Help with eval

Am I missing something with eval? Why does this code not work?

eval(“foo = 1”)
puts foo

Gives:

C:\example\trunk>ruby script/runner lib/aggregate/keith.rb
C:/example/trunk/vendor/rails/railties/lib/commands/runner.rb:45:
undefined local variable or method foo' for main:Obje ct (NameError) from c:/ruby/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:ineval’
from
C:/noozler/trunk/vendor/rails/railties/lib/commands/runner.rb:45
from
c:/ruby/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in
gem_original_require' from c:/ruby/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:inrequire’
from script/runner:3

Keith C. wrote:

Am I missing something with eval? Why does this code not work?

Maybe it’s a scoping issue with where you’re running your code…works
fine for me in irb:

eval(“foo = 1”)
=> 1

foo
=> 1

Hm. You’re right, that did work. How come the second bit below doesn’t
work?

Loading development environment (Rails 2.0.2)

eval(“foo = 1”)
=> 1

foo
=> 1

begin
?> eval(“bar=1”)

bar
end
NameError: undefined local variable or method `bar’ for
#Object:0x27bf9ec
from (irb):5

yermej wrote:

Sorry, but I don’t know why, only that it seems odd.

Wow. That is weird. I think evals are scoped as blocks. I’m going to
play with this for a bit.

On Jan 23, 4:13 pm, Keith C. [email protected] wrote:

end

NameError: undefined local variable or method `bar’ for
#Object:0x27bf9ec
from (irb):5

For even more fun, evaluate bar once you’ve gotten the NameError.

bar
=> 1

Sorry, but I don’t know why, only that it seems odd.

On Jan 23, 2008, at 4:06 PM, Keith C. wrote:

Wow. That is weird. I think evals are scoped as blocks. I’m going to
play with this for a bit.

variables created in eval can only been seen from eval. when you are
irb you are already inside eval. there are some exceptions - search
the archives.

if you post what you are trying to accomplish perhaps someone can
help - no pattern that relies on eval creating vars in the local
scope is going to pan out too well without a little dsl or some
hackery - eval just won’t suffice.

regards.

a @ http://codeforpeople.com/

ara.t.howard wrote:

if you post what you are trying to accomplish perhaps someone can
help - no pattern that relies on eval creating vars in the local
scope is going to pan out too well without a little dsl or some
hackery - eval just won’t suffice.

I’m not trying to accomplish anything too fancy, and I was able to skirt
around the problem by declaring my variables outside of the eval block.

But now, my interest is piqued. What is dsl? What if I wanted to declare
a variable with a dynamic name using an eval block? (And I wanted to use
it outside of the eval block)

On Jan 24, 5:50 am, “ara.t.howard” [email protected] wrote:

help - no pattern that relies on eval creating vars in the local
scope is going to pan out too well without a little dsl or some
hackery - eval just won’t suffice.

regards.

a @http://codeforpeople.com/

share your knowledge. it’s a way to achieve immortality.
h.h. the 14th dalai lama

begin
eval(“bar=1”)
puts bar
end

Gives me and error too. but if I do

bar = 0
begin
eval(“bar=1”)
puts bar
end

It works! I am a ruby rookie so I dont know why this is so.

“K” == Keith C. [email protected] writes:

K> evals are scoped as blocks, and so variables created in them are not
K> available outside of the block.

No, not really. Try this

moulon% ruby -e ‘eval “a = 12”; eval “p a”’
12
moulon%

if the variable `a’ was scoped, the second eval won’t see it.

This is at compile time that ruby make the difference between an access
to
a variable and a method call.

When you write :

eval “a = 12”
a

at compile time, ruby has not yet seen a variable with the name a' and it think that on the second line you try to call the methoda’

Something like this

def a
p 24
end

eval “a = 12”
a

When you write it

a = 0
eval “a = 12”
a

when ruby compile the third line, it know that it exist a variable with
the
name a' (it has seen it at the first line) and this time it try to access the variablea’ rather than call the method #a

Guy Decoux

Chirantan wrote:

It works! I am a ruby rookie so I dont know why this is so.

In the first example, you are defining the variable inside of the block.

In the second example, you’ve defined the variable outside of the block.

evals are scoped as blocks, and so variables created in them are not
available outside of the block.

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