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comp.lang.ruby

Limitations of eval

Adelle Hartley

5/8/2005 1:20:00 PM

Hi all,

I have been using eval to execute code fragments that I fetch from a
database.

Imagine my surprise when I discovered that the following does not work when
executed by eval:

unless thing.nil?
puts "I have something"
else
puts "nothing here"
end

This works as expected when executed "properly" (from within a regular
normal .rb file).

I don't seem able to make any conditional code work correctly with eval,
unless the conditinoal branch is a one liner:

puts "I have something" unless thing.nil?

Are the limitations of eval documented somewhere (or is it possible that I'm
just not doing it right)?

Adelle.
3 Answers

dblack

5/8/2005 1:53:00 PM

0

Robert Klemme

5/8/2005 9:25:00 PM

0


"David A. Black" <dblack@wobblini.net> schrieb im Newsbeitrag
news:Pine.LNX.4.61.0505080651580.23590@wobblini...
> Hi --
>
> On Sun, 8 May 2005, Adelle Hartley wrote:
>
>> Hi all,
>>
>> I have been using eval to execute code fragments that I fetch from a
>> database.
>
> (shudder :-) But I'm sure you've heard all of that already :-)

*eval grin*

>> Imagine my surprise when I discovered that the following does not work
>> when
>> executed by eval:
>>
>> unless thing.nil?
>> puts "I have something"
>> else
>> puts "nothing here"
>> end
>>
>> This works as expected when executed "properly" (from within a regular
>> normal .rb file).
>>
>> I don't seem able to make any conditional code work correctly with eval,
>> unless the conditinoal branch is a one liner:
>>
>> puts "I have something" unless thing.nil?
>>
>> Are the limitations of eval documented somewhere (or is it possible that
>> I'm
>> just not doing it right)?
>
> I can't duplicate the problem:
>
> $ ruby
> thing = 1
> eval '
> unless thing.nil?
> puts "I have something"
> else
> puts "nothing here"
> end'
> ^D
> I have something

My guess is that the string probably contained something different than
Adelle expected. Adelly, did you try to output your string with "p" before
you eval it? Might be interesting to see what's really in there. As an
illustration what I mean:

>> eval "unless thing puts 'y' else puts 'n' end"
(eval):1: warning: parenthesize argument(s) for future version
SyntaxError: (eval):1:in `irb_binding': compile error
(eval):1: syntax error
unless thing puts 'y' else puts 'n' end
^
(eval):1: syntax error
unless thing puts 'y' else puts 'n' end
^
from (irb):9
from (irb):9
>> eval "unless thing then puts 'y' else puts 'n' end"
n
=> nil

I.e. you probably do not have the line breaks in your string and since you
don't use "then" syntax is not correct. It doesn't work in a normal line
without "then" also:

>> unless thing puts 'y' else puts 'n' end
(irb):11: warning: parenthesize argument(s) for future version
SyntaxError: compile error
(irb):11: syntax error
unless thing puts 'y' else puts 'n' end
^
(irb):11: syntax error
from (irb):11

Kind regards

robert

Adelle Hartley

5/14/2005 4:53:00 AM

0

David A. Black wrote:
> On Sun, 8 May 2005, Adelle Hartley wrote:
>
> > Hi all,
> >
> > I have been using eval to execute code fragments that I
> fetch from a
> > database.
>
> (shudder :-) But I'm sure you've heard all of that already :-)

Don't worry, it's a highly experimental prototype of something that I don't
intend to release.

> > I don't seem able to make any conditional code work correctly with
> > eval, unless the conditinoal branch is a one liner:
<snip>
> I can't duplicate the problem:

I don't know what I was doing wrong, since it looked correct when I output
the code using puts. Anyway, it's working now.

Thanks.

Adelle.