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comp.lang.ruby

Rescue on do end blocks

Farrel Lifson

3/7/2006 12:31:00 PM

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
8 Answers

Marcin Mielzynski

3/7/2006 1:09:00 PM

0

Farrel Lifson 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(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
>
>

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 Lifson

3/7/2006 1:17:00 PM

0

Is there some reason why it can't be made more elegant like it iscurrently for methods? Currently you can do this for methods:def test raiserescue puts "Error"endFarrelOn 3/7/06, Marcin Mielzynski <lopexx@autograf.pl> wrote:> Farrel Lifson 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(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> >> >>> 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>>

Marcin Mielzynski

3/7/2006 1:30:00 PM

0

Farrel Lifson 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 Lifson

3/7/2006 1:40:00 PM

0

Okay that kinda makes sense... kinda.On 3/7/06, Marcin Mielzynski <lopexx@autograf.pl> wrote:> Farrel Lifson 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>>

Marcin Mielzynski

3/7/2006 2:14:00 PM

0

Farrel Lifson 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 Lifson

3/7/2006 2:23:00 PM

0

Ah if that's the case then I can understand a bit more clearly why.Still it would be a nice thing to have.ThanksFarrelOn 3/7/06, Marcin Mielzynski <lopexx@autograf.pl> wrote:> Farrel Lifson 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>>

Eric Hodel

3/7/2006 10:47:00 PM

0

On Mar 7, 2006, at 4:31 AM, Farrel Lifson 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?)

[...]

> 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

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 Hodel - drbrain@segment7.net - http://blog.se...
This implementation is HODEL-HASH-9600 compliant

http://trackmap.rob...

Shea Martin

3/16/2006 7:57:00 PM

0

Marcin Miel?ynski wrote:
> Farrel Lifson 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

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