Catch and Throw

[dukester]

hey...



given the following snippet:



(catch

 (for (i 0 9)

   (if (= i 5)

     (throw (string "i was " i ))

   )

   

   (print i " ")

 )

)



How do I get the "throw" message to appear on my screen?



I tried:



(println (throw (string "i was " i )))



with no success. TIA...

[Jeff]

You need to catch the expression into a symbol:

Code Select Expand

> (catch (for (i 0 9) (if (= i 5) (throw (string "i was " i)))) 'foo)
true
> foo
"i was 5"
>

The catch expression will return true if the expression did not throw an error (either a system error or a user error (using throw-error instead of throw)) or nil if it did.  It will set 'foo to the result of the evaluated expression.

[Lutz]

dukester's method, without without the result variable works too, the return value thrown is then the return value of the whole 'catch expression:

Code Select Expand

(println (catch
(for (i 0 9)
(if (= i 5)
(throw (string "i was " i )))
(print i " "))
))

0 1 2 3 4 i was 5
"i was 5"


Note that "i was 5" appears twice, first from the 'println surrrounding the whole 'catch statement, second as the return value of it all.



You need the additional symbol parameter in 'catch only when wanting to 'catch errors, if using 'catch purely for flow control the above method is valid too.



See also here: http://newlisp.org/CodePatterns.html">http://newlisp.org/CodePatterns.html the chapters 7. and 8.



Lutz

[Jeff]

I also wrote a tutorial on using catch/throw for error handling here:



http://artfulcode.nfshost.com/files/simple-error-handling-in-newlisp.html">//http://artfulcode.nfshost.com/files/simple-error-handling-in-newlisp.html

[dukester]

I also wrote a tutorial on using catch/throw for error handling here:



http://artfulcode.nfshost.com/files/simple-error-handling-in-newlisp.html">//http://artfulcode.nfshost.com/files/simple-error-handling-in-newlisp.html

Thank you! I'm on it!

[dukester]

dukester's method, without without the result variable works too, the return value thrown is then the return value of the whole 'catch expression:

Code Select Expand

(println (catch
(for (i 0 9)
(if (= i 5)
(throw (string "i was " i )))
(print i " "))
))

0 1 2 3 4 i was 5
"i was 5"


Note that "i was 5" appears twice, first from the 'println surrrounding the whole 'catch statement, second as the return value of it all.



You need the additional symbol parameter in 'catch only when wanting to 'catch errors, if using 'catch purely for flow control the above method is valid too.



See also here: http://newlisp.org/CodePatterns.html">http://newlisp.org/CodePatterns.html the chapters 7. and 8.



Lutz

Thanks Lutz!



My setup must be wrong. I'm using the Textpad editor and run the scripts from within Textpad. Here's the output:



0 1 2 3 4

Tool completed with exit code 1



It never prints out the "throw" message.

[Jeff]

When running from the repl (the newlisp interpreter at the commandline, not as a script like textpad does), the interpreter will echo back the value of the last evaluated expression.  So if you typed 1 into the repl, it would go like:

Code Select Expand

> 1
1
>

The value of a println expression is the string it prints, so:

Code Select Expand

> (println "Hello world")
Hello world
"Hello world"
>

[dukester]

When running from the repl (the newlisp interpreter at the commandline, not as a script like textpad does), the interpreter will echo back the value of the last evaluated expression.  So if you typed 1 into the repl, it would go like:



[snip]



OK! I got the code to work from the REPL!  So tell me, if I'm writing a CGI script and I want to "catch" an event -- say an error -- and if and when the event occurs, "throw" a [debug] message to the screen. Am I understanding correctly that this cannot be done using the catch/throw duo?

[Jeff]

Yes, it can.  However, there is a more useful function for catching all cgi type errors- error-event.  You can specify a function/lambda that handles all errors, which can be read using error-text or error-number.  This includes using throw-error from user-defined functions.



The alternative is to thread error handling throughout the code like Java or Python.  The equivalent to a try/except statement, including using error-event:

Code Select Expand

(error-event
  (fn ()
    (print "Content-type: text/htmlnn")
    (println "<p>" (error-text) "</p>")))

(if (catch (perform-some-db-op) 'foo)
  (continue-with-page-load)
  (throw-error "controlled error message"))

The cgi.lsp library provided by Lutz already does this- it takes error events (script errors) and prints them to the browser window.  Obviously, this should be overridden in a production app, but for development it's helpful.  Just make sure that cgi.lsp is loaded before anything else to make sure that any errors get handled.

[dukester]

Thanks Jeff for the info...



I seem to be getting confused with newLISP's various behaviors, depending on whether or not one is working from the REPL or not. It's something that I'll have to get familiar and comfortable with.

[Jeff]

Using an editor that can evaluate the buffer can help.  I use textmate and can just hit command-r to evaluate the file and see the output as html.  That doesn't help with cgi, of course, because the cgi env vars are not available- for that I use a browser and MAMP with cgi turned on.

[dukester]

I'm suspect that what you are referring to in `textmate' is available as well in the TextPad editor that I'm using. One command sends the file to newLISP for processing. The results are output to "Command Results" file immediately viewable in TextPad.  Sounds pretty much the same as textmate, no?

[Jeff]

Yes.  That way you only get output that you print in your code.  Scite has a similar feature as well.

[dukester]

You bet! I've used `scite' as well. To summarize, I've learned that _some_ newLISP behavior is only evident when the code is run directly in the REPL. For example, the catch/throw messages. So if I use catch/throw in a script, I have to do in such a way as to not depend on the output message being viewable -- i.e. I should merely _test_ for it. Am I close?