Rescue on do end blocks

[Farrel Lifson]

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 Mielżyński]

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 Miel¿yñski]

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]

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b3RlOgo+ID4gSXMgdGhlcmUgc29tZSByZWFzb24gd2h5IGl0IGNhbid0IGJlIG1hZGUgbW9yZSBl
bGVnYW50IGxpa2UgaXQgaXMKPiA+IGN1cnJlbnRseSBmb3IgbWV0aG9kcz8gQ3VycmVudGx5IHlv
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fQo+Cj4gd2hpY2ggZG9lc250IG1ha2Ugc2Vuc2UgZm9yIG1lCj4KPiBsb3BleAo+Cj4K

 

[Marcin Miel¿yñski]

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]

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ZiwgYmVnaW4sCj4gaWYsIHdoaWxlIGV0Yy4uCj4KPiBsb3BleAo+Cj4K

 

[Eric Hodel]

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.

 

[Shea Martin]

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