Specify a mandatory block parameter


#1

I think I know the answer to this already, but…

Is there a what to make a block parameter mandatory?

def no_block( p_str ) puts "#{p_str}" end

def block( &p_block )
puts “#{p_block}”
end

no_block #this will cause an exception
block #this doesn’t, but is still missing parameter?

I know, I could just raise my own exception if p_block == nil, just
wondering why the difference? Or am I wrong?

Thanks,

~S


#2

Hi –

On Fri, 17 Mar 2006, Shea M. wrote:

I think I know the answer to this already, but…

Is there a what to make a block parameter mandatory?

def no_block( p_str ) puts "#{p_str}"

I believe that in every case, that’s exactly equivalent to:

puts p_str

because #{…} interpolates the results of a to_s call, and puts also
calls to_s.

I know, I could just raise my own exception if p_block == nil, just wondering
why the difference? Or am I wrong?

The block is really in its own category as a semantic (and syntactic)
thing. You can hook into it in the arglist, but it’s really a
separate construct – more on the logical level of the arglist itself,
rather than a particular argument.

You can test for the presence of a block with block_given? or its
synonym, iterator?

David


David A. Black (removed_email_address@domain.invalid)
Ruby Power and Light, LLC (http://www.rubypowerandlight.com)

“Ruby for Rails” chapters now available
from Manning Early Access Program! http://www.manning.com/books/black


#3

Shea M. wrote:

puts "#{p_block}"

end

no_block #this will cause an exception
block #this doesn’t, but is still missing parameter?

I know, I could just raise my own exception if p_block == nil, just
wondering why the difference? Or am I wrong?

Do you mean something like this?

def f
throw “D’oh! No block.” unless block_given?
puts “OK, got a block.”
end

begin
f { }
f
rescue
puts $!
end


#4

Hi –

On Fri, 17 Mar 2006, Jeffrey S. wrote:

def block( &p_block )
Do you mean something like this?

def f
throw “D’oh! No block.” unless block_given?

I think you mean “raise” :slight_smile:

David


David A. Black (removed_email_address@domain.invalid)
Ruby Power and Light, LLC (http://www.rubypowerandlight.com)

“Ruby for Rails” chapters now available
from Manning Early Access Program! http://www.manning.com/books/black


#5

Shea M. wrote:

puts "#{p_block}"

~S

I think you guys missed part of my post:
“I know, I could just raise my own exception if p_block == nil, just
wondering why the difference?”

Although thank you for reinforcing the concept of raising my own
exception.

~S


#6

Shea M. wrote:

def block( &p_block )
Thanks,

~S

I think you guys missed part of my post:
“I know, I could just raise my own exception if p_block == nil, just
wondering why the difference?”

That’s not the same thing. If you use block_given? as David Black and I
suggested, there is no p_block to check for nil.

Although thank you for reinforcing the concept of raising my own exception.

So… You want an exception to be thrown, but you don’t want to throw
an exception? Please show me what I’ve missed. Are you saying that
specifying the &p_block parameter should make the block mandatory
automatically? Hmmm… I don’t know of a way to enforce the presence
of a mandatory block without raising any exceptions.


#7

Jeffrey S. wrote:

So… You want an exception to be thrown, but you don’t want to throw
an exception? Please show me what I’ve missed. Are you saying that
specifying the &p_block parameter should make the block mandatory
automatically?

I am not saying it should, but just curious why specifying a block,
does not make it mandatory (automatically)? Specifying a normal
parameter, makes it mandatory (automatically. It just seems odd that
the two are treated differently. Just my $0.02.

~S


#8

removed_email_address@domain.invalid wrote:

def f
throw “D’oh! No block.” unless block_given?

I think you mean “raise” :slight_smile:

I reiterate: D’oh!


#9

Hi –

On Tue, 21 Mar 2006, Shea M. wrote:

differently. Just my $0.02.
It’s hard for me to answer, because I don’t understand why one would
assume, a priori, that they should be treated the same :slight_smile: But in
practice, the code-block facility works out better, I think, the way
it is. The method can easily branch on existence or non-existence of
a block, and it’s easy to write methods that can work with or without
a block. I guess I view the &block thing as a special concession to
the possibility that one might want to handle the block as an object
(rather than as a syntactic construct), but not the heart of the
matter.

David


David A. Black (removed_email_address@domain.invalid)
Ruby Power and Light, LLC (http://www.rubypowerandlight.com)

“Ruby for Rails” chapters now available
from Manning Early Access Program! http://www.manning.com/books/black


#10

Hi,

Blocks are actually objects of class Proc, so if you need to enforce
presence of block parameter then you can do something like this:

irb(main):001:0> p = Proc.new { puts ‘block’ }
=> #Proc:0x02ba5b00@:1(irb)
irb(main):002:0> def method_with_mandatory_block(block)
irb(main):003:1> block.call
irb(main):004:1> end
irb(main):005:0> method_with_mandatory_block p
block


Martins


#11

13 removed_email_address@domain.invalid writes:

Blocks are actually objects of class Proc

Are you sure? I did a lot of reading up on blocks lately, and from
what I learned, blocks are one of the very few things in Ruby that are
not objects. You can, however, automatically convert them to Proc
objects if you need to; maybe that’s what you’re noticing?

-=Eric


#12

Hi –

On Thu, 23 Mar 2006, Eric S. wrote:

13 removed_email_address@domain.invalid writes:

Blocks are actually objects of class Proc

Are you sure? I did a lot of reading up on blocks lately, and from
what I learned, blocks are one of the very few things in Ruby that are
not objects. You can, however, automatically convert them to Proc
objects if you need to; maybe that’s what you’re noticing?

That’s correct; the block is a syntactic construct (like argument
list, which also isn’t an object). I think there may have once been a
Block class in CVS once for a day or two, as part of the evolution of
the whole block/lambda/Proc sphere, but if so it was just an
experiment and not pursued.

David


David A. Black (removed_email_address@domain.invalid)
Ruby Power and Light, LLC (http://www.rubypowerandlight.com)

“Ruby for Rails” chapters now available
from Manning Early Access Program! http://www.manning.com/books/black


#13

Hi,

I’m not sure how blocks are treated internally by Ruby (if some expert
can tell us, then it would be great) but when we are using blocks in
our Ruby code then they are objects of class Proc (again, please,
confirm someone if I’m wrong).


Martins


#14

13 wrote:

On 3/23/06, Eric S. removed_email_address@domain.invalid wrote:

13 removed_email_address@domain.invalid writes:

Blocks are actually objects of class Proc
Are you sure? I did a lot of reading up on blocks lately, and from
what I learned, blocks are one of the very few things in Ruby that are
not objects. You can, however, automatically convert them to Proc
objects if you need to; maybe that’s what you’re noticing?

-=Eric

Actually Eric is right. They are only objects if you request them to
be.

def foo

no object

yield 1
end

def bar(&b)

Proc instance

b[2]
end

Kind regards

robert

#15

Hi –

On Fri, 24 Mar 2006, 13 wrote:

Hi,

I’m not sure how blocks are treated internally by Ruby (if some expert
can tell us, then it would be great) but when we are using blocks in
our Ruby code then they are objects of class Proc (again, please,
confirm someone if I’m wrong).

Consider this:

a.each {|b| puts b.capitalize }

That’s a method call with a block. But the block is just a syntactic
construct; it’s not a Proc object.

You can convert back and forth between blocks and Procs, in various
ways, but there is such a thing as a block that is just a block.

David

not objects. You can, however, automatically convert them to Proc
objects if you need to; maybe that’s what you’re noticing?

-=Eric


David A. Black (removed_email_address@domain.invalid)
Ruby Power and Light, LLC (http://www.rubypowerandlight.com)

“Ruby for Rails” chapters now available
from Manning Early Access Program! http://www.manning.com/books/black