Forum: Ruby Rescue on do end blocks

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Farrel L. (Guest)
on 2006-03-07 14:32
(Received via mailing list)
I've been playing around with rescue expressions and besides being put
on functions they also seem to work when defining classes (useful for
metaprogramming perhaps?)

irb(main):001:0> class Test
irb(main):002:1>   raise
irb(main):003:1> rescue
irb(main):004:1>   puts "Error"
irb(main):005:1> ensure
irb(main):006:1*   puts "Cleaned up"
irb(main):007:1> end
Error
Cleaned up

However, why can they not be applied to do end blocks?

irb(main):016:0> [1,2,3].each do |num|
irb(main):017:1*   num.upcase
irb(main):018:1> rescue
irb(main):019:1>   puts "Error"
irb(main):020:1> ensure
irb(main):021:1*   puts "Clened up"
irb(main):022:1> end
SyntaxError: compile error
(irb):18: syntax error
(irb):20: syntax error
        from (irb):22

Farrel
Marcin MielżyÅ?ski (Guest)
on 2006-03-07 15:09
(Received via mailing list)
Farrel L. wrote:
> irb(main):007:1> end
> irb(main):021:1*   puts "Clened up"
> irb(main):022:1> end
> SyntaxError: compile error
> (irb):18: syntax error
> (irb):20: syntax error
>         from (irb):22
>
> Farrel
>
>

rescue must be enclosed in begin/end

[1,2,3].each do |num|
  begin #!
    num.upcase
  rescue
    puts "Error"
  ensure
    puts "Clened up"
  end
end

lopex
Farrel L. (Guest)
on 2006-03-07 15:18
(Received via mailing list)
Is there some reason why it can't be made more elegant like it is
currently for methods? Currently you can do this for methods:

def test
  raise
rescue
  puts "Error"
end

Farrel
Marcin MielżyÅ?ski (Guest)
on 2006-03-07 15:36
(Received via mailing list)
Farrel L. wrote:
> Is there some reason why it can't be made more elegant like it is
> currently for methods? Currently you can do this for methods:
>
> def test
>   raise
> rescue
>   puts "Error"
> end
>

Because do/end is another form of { } (the difference is associativity)
and those are used to compose blocks.

Imagine:

sth.each{
    raise
  rescue
    puts "Error"
}

which doesnt make sense for me

lopex
Farrel L. (Guest)
on 2006-03-07 15:42
(Received via mailing list)
Okay that kinda makes sense... kinda.
Marcin MielżyÅ?ski (Guest)
on 2006-03-07 16:16
(Received via mailing list)
Farrel L. wrote:
> Okay that kinda makes sense... kinda.
>

:)

Maybe somebody else will explain this better later on, but I'm sure that
the 'end' of 'do' has nothing to do with the 'end' of class, def, begin,
if, while etc..

lopex
Farrel L. (Guest)
on 2006-03-07 16:25
(Received via mailing list)
Ah if that's the case then I can understand a bit more clearly why.
Still it would be a nice thing to have.

Thanks
Farrel
Eric H. (Guest)
on 2006-03-08 00:50
(Received via mailing list)
On Mar 7, 2006, at 4:31 AM, Farrel L. wrote:

> I've been playing around with rescue expressions and besides being put
> on functions they also seem to work when defining classes (useful for
> metaprogramming perhaps?)

[...]

> (irb):18: syntax error
> (irb):20: syntax error
>         from (irb):22

This will make blocks much more expensive to call because you'll have
to perform the exception handler setup and teardown for each
invocation of the block which will give a slight performance
degradation:

require 'benchmark'

N = 10_000_000

Benchmark.bmbm do |bm|
   bm.report 'without begin' do N.times { 0 } end
   bm.report 'with begin' do N.times { begin 0 end } end
   bm.report 'with rescue' do N.times { begin 0; rescue; end } end
end

                     user     system      total        real
without begin   4.520000   0.020000   4.540000 (  4.939785)
with begin      5.000000   0.030000   5.030000 (  6.050101)
with rescue     6.720000   0.030000   6.750000 (  7.716410)

While having an implicit begin won't give the same level of slowdown
if implemented in the compiler, there's no pressing need to add the
complexity to Ruby because it isn't the common case.

--
Eric H. - removed_email_address@domain.invalid - http://blog.segment7.net
This implementation is HODEL-HASH-9600 compliant

http://trackmap.robotcoop.com
Shea M. (Guest)
on 2006-03-16 22:01
(Received via mailing list)
Marcin Miel¿yñski wrote:
>
>
> which doesnt make sense for me
>
> lopex

what about this

sth.each{

	#my code
    	raise
	#don't reach here

	}rescue{

	puts "Error"

	}

kind like c++/java?  I don't think it looks that great either, but then
I don't like the {} block in my (ruby) code anyway.

just my $0.02

~S
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