From: Tom Werner <pubsub@...>
Date: 2007-08-10T05:02:32+09:00
Subject: Re: define_method with default parameters

dblack@rubypal.com wrote:
> Hi --
>
> On Fri, 10 Aug 2007, Tom Werner wrote:
>
>> Gordon Thiesfeld wrote:
>>> On Aug 9, 11:40 am, Emmanuel Oga <oga_emmanuel_...@yahoo.com.ar>
>>> 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
>>>
>>>
>>>
>>
>>
>> Has the addition of block argument defaults to the language been 
>> considered?
>
> Yes, quite often :-)
>
>> 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?
>
> The problem is with something like:
>
>   m do |a, b = 1 | 2 | 3; end
>
> 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