From: "J.Hawkesworth" <J.Hawkesworth@...>
Date: 2003-01-08T22:56:57+09:00
Subject: Re: Can you detect if a method takes a block argument?

Martin,

I'm still new to ruby but I've noticed on http://dev.rubycentral.com/ref/ that there is a Kernel method called block_given? which I imagine might be of use to you, although it might require standing some of your logic on its head.  

Snowing here in Birmingham (UK) too (sorry to everyone in the rest of the world - snow is rare enough to be noteworthy to some of us weather-obsessed Brits).

cheers,

Jon


-----Original Message-----
From: Martin Hart [mailto:martin@zsdfherg.com]
Sent: 08 January 2003 13:28
To: ruby-talk ML
Subject: Can you detect if a method takes a block argument?


Hi all, hope everybody had a great new year - it is snowing outside for 
the first time this winter :-)

Sorry if this is common knowledge, I have done some searching of archives 
but cannot find information...
Is there a way to tell whether or not a method takes a block argument?  
The sort of thing that I want to do is write the Client class below (from 
the pseudocode example given):

class Server
	def methodWithNoBlock(arg)
		return 1
	end

	def methodWithBlock(arg, &blk)
		blk.call(1)
		blk.call(2)
	end
end

# Client is the class that I will write - sort of a delegator with knobs 
on
client = Client.new(Server.new)

# If you call the method "normally" - then everything behaves as expected
client.methodWithNoBlock => 1

# If you call the method with a block and it doesn't take a block, then 
the method's return value
# is passed as block argument. 
client.methodWithNoBlock { |arg|
	# here arg is 1, and the method returns nil
}

# If you call a method that takes a block, and you don't pass one, then 
the method's return value
# is an array of the block arguments:
client.methodWithBlock => [1, 2]

# If you call a method that takes a block, and you do pass one, then 
everything behaves as expected
client.methodWithBlock { |arg|
	# first time arg is 1, second time it is 2
	# method returns nil
}

The bit that I don't know how to do is:
1. If client detects that the requested method takes a block argument, it 
should:
 a) pass the block given to it (if there was one), or
 b) create a dummy block that fills a return array, because the caller 
didn't provide a block.

Possible? Sensible? etc.

Cheers for any help you can give,


-- 
Martin Hart              Tel:    +44 (0) 1582 618468
Arnclan                Fax:    +44 (0) 1582 619596
Union Street,          E-mail: martin@zsdfherg.com
Dunstable, Beds LU6 1EX


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