Asp Forum - proc to block - comp.lang.ruby

L A

1/8/2008 4:06:00 AM

[Note: parts of this message were removed to make it a legal post.]

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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7 Answers

Gary Wright

1/8/2008 4:35:00 AM

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 Wright

botp

1/8/2008 4:36:00 AM

On Jan 8, 2008 12:06 PM, L A <loren.abrams@yahoo.com> 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@(irb):1>
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@(irb):30>
(proc {p}).call
#=> #<Proc:0x028cc40c@(irb):1>
(proc {p}).call.call
#=> "hi, i'm proc"

kind regards -botp

Rick DeNatale

1/8/2008 1:25:00 PM

On Jan 7, 2008 11:35 PM, Gary Wright <gwtmp01@mac.com> wrote:
>
> 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

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.denh...

noah.easterly@gmail.com

1/8/2008 2:04:00 PM

On Jan 8, 8:24 am, Rick DeNatale <rick.denat...@gmail.com> wrote:
> On Jan 7, 2008 11:35 PM, Gary Wright <gwtm...@mac.com> wrote:
>
>
>
> > 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
>
> Except that this isn't part of the standard ruby library for 1.8. It
> relies on having a Symbol#to_proc method
>
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 http://www.ruby-doc.org/docs/ProgrammingRuby/html/tut_metho...)

Rick DeNatale

1/8/2008 3:39:00 PM

On Jan 8, 2008 9:05 AM, Noah Easterly <noah.easterly@gmail.com> wrote:
> On Jan 8, 8:24 am, Rick DeNatale <rick.denat...@gmail.com> wrote:
> > On Jan 7, 2008 11:35 PM, Gary Wright <gwtm...@mac.com> wrote:
> > > 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
> >
> > Except that this isn't part of the standard ruby library for 1.8. It
> > relies on having a Symbol#to_proc method
> >
> 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."

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/t...

--
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denh...

Sebastian Hungerecker

1/8/2008 3:56:00 PM

Rick DeNatale wrote:
> Gary suggested using a symbol as a proc

No, he didn't.

--
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ICQ: 205544826

Gary Wright

1/8/2008 4:33:00 PM

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 Wright

comp.lang.ruby

proc to block

L A

Gary Wright

botp

Rick DeNatale

noah.easterly@gmail.com

Rick DeNatale

Sebastian Hungerecker

Gary Wright

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