# Blocks in Procs?

**URL:** <https://www.ruby-forum.com/t/blocks-in-procs/60739>\
**Category:** Ruby\
**Created:** [May 17, 2006, 4:17am UTC](https://www.ruby-forum.com/t/blocks-in-procs/60739 "2006-05-17T04:17:57Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![travis\_michel](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/e47c2d/32.png) [@travis\_michel](https://www.ruby-forum.com/u/travis_michel)\
**Post date:** [May 17, 2006, 4:17am UTC](https://www.ruby-forum.com/t/blocks-in-procs/60739/1 "2006-05-17T04:17:57Z")

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Hi all, I have been using a lot of Proc’s and lambda’s lately and I have  
encountered something a little baffling. It might be syntax or a  
misunderstanding of the language; would anyone mind helping me out?

I expect the following to print “Hello World”, but it does not:

#!/usr/bin/env ruby

x = Proc.new { |a| p( block\_given? ? yield(a) : a ) }  
x.call( ‘World’ ) { |b| p “Hello #{b}” }

# Likewise, in continuation:

y = Proc.new { |b| p “Hello #{b}” }  
p x.call( ‘World’, &y )

# this may be related, it returns false instead of nil… what gives?

p x.call( ‘World’ ) &y

**END**

I have nothing to offer but a thank you.

- travis

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**Author:** ![travis\_michel](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/e47c2d/32.png) [@travis\_michel](https://www.ruby-forum.com/u/travis_michel)\
**Post date:** [May 17, 2006, 4:36am UTC](https://www.ruby-forum.com/t/blocks-in-procs/60739/2 "2006-05-17T04:36:29Z")

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On May 16, 2006, at 10:17 PM, travis michel wrote:

> #!/usr/bin/env ruby
> 
> x = Proc.new { |a| p( block\_given? ? yield(a) : a ) }  
> x.call( ‘World’ ) { |b| p “Hello #{b}” }

I don’t believe you can pass a block argument when calling a Proc.  
It is silently ignored.  
In your case the call to block\_given? would be relative to the scope  
where Proc.new is called and not relative to the arguments passed  
when the block is executed via Proc#call. This is changing in Ruby  
1.9 I believe so that block arguments can be provided to blocks when  
called via yield and/or Proc#call.

> # Likewise, in continuation:
> 
> y = Proc.new { |b| p “Hello #{b}” }  
> p x.call( ‘World’, &y )

Same problem, a block argument is ignored by Proc#call.

> # this may be related, it returns false instead of nil… what gives?
> 
> p x.call( ‘World’ ) &y

x.call(‘World’) returns nil because the return value of the Proc is  
the last expression evaluated in the block which is a call to  
Kernel#p, which always returns nil.

So now you have:

p nil &y

which is getting parsed as a call to the bitwise and operator (nil &  
y), which returns false.

Gary W.

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**Author:** ![travis\_michel](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/e47c2d/32.png) [@travis\_michel](https://www.ruby-forum.com/u/travis_michel)\
**Post date:** [May 17, 2006, 4:57am UTC](https://www.ruby-forum.com/t/blocks-in-procs/60739/3 "2006-05-17T04:57:52Z")

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Thank you very much Gary! /me crosses fingers for support in 1.9

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**Author:** ![travis\_michel](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/e47c2d/32.png) [@travis\_michel](https://www.ruby-forum.com/u/travis_michel)\
**Post date:** [May 19, 2006, 8:10am UTC](https://www.ruby-forum.com/t/blocks-in-procs/60739/4 "2006-05-19T08:10:17Z")

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travis michel wrote:

> Thank you very much Gary! /me crosses fingers for support in 1.9

It’s already there, I think.

Cheers,  
Dave

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**Author:** ![travis\_michel](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/e47c2d/32.png) [@travis\_michel](https://www.ruby-forum.com/u/travis_michel)\
**Post date:** [May 17, 2006, 11:55am UTC](https://www.ruby-forum.com/t/blocks-in-procs/60739/5 "2006-05-17T11:55:52Z")

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On 17/05/06, travis michel [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> I expect the following to print “Hello World”, but it does not:
> 
> #!/usr/bin/env ruby
> 
> x = Proc.new { |a| p( block\_given? ? yield(a) : a ) }  
> x.call( ‘World’ ) { |b| p “Hello #{b}” }

Is this any use to you?

x = Proc.new { |\*a| a[1] ? a[1].call(a[0]) : a[0] }  
p x.call( ‘Boo’ )  
p x.call( ‘World’, Proc.new{ |b| “Hello #{b}” } )

“Boo”  
“Hello World”

Regards,  
Paul.