Specify a mandatory block parameter

ruby-talk
1

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

Is there a what to make a block parameter mandatory?

<code>
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?
</code>

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 --

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

Is there a what to make a block parameter mandatory?

<code>
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.

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?
</code>

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

···

On Fri, 17 Mar 2006, Shea Martin wrote:

--
David A. Black (dblack@wobblini.net)
Ruby Power and Light, LLC (http://www.rubypowerandlight.com)

"Ruby for Rails" chapters now available
from Manning Early Access Program! Ruby for Rails

3

Shea Martin wrote:

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

Is there a what to make a block parameter mandatory?

<code>
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?
</code>

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

Shea Martin wrote:

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

Is there a what to make a block parameter mandatory?

<code>
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?
</code>

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

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?"

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

~S

5

Hi --

···

On Fri, 17 Mar 2006, Jeffrey Schwab wrote:

Shea Martin wrote:

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

Is there a what to make a block parameter mandatory?

<code>
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?
</code>

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?

I think you mean "raise" :slight_smile:

David

--
David A. Black (dblack@wobblini.net)
Ruby Power and Light, LLC (http://www.rubypowerandlight.com)

"Ruby for Rails" chapters now available
from Manning Early Access Program! Ruby for Rails

6

Shea Martin wrote:

Shea Martin wrote:

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

Is there a what to make a block parameter mandatory?

<code>
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?
</code>

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

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

dblack@wobblini.net wrote:

def f
   throw "D'oh! No block." unless block_given?

I think you mean "raise" :slight_smile:

I reiterate: D'oh!

8

Jeffrey Schwab 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

9

Hi --

Jeffrey Schwab 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.

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

···

On Tue, 21 Mar 2006, Shea Martin wrote:

--
David A. Black (dblack@wobblini.net)
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@(irb):1>
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

On 3/20/06, dblack@wobblini.net <dblack@wobblini.net> wrote:

Hi --

On Tue, 21 Mar 2006, Shea Martin wrote:

> Jeffrey Schwab 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.

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 (dblack@wobblini.net)
Ruby Power and Light, LLC (http://www.rubypowerandlight.com)

"Ruby for Rails" chapters now available
from Manning Early Access Program! Ruby for Rails

11

13 <one.three@gmail.com> 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,

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

On 3/23/06, Eric Schwartz <emschwar@fc.hp.com> wrote:

13 <one.three@gmail.com> 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

13

Hi --

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

···

On Fri, 24 Mar 2006, 13 wrote:

--
Martins

On 3/23/06, Eric Schwartz <emschwar@fc.hp.com> wrote:

13 <one.three@gmail.com> 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

--
David A. Black (dblack@wobblini.net)
Ruby Power and Light, LLC (http://www.rubypowerandlight.com)

"Ruby for Rails" chapters now available
from Manning Early Access Program! Ruby for Rails

14

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).

--
Martins

13 <one.three@gmail.com> 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

···

On 3/23/06, Eric Schwartz <emschwar@fc.hp.com> wrote: