Define_method with default parameters

Ruby
1

how can i use define_method to assign default parameters?

I want to do this:

a_helper_module.module_eval do

define_method(:method_name) do |parameter, parameter2= “default”|
puts parameter
puts parameter2
end

end

But parameter2= “default” is not a valid!

Thanks!

2

On Aug 9, 11:40 am, Emmanuel O. [email protected]
wrote:

how can i use define_method to assign default parameters?

Not sure if this is the most elegant way, but:

define_method(:method_name) do |*args|
parameter, parameter2 = *args
parameter2 ||= ‘default’
puts parameter
puts parameter2
end

3

mmmm better than my method! :slight_smile: :

    eval <<-EOMETHDEF
    def method_name parameter, parameter2= 'default'
      puts parameter
      puts parameter2
    end
    EOMETHDEF

thanks.

Gordon T. wrote:

On Aug 9, 11:40 am, Emmanuel O. [email protected]
wrote:

how can i use define_method to assign default parameters?

Not sure if this is the most elegant way, but:

define_method(:method_name) do |*args|
parameter, parameter2 = *args
parameter2 ||= ‘default’
puts parameter
puts parameter2
end

4

Gordon T. wrote:

parameter, parameter2 = *args
parameter2 ||= 'default'
puts parameter
puts parameter2

end

Has the addition of block argument defaults to the language been
considered? There are a number of places (especially in define_method,
Proc.new, and lambda) where it would come in handy. The syntax would of
course be:

def foo
yield ‘from method’
end

foo do |x, y = ‘from default’|
puts x
puts y
end

Which would output:

from method
from default

I’ve seen this come up enough that its addition would seem welcome. Are
there arguments against it?

Tom

5

On Thursday 09 August 2007 11:38:06 am Tom W. wrote:

define_method(:method_name) do |*args|
Has the addition of block argument defaults to the language been
puts y
Tom
I believe this has come up lots. IIRC, the current response is that
such
block defaults are not possible with the lexical parser / grammar ruby
currently uses.

6

Hi –

On Fri, 10 Aug 2007, Tom W. wrote:

define_method(:method_name) do |*args|
Has the addition of block argument defaults to the language been considered?
Yes, quite often :slight_smile:

end

Which would output:

from method
from default

I’ve seen this come up enough that its addition would seem welcome. Are there
arguments against it?

The problem is with something like:

m do |a, b = 1 | 2 | 3; end

you can’t tell which | is doing what.

David

7

[email protected] wrote:

you can’t tell which | is doing what.

David

That’s never stopped Ruby from doing other things on a single line.
Single line ‘if’ statements need a ‘then’ or a semicolon (being syntax
errors otherwise).

if x | y; ‘foo’; end

Or consider the following single line:

x = 10 - 5 - 2

Perfectly valid, but wait! What I really meant was:

x = 10 - 5; -2

The onus is on the programmer to write code that works in the face of
possibly ambiguous syntax.

Simply require a semicolon in your example case and there’s no more
problem:

m do |a, b = 1|; 2 | 3; end

or

m do |a, b = 1 | 2|; 3; end

Just because a certain functionality might produce ambiguous code
seems a poor reason to exclude it from consideration!

Tom

8

On Aug 9, 2007, at 11:20 AM, Gordon T. wrote:

i prefer

define_method ‘method_name’ do |required, *optional|
one, two, *ignored = *optional
end

because you an error will be thrown if required is not passed and you
don’t risk slurping ten arguments into ‘two’

alternatively just use a hash

define_method ‘method_name’ do |required, *options|
options = options.first || Hash.new
foobar = options[:foobar]
end

method_name ‘required’
method_name ‘required’, :foobar => 42

kind regards.

a @ http://drawohara.com/

9

Hi –

On Fri, 10 Aug 2007, ara.t.howard wrote:

puts parameter2

because you an error will be thrown if required is not passed and you don’t
risk slurping ten arguments into ‘two’

You can do that just with the comma:

one, two, = *optional

David

10

On Aug 9, 2007, at 2:42 PM, [email protected] wrote:

one, two, = *optional

i used to use that, but people ‘correct it’ to

one, two = *optional

and lo it works so long as there are two, then, when there are three
it blows up so i’ve take to ‘doccumenting’ it with ‘*ignored’

paranoid i guess :wink:

a @ http://drawohara.com/

11

Hi –

On Fri, 10 Aug 2007, ara.t.howard wrote:

up so i’ve take to ‘doccumenting’ it with ‘*ignored’

paranoid i guess :wink:

Or maybe I’m too sanguine :slight_smile:

David

12

Hi –

On Fri, 10 Aug 2007, Daniel DeLorme wrote:

end

Why don’t you just use the regular “def”? Am I missing something?

In this particular example you could, but the issue of defaults for
block parameters is still a real one, for #define_method and other
cases.

David

13

Emmanuel O. wrote:

how can i use define_method to assign default parameters?

I want to do this:

a_helper_module.module_eval do
define_method(:method_name) do |parameter, parameter2= “default”|
puts parameter
puts parameter2
end
end

Why don’t you just use the regular “def”? Am I missing something?

Daniel