Default block parameter?

ruby-talk
1

Okay, this is probably a dumb question, but how do I declare an
optional block parameter with a default value?

I tried several variations on this basic theme:

        def meth(&block = lambda { |i| ... })
        ...
        end

But I keep getting syntax errors. Help?

2

Okay, this is probably a dumb question, but how do I declare an
optional block parameter with a default value?

I tried several variations on this basic theme:

        def meth(&block = lambda { |i| ... })
        ...
        end

But I keep getting syntax errors. Help?

Hopefully this will give you some ideas:

>> def default
>> if block_given?
>> yield
>> else
?> lambda { puts "Default block" }.call
>> end
>> end
=> nil
>> default
Default block
=> nil
>> default { p 2 + 2 }
4
=> nil

James Edward Gray II

···

On Jan 4, 2006, at 6:52 PM, Mark J.Reed wrote:

3

Mark J.Reed wrote:

Okay, this is probably a dumb question, but how do I declare an
optional block parameter with a default value?

I tried several variations on this basic theme:

        def meth(&block = lambda { |i| ... })
        ...
        end

But I keep getting syntax errors. Help?

I think the & is your problem. You can do something like:

def foo(block = lambda {|i| puts i})
    block.call("Hi")
end

foo()
foo(lambda {|i| puts i; puts i})

Best,
Paul

4

Hi --

Okay, this is probably a dumb question, but how do I declare an
optional block parameter with a default value?

I tried several variations on this basic theme:

       def meth(&block = lambda { |i| ... })
       ...
       end

But I keep getting syntax errors. Help?

The &block thing is a special dispensation from Ruby, letting you grab
the block but not serving as a normal argument. The way I've always
seen this done is:

   def meth(&block)
     block ||= lambda { ... }
     ...
   end

I don't think there's a way to do it inside the arglist.

David

···

On Thu, 5 Jan 2006, Mark J.Reed wrote:

--
David A. Black
dblack@wobblini.net

"Ruby for Rails", from Manning Publications, coming April 2006!

5

> Okay, this is probably a dumb question, but how do I declare an
> optional block parameter with a default value?
>
> I tried several variations on this basic theme:
>
> def meth(&block = lambda { |i| ... })
> ...
> end

The &block thing is a special dispensation from Ruby, letting you grab
the block but not serving as a normal argument. The way I've always
seen this done is:

   def meth(&block)
     block ||= lambda { ... }
     ...
   end

But keep in mind that assigning to block inside the method doesn't
affect the behavior of yield:

def test(&block)
  block ||= lambda{ puts "default" }
  yield
end

=> nil

test

LocalJumpError: no block given

So if you need a default block and currently use yield, you'll either
need to branch on block_given? (as suggested by James), or just use
block.call instead of yield. The latter is probably preferrable, but
may have subtle differences in parameter assignment if it matters.

Jacob Fugal

···

On 1/4/06, dblack@wobblini.net <dblack@wobblini.net> wrote:

On Thu, 5 Jan 2006, Mark J.Reed wrote:

6

Hi --

···

On Fri, 6 Jan 2006, Jacob Fugal wrote:

On 1/4/06, dblack@wobblini.net <dblack@wobblini.net> wrote:

On Thu, 5 Jan 2006, Mark J.Reed wrote:

Okay, this is probably a dumb question, but how do I declare an
optional block parameter with a default value?

I tried several variations on this basic theme:

       def meth(&block = lambda { |i| ... })
       ...
       end

The &block thing is a special dispensation from Ruby, letting you grab
the block but not serving as a normal argument. The way I've always
seen this done is:

   def meth(&block)
     block ||= lambda { ... }
     ...
   end

But keep in mind that assigning to block inside the method doesn't
affect the behavior of yield:

> def test(&block)
> block ||= lambda{ puts "default" }
> yield
> end
=> nil
> test
LocalJumpError: no block given

Right -- all that happens in my version is assignment to a variable.

David

--
David A. Black
dblack@wobblini.net

"Ruby for Rails", from Manning Publications, coming April 2006!

7

Thanks to all who posted. The upshot seems to be that I need to declare
the method with no block in the signature, and then check block_given?
within the body and manually invoke a default block if none was passed in.

Which is basically the solution I arrived at, although I had forgotten about
block_given? and was trapping the LocalJumpError to achieve the same result.

Thanks again!