Question

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 1

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!