Home Accessibility Courses Twitter The Mouth Facebook Resources Site Map About Us Contact
 
Python and Tcl - public course schedule [here]
Private courses on your site - see [here]
Please ask about maintenance training for Perl, PHP, Lua, etc
 
Exception handling in Perl - using eval

Exception handling is the trapping of nonstandard results from blocks of code or functions - for example, if I draw a card from a pack and ask you "what suite is this?" you'll usually be able to tell me "it's a heart" or "it's a club" ... but if I had drawn a joker, there would have been no appropriate answer to give and you might have thrown an exception.

Nonstandard results may also be generated where a piece of code fails for some reason - if you try to open a file for reading and the file doesn't exist, or if you perform a piece of arithmetic in which you attempt to divide by zero, for example.

Keywords like try and catch or except, or begin and rescue are provided within many languages (Java, Python, Lua ...) to formalise exception handling, where code which may throw an exception is placed into the first (try or begin) block, and code to be performed if it's trapped is put into the following (catch, except, rescue) block.

Perl doesn't come with any of these keywords (although you can add the syntactic icing via CPAN modules), and by default Perl will exit the program with an error message if your code fails for certain reasons, such as if you divide by zero, or try to set a list member with a position number that's so negative that it's back off the start of a list. However, you can provide the same logic by placing the code from which you want to trap errors into an eval block:

  eval {
    $n = 30 / $k;
  }


If the block fails, it returns a false value - and places the reason for the failure into the special variable $@, so that you can check the return and provide an extra block to handle the 'excpetion' if you wish.

There's a complete example [here]; note the 1; in the end of the eval block which is an excellent general way to ensure that the block will always return a true value if it completes (as is usually the case in Perl, the value returned by a block is the result of the last operation performed in the block).
(written 2010-10-23)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P303 - Perl - Miscellany
  [3159] Returning multiple values from a function call in various languages - a comparison - (2011-02-06)
  [2877] Further more advanced Perl examples - (2010-07-19)
  [2876] Different perl examples - some corners I rarely explore - (2010-07-18)
  [2427] Operator overloading - redefining addition and other Perl tricks - (2009-09-27)
  [2219] Configuring httpd, or Tomcat, to run CGI scripts in Perl - (2009-06-05)


Back to
What are .pid files?
Previous and next
or
Horse's mouth home
Forward to
Audio equipment
Some other Articles
The legal considerations of your web presence - revisited
Logging the performance of the Apache httpd web server
Well House Manor - the next six years
Audio equipment
Exception handling in Perl - using eval
What are .pid files?
Children, zombies, and reaping processes
Expect in Perl - a short explanation and a practical example
Dulwich College Preparatory, and Sevenoaks, Schools
Setting up a matrix of data (2D array) for processing in your program
4759 posts, page by page
Link to page ... 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96 at 50 posts per page


This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

Link to Ezine home page (for reading).
Link to Blogging home page (to add comments).

You can Add a comment or ranking to this page

© WELL HOUSE CONSULTANTS LTD., 2019: 404 The Spa • Melksham, Wiltshire • United Kingdom • SN12 6QL
PH: 01225 708225 • EMAIL: info@wellho.net • WEB: http://www.wellho.net • SKYPE: wellho

PAGE: http://www.wellho.net/mouth/3012_Exc ... -eval.html • PAGE BUILT: Sat May 27 16:49:10 2017 • BUILD SYSTEM: WomanWithCat