Proc to block

Ruby
1

Hello all,

Is there a way to turn a Proc into a block for the purposes of methods
that yield to blocks? It’s not a necessity as I can always wrap the
Proc in a block, I’m just curious.

Thanks,
Loren

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2

On Jan 7, 2008, at 11:06 PM, L A wrote:

Hello all,

Is there a way to turn a Proc into a block for the purposes of
methods that yield to blocks? It’s not a necessity as I can always
wrap the Proc in a block, I’m just curious.

Just prefix the object you want to be passed as a block with ‘&’ in
the argument list.

p = proc { |x| x > 10 }

[1,5,10,20, 30].select &p

Gary W.

3

On Jan 8, 2008 12:06 PM, L A [email protected] wrote:

Is there a way to turn a Proc into a block for the purposes of methods that yield to blocks? It’s not a necessity as I can always wrap the Proc in a block, I’m just curious.

maybe use the & op, like

p=proc{“hi, i’m proc”}
#=> #Proc:0x028cc40c@:1(irb)
p.call
#=> “hi, i’m proc”
def q
yield
end
#=> nil
q
LocalJumpError: no block given
from (irb):7:in `q’
from (irb):9
from :0
q &p
#=> “hi, i’m proc”
(proc &p).call
#=> “hi, i’m proc”

i am not sure why you want to wrap a proc with a block though

proc {p}
#=> #Proc:0x02898418@:30(irb)
(proc {p}).call
#=> #Proc:0x028cc40c@:1(irb)
(proc {p}).call.call
#=> “hi, i’m proc”

kind regards -botp

4

On Jan 8, 8:24 am, Rick DeNatale [email protected] wrote:

wrap the Proc in a block, I’m just curious.

Wait… what?

No it doesn’t.

[ 1, 5, 10, 20, 30 ].map &:to_a

uses Symbol#to_proc

p = proc { |x| x > 10 }
[1,5,10,20, 30].select &p

is standard ruby syntax, even from 1.6:

“If the last argument to a method is preceded by an ampersand, Ruby
assumes that it is a Proc object. It removes it from the parameter
list, converts the Proc object into a block, and associates it with
the method.”
(see
Programming Ruby: The Pragmatic Programmer's Guide)

5

On Jan 8, 2008 9:05 AM, Noah E. [email protected] wrote:

relies on having a Symbol#to_proc method
[1,5,10,20, 30].select &p

is standard ruby syntax, even from 1.6:

“If the last argument to a method is preceded by an ampersand, Ruby
assumes that it is a Proc object. It removes it from the parameter
list, converts the Proc object into a block, and associates it with
the method.”

This isn’t what I said.

Gary suggested using a symbol as a proc, which requires that Symbol
implement the to_proc method, which is NOT part of standard Ruby prior
to 1.9.

$ qri to_proc
------------------------------------------------------ Multiple choices:

 Method#to_proc, Proc#to_proc, Test::Unit::Util::ProcWrapper#to_proc

Now as I pointed out extending Symbol to implement to_proc is quite
common, and many Rails programmers tend to think of the stuff in
ActiveSupport as being standard Ruby, but it isn’t.

It’s likely that Rails will change ActiveSupport to only add
Symbol#to_proc if using Ruby < 1.9 since 1.9 includes it, but that
doesn’t seem to have happened yet.
http://dev.rubyonrails.org/ticket/8818

–
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/

6

On Jan 7, 2008 11:35 PM, Gary W. [email protected] wrote:

the argument list.

p = proc { |x| x > 10 }

[1,5,10,20, 30].select &p

Except that this isn’t part of the standard ruby library for 1.8. It
relies on having a Symbol#to_proc method

It is a common extension, for example Rails includes it.

It IS part of Ruby 1.9 though.

I’d suggest googling for “ruby to_proc”

–
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/

7

On Jan 8, 2008, at 10:39 AM, Rick DeNatale wrote:

Gary suggested using a symbol as a proc, which requires that Symbol
implement the to_proc method, which is NOT part of standard Ruby prior
to 1.9.

Rick, you just misread my original post a bit. I didn’t mention
Symbol#to_proc at all but I did hint at it by saying you ‘just’
prefix the argument with ‘&’. That sort of begs the question of
what happens if the argument isn’t an instance of Proc but I didn’t
want to muddy the waters at that point. My example used a proc.

Gary W.

8

Rick DeNatale wrote:

Gary suggested using a symbol as a proc

No, he didn’t.