# Assign blocks to variables

**URL:** <https://www.ruby-forum.com/t/assign-blocks-to-variables/192172>\
**Category:** Ruby\
**Created:** [July 25, 2010, 5:00pm UTC](https://www.ruby-forum.com/t/assign-blocks-to-variables/192172 "2010-07-25T17:00:57Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![poseid](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/dc4da7/32.png) [@poseid](https://www.ruby-forum.com/u/poseid)\
**Post date:** [July 25, 2010, 5:00pm UTC](https://www.ruby-forum.com/t/assign-blocks-to-variables/192172/1 "2010-07-25T17:00:57Z")

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If you want to work on a function that takes a block as input, is it  
possible to assign the block to a variable?  
In other words, how would you parameterize this in irb:

myfunc {  
otherfunc { |a|  
print a  
}  
}

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<div class="post-metadata">

**Author:** ![poseid](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/dc4da7/32.png) [@poseid](https://www.ruby-forum.com/u/poseid)\
**Post date:** [July 25, 2010, 5:31pm UTC](https://www.ruby-forum.com/t/assign-blocks-to-variables/192172/2 "2010-07-25T17:31:58Z")

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On Jul 25, 2010, at 9:00 AM 7/25/10, poseid wrote:

> 

You hand blocks in as a variable to a function by prepending a variable  
name with an ampersand:

myfunc(&block)

Is that what you’re looking for?

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<div class="post-metadata">

**Author:** ![poseid](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/dc4da7/32.png) [@poseid](https://www.ruby-forum.com/u/poseid)\
**Post date:** [July 25, 2010, 5:40pm UTC](https://www.ruby-forum.com/t/assign-blocks-to-variables/192172/3 "2010-07-25T17:40:33Z")

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poseid wrote:

> If you want to work on a function that takes a block as input, is it  
> possible to assign the block to a variable?  
> In other words, how would you parameterize this in irb:
> 
> myfunc {  
> otherfunc { |a|  
> print a  
> }  
> }

You can assign blocks to variables like this:  
irb(main):001:0\> b1 = lambda{puts “hello”}  
=\> #\<Proc:0x00000001d83b18@(irb):1 (lambda)\>  
irb(main):002:0\> b1.call  
hello  
=\> nil  
irb(main):003:0\> b2 = proc{puts “hello”}  
=\> #Proc:0x00000001d4de88@:3(irb)  
irb(main):004:0\> b2.call  
hello  
=\> nil  
irb(main):005:0\>

Notice that there’s a subtle difference between #proc and #lambda if the  
block takes parameters:

irb(main):005:0\> b3 = lambda{|arg| p arg}  
=\> #\<Proc:0x00000001d357d8@(irb):5 (lambda)\>  
irb(main):006:0\> b4 = proc{|arg| p arg}  
=\> #Proc:0x00000001d1fd20@:6(irb)  
irb(main):007:0\> b3.call(1)  
1  
=\> 1  
irb(main):008:0\> b4.call(1)  
1  
=\> 1  
irb(main):009:0\> b3.call  
ArgumentError: wrong number of arguments (0 for 1)  
from (irb):9:in `call' from (irb):9 from /home/marvin/Programmieren/Programme/irb_/irb_.rb:37:in`’  
irb(main):010:0\> b4.call  
nil  
=\> nil  
irb(main):011:0\>

#lambda acts more like a method, checking wheather it was given all  
necessary arguments, whereas #proc just assigns nil to parameters that  
weren’t provided.

However, to write a function that takes a block you don’t have to take  
care about that[\*]. There’s the “yield” keyword that invokes a supplied  
block and raises a LocalJumpError if no block was supplied.

irb(main):011:0\> def foo  
irb(main):012:1\> yield  
irb(main):013:1\> end  
=\> nil  
irb(main):014:0\> foo{puts “Hello, World!”}  
Hello, World!  
=\> nil  
irb(main):015:0\>

Additionally, with #block\_given? you can check wheather a block has been  
supplied.

Marvin

[\*] You could define a method like as “def foo(&block)”, than you get a  
#proc-ed block in the variable “block”.

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<div class="post-metadata">

**Author:** ![poseid](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/dc4da7/32.png) [@poseid](https://www.ruby-forum.com/u/poseid)\
**Post date:** [July 25, 2010, 5:40pm UTC](https://www.ruby-forum.com/t/assign-blocks-to-variables/192172/4 "2010-07-25T17:40:45Z")

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On Jul 25, 8:58 am, poseid [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> If you want to work on a function that takes a block as input, is it  
> possible to assign the block to a variable?

You can capture a block parameter passed to a method using an  
ampersand name at the end. You can pass a proc as a block to a method  
by throwing an ampersand before it. There are also other ways to  
create a proc value, such as using the ‘proc’ or ‘lambda’ methods, or  
the arrow syntax available in 1.9:

> > def capture\_block(&my\_block); my\_block; end  
> > def with42; yield 42; end  
> > b1 = capture\_block{ |n| puts “N is #{n}” }  
> > =\> #Proc:0x00000100844200@:3(irb)

> > with42( &b1 )  
> > N is 42

> > b2 = proc{ |n| puts “I said, it is #{n}” }  
> > =\> #Proc:0x00000100c061b0@:5(irb)

> > with42( &b2 )  
> > I said, it is 42

> > b3 = -\>(n){ puts “Oh my #{n}” }  
> > =\> #\<Proc:0x00000100be7cf0@(irb):11 (lambda)\>  
> > with42( &b3 )  
> > Oh my 42

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<div class="post-metadata">

**Author:** ![poseid](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/dc4da7/32.png) [@poseid](https://www.ruby-forum.com/u/poseid)\
**Post date:** [July 25, 2010, 6:05pm UTC](https://www.ruby-forum.com/t/assign-blocks-to-variables/192172/5 "2010-07-25T18:05:55Z")

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> #lambda acts more like a method, checking wheather it was given all  
> necessary arguments, whereas #proc just assigns nil to parameters that  
> weren’t provided.

Thank you all.  
#proc is solving my problem!