Brent Chapman Brent Chapman - 1 month ago 15
Ruby Question

How to rescue an eval in Ruby?

I'm trying to figure out how to rescue syntax errors that come up when

eval()
ing code in Ruby 1.8.6.

I would expect the following Ruby code:

#!/usr/bin/ruby

good_str = "(1+1)"
bad_str = "(1+1" # syntax error: missing closing paren

begin
puts eval(good_str)
puts eval(bad_str)
rescue => exc
puts "RESCUED!"
end


to produce the following result when run:

2
RESCUED!


Instead, what I get is:

2
eval_rescue.rb:8: (eval):1: compile error (SyntaxError)
(eval):1: syntax error, unexpected $end, expecting ')'


It appears that the SyntaxError raised by the eval method is being rescued somewhere within the eval, without giving me a chance to handle it myself.

Anybody have any idea how to get the behavior I want (i.e., for my 'rescue' clause to catch the error from the 'eval')?

Answer

Well, that was easy...

It turns out that, by default, the "rescue" statement does not catch all exceptions, but only those that are subclasses of StandardError. SyntaxError is a sibling/cousin of StandardError, not a subclass of it, so the rescue statement doesn't capture it unless explicitly told to.

To have the rescue block capture all exceptions, you need to change the code to the following:

#!/usr/bin/ruby

good_str = "(1+1)"
bad_str = "(1+1"    # syntax error: missing closing paren

begin
    puts eval(good_str)
    puts eval(bad_str)
rescue Exception => exc
    puts "RESCUED!"
end

Note the change in the "rescue" line, from "rescue => exc" to "rescue Exception => exc".

Now, when you run the code, you get the desired results:

2
RESCUED!