(0,1) # returns 0 according to the web page
p proc_two.call(1) # returns 1,nil according to the web page

p meth_one(0,1) # ???
p meth_two(1) # ???

Would the last two work and return the same result as the direct proc
calls?
If so, how would we define a method via define_method that takes a
fixed number of parameters?
If not, where does the magic happen that while the proc accepts the
arguments,
the method defined by the proc does not?



Thanks for your help,

Tammo

Rick DeNatale

8/4/2006 1:35:00 AM

0

On 8/3/06, Tammo Freese <freese@acm.org> wrote:
>
> Robert Dober wrote:
> [...]
> > and it seems to make it into Ruby2 since June
> > http://pub.cozmixng.org/~t...
> > cmd=view;name=Ruby2.0BlockParameterAndMultipleValueAssignment
> > which I consider extremly good news :)
>
>
> I am suprised that the blocks will work in such a way.
> How do these play together with the new define_method ?
>
> Examples from
> http://pub.cozmixng.org/~t...
> cmd=view;name=Ruby2.0BlockParameterAndMultipleValueAssignment
> :
>
> - Proc.new{|x| x}.call(0,1) returns 0.
> - Proc.new{|x,y| [x,y]}.call(0) returns [0,nil].
>
> So missing arguments seem to be set to nil, and additional arguments
> seem to be ignored. What would happen for this code fragment:

That first example is a change from 1.8

Proc.new {|x| x}.call(0,1)
(irb):58: warning: multiple values for a block parameter (2 for 1)
from (irb):58
=> [0, 1]

I'm not sure that I like this change.


--
Rick DeNatale

IPMS/USA Region 12 Coordinator
http://ipmsr12.denh...

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