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comp.lang.ruby

unexpected return (LocalJumpError

Kyung won Cheon

11/12/2008 4:38:00 AM

def a
blk = Proc.new { return }
blk.call
end

a # => It's OK

def a(blk)
blk.call
end

blk = lambda { return }
a blk # => It's OK

blk = Proc.new { return }
a blk # => unexpected return (LocalJumpError)


############
# Help Me^^
############
--
Posted via http://www.ruby-....
4 Answers

Mike Gold

11/12/2008 6:24:00 AM

0

Kyung won Cheon wrote:
> def a
> blk = Proc.new { return }
> blk.call
> end
>
> a # => It's OK
>
> def a(blk)
> blk.call
> end
>
> blk = lambda { return }
> a blk # => It's OK
>
> blk = Proc.new { return }
> a blk # => unexpected return (LocalJumpError)

% ruby -e 'return'
-e:1: unexpected return (LocalJumpError)

That is what you are seeing. 'return' interrupts a method, but there is
no method at the top-level scope.

def a(blk)
blk.call
end

def main
blk = Proc.new { return }
p "before"
a blk
p "after" # never gets here
end

main # => "before"
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Brian Candler

11/12/2008 8:52:00 AM

0

For the differences between lambda and Proc, including a lot of gory
detail, see
http://innig.net/software/ruby/closures-...
--
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Robert Dober

11/12/2008 10:07:00 AM

0

On Wed, Nov 12, 2008 at 9:52 AM, Brian Candler <b.candler@pobox.com> wrote:
> For the differences between lambda and Proc, including a lot of gory
> detail, see
> http://innig.net/software/ruby/closures-...
or better dont,
All you need to know is that lambda and Proc::new behave as they are
designed to behave, therefore the behavior you encountered is not a
bug but a feature as surprising as that comes to most of us ;). IIRC
this will change in 1.9
To better understand what really happens just run this code:
def a blk
b blk
puts :in_a
end

def b blk
c blk
puts :in_b
end

def c blk
blk.call
puts :in_c
end

def main

#a lambda{ puts :in_lambda; return; puts :nonsense}
#a Proc::new{ puts :in_proc; return; puts :nonsense}
puts :in_main
end

--
> Posted via http://www.ruby-....
>
>



--=20
C'est v=E9ritablement utile puisque c'est joli.

Antoine de Saint Exup=E9ry

Brian Candler

11/12/2008 5:13:00 PM

0

Robert Dober wrote:
> IIRC this will change in 1.9

In what way?

proc is now an alias for Proc.new instead of lambda. But what other
changes are in the pipeline?

BTW I agree that this is a feature, not a bug. But it's not obvious to
the newcomer that there are two, rather different, closure-like objects:

- lambda is standalone. A 'return' returns from the lambda.

- Proc.new / block is created within the context of a method. A 'return'
returns from that method. If the method has already terminated, then
'return' makes no sense and causes an error.

The first is what Lisp programmers expect. The second is what makes it
possible to write

def calculate_something(foo)
foo.each do |params|
...
return val if cond
end
end
--
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