Trapping errors

[Brad Knotwell <knotwell at ix dot netcom dot com>]

Hello all--

The short (unhelpful) answer is to use the file-exists? primitive:

   (file-exists? "/etc/passwd")

The other relevant primitive you'd probably find interesting is named access?:

    (access? "/etc/passwd" R_OK)
    (access? "/etc/passwd" W_OK)
    (access? "/etc/passwd" X_OK)

The "real" answer to your question involves the catch/throw primitives.
I've included below the relevant section from the guile-reference manual:

 - primitive: catch KEY THUNK HANDLER
     Invoke THUNK in the dynamic context of HANDLER for exceptions
     matching KEY.  If thunk throws to the symbol KEY, then HANDLER is
     invoked this way:

          (handler key args ...)

     KEY is a symbol or #t.

     THUNK takes no arguments.  If THUNK returns normally, that is the
     return value of `catch'.

     Handler is invoked outside the scope of its own `catch'.  If
     HANDLER again throws to the same key, a new handler from further
     up the call chain is invoked.

     If the key is `#t', then a throw to *any* symbol will match this
     call to `catch'.

 - primitive: throw KEY &rest ARGS ...
     Invoke the catch form matching KEY, passing ARGS to the HANDLER.

     KEY is a symbol.  It will match catches of the same symbol or of
     #t.

     If there is no handler at all, an error is signaled.

 - procedure: error MSG ARGS ...
     Raise an error with key `misc-error' and a message constructed by
     displaying MSG and writing ARGS.

Hope this helps.

--Brad


Steven Greenberg writes:
 > Hi.  I'm sorry if this is an RTFM but I've checked all of the FMs I can find and it's not covered.
 > 
 > Anyway...
 > 
 > My application gets a file name and then tries to open the file.  If the file dosen't exist then the application stops.  I'd like to be able to catch the error and handle it a bit more gracefully.  Can someone point me to a way to accomplish this?
 > 
 >     Many thanks,
 >     Steve
 > 
 > 
 > begin:vcard 
 > n:Greenberg;Steven
 > tel;fax:650.210.0710
 > tel;work:650.210.2335
 > x-mozilla-html:TRUE
 > url:http://www.epinions.com/
 > org:Epinions
 > adr:;;2037 Landings Drive;Mountain View;CA;94043;
 > version:2.1
 > email;internet:greenbes@epinions.com
 > title:Engineer, iconoclast
 > fn:Steven Greenberg
 > end:vcard