Re: Exception handling

Re: Exception handling

[Peter Stephenson]

Here's a suggestion for adding the exception handling functions with
Bart's changes.  It seemed to warrant a separate function directory
rather than "Misc".  A single init-exceptions function in Misc would be
another way of doing it, but I think adding a third function name is an
unnecessary complication.

Index: Doc/Zsh/contrib.yo
===================================================================
RCS file: /cvsroot/zsh/zsh/Doc/Zsh/contrib.yo,v
retrieving revision 1.39
diff -u -r1.39 contrib.yo
--- Doc/Zsh/contrib.yo	13 Jan 2005 15:31:52 -0000	1.39
+++ Doc/Zsh/contrib.yo	14 Feb 2005 14:35:05 -0000
@@ -13,6 +13,7 @@
 menu(Utilities)
 menu(Prompt Themes)
 menu(ZLE Functions)
+menu(Exception Handling)
 menu(MIME Functions)
 menu(Other Functions)
 endmenu()
@@ -345,7 +346,7 @@
 )
 enditem()
 
-texinode(ZLE Functions)(MIME Functions)(Prompt Themes)(User Contributions)
+texinode(ZLE Functions)(Exception Handling)(Prompt Themes)(User Contributions)
 sect(ZLE Functions)
 
 subsect(Widgets)
@@ -1033,7 +1034,93 @@
 )
 enditem()
 
-texinode(MIME Functions)(Other Functions)(ZLE Functions)(User Contributions)
+texinode(Exception Handling)(MIME Functions)(ZLE Functions)(User Contributions)
+sect(Exception Handling)
+
+Two functions are provided to enable zsh to provide exception handling in a
+form that should be familiar from other languages.
+
+startitem()
+findex(throw)
+item(tt(throw) var(exception))(
+The function tt(throw) throws the named var(exception).  The name is
+an arbitrary string and is only used by the tt(throw) and tt(catch)
+functions.  An exception is for the most part treated the same as a
+shell error, i.e. an unhandled exception will cause the shell to abort all
+processing in a function or script and to return to the top level in an
+interative shell.
+)
+item(tt(catch) var(exception-pattern))(
+The function tt(catch) returns status zero if an exception was thrown and
+the pattern var(exception-pattern) matches its name.  Otherwise it
+returns status 1.  var(exception-pattern) is a standard
+shell pattern, respecting the current setting of the tt(EXTENDED_GLOB)
+option.  An alias tt(catch) is also defined to prevent the argument to the
+function from matching filenames, so patterns may be used unquoted.  Note
+that as exceptions are not fundamentally different from other shell errors
+it is possible to catch shell errors by using an empty string as the
+exception name.  The shell variable tt(CAUGHT) is set by tt(catch) to the
+name of the exception caught.  It is possible to rethrow an exception by
+calling the tt(throw) function again once an exception has been caught.
+findex(catch)
+)
+enditem()
+
+The functions are designed to be use together with the tt(always) construct
+described in
+ifzman(zmanref(zshmisc))\
+ifnzman(noderef(Complex Commands)).  This is important as only this
+construct provides the required support for exceptions.  A typical example
+is as follows.
+
+example({
+  # "try" block
+  # ... nested code here calls "throw MyExcept"
+} always {
+  # "always" block
+  if catch MyExcept; then
+    print "Caught exception MyExcept"
+  elif catch ''; then
+    print "Caught a shell error.  Propagating..."
+    throw ''
+  fi
+  # Other exceptions are not handled but may be caught further
+  # up the call stack.
+})
+
+If all exceptions should be caught, the following idiom might be
+preferable.
+
+example({
+  # ... nested code here throws an exception
+} always {
+  if catch *; then
+    case $CAUGHT in
+      LPAR()MyExcept+RPAR()
+      print "Caught my own exception"
+      ;;
+      LPAR()*RPAR()
+      print "Caught some other exception"
+      ;;
+    esac
+  fi
+})
+
+In common with exception handling in other languages, the exception may be
+thrown by code deeply nested inside the `try' block.  However, note that it
+must be thrown inside the current shell, not in a subshell forked for a
+pipline, parenthesised current-shell construct, or some form of
+substitution.
+
+The system internally uses the shell variable tt(EXCEPTION) to record the
+name of the exception between throwing and catching.  One drawback of this
+scheme is that if the exception is not handled the variable tt(EXCEPTION)
+remains set and may be incorrectly recognised as the name of an exception
+if a shell error subsequently occurs.  Adding tt(unset EXCEPTION) at the
+start of the outermost layer of any code that uses exception handling will
+eliminate this problem.
+
+texinode(MIME Functions)(Other Functions)(Exception Handling)(User Contributions)
 sect(MIME Functions)
 
 Three functions are available to provide handling of files recognised by
Index: Functions/Exceptions/catch
===================================================================
RCS file: Functions/Exceptions/catch
diff -N Functions/Exceptions/catch
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ Functions/Exceptions/catch	14 Feb 2005 14:35:05 -0000
@@ -0,0 +1,41 @@
+# Catch an exception.  Returns 0 if the exception in question was caught.
+# The first argument gives the exception to catch, which may be a
+# pattern.
+# This must be within an always-block.  A typical set of handlers looks
+# like:
+#   {
+#     # try block; something here throws exceptions
+#   } always {
+#      if catch MyExcept; then
+#         # Handler code goes here.
+#         print Handling exception MyExcept
+#      elif catch *; then
+#         # This is the way to implement a catch-all.
+#         print Handling any other exception
+#      fi
+#   }
+# As with other languages, exceptions do not need to be handled
+# within an always block and may propagate to a handler further up the
+# call chain.
+#
+# It is possible to throw an exception from within the handler by
+# using "throw".
+#
+# The shell variable $CAUGHT is set to the last exception caught,
+# which is useful if the argument to "catch" was a pattern.
+#
+# Use "function" keyword in case catch is already an alias.
+function catch {
+  if [[ $TRY_BLOCK_ERROR -gt 0 && $EXCEPTION = ${~1} ]]; then
+    (( TRY_BLOCK_ERROR = 0 ))
+    CAUGHT="$EXCEPTION"
+    unset EXCEPTION
+    return 0
+  fi
+
+  return 1
+}
+# Never use globbing with "catch".
+alias catch="noglob catch"
+
+catch "$@"
Index: Functions/Exceptions/throw
===================================================================
RCS file: Functions/Exceptions/throw
diff -N Functions/Exceptions/throw
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ Functions/Exceptions/throw	14 Feb 2005 14:35:05 -0000
@@ -0,0 +1,29 @@
+# Throw an exception.
+# The first argument is a string giving the exception.  Other arguments
+# are ignored.
+#
+# This is designed to be called somewhere inside a "try-block", i.e.
+# some code of the form:
+#   {
+#     # try-block
+#   } always {
+#     # always-block
+#   }
+# although as normal with exceptions it might be hidden deep inside
+# other code.  Note, however, that it must be code running within the
+# current shell; with shells, unlike other languages, it is quite easy
+# to miss points at which the shell forks.
+#
+# If there is nothing to catch an exception, this behaves like any
+# other shell error, aborting to the command prompt or abandoning a
+# script.
+
+# The following must not be local.
+EXCEPTION="$1"
+if (( TRY_BLOCK_ERROR == 0 )); then
+  # We are throwing an exception from the middle of an always-block.
+  # We can do this by restoring the error status from the try-block.
+  (( TRY_BLOCK_ERROR = 1 ))
+fi
+# Raise an error, but don't show an error message.
+{ ${:?THROW} } 2>/dev/null
Index: Src/zsh.mdd
===================================================================
RCS file: /cvsroot/zsh/zsh/Src/zsh.mdd,v
retrieving revision 1.11
diff -u -r1.11 zsh.mdd
--- Src/zsh.mdd	6 Oct 2003 18:39:58 -0000	1.11
+++ Src/zsh.mdd	14 Feb 2005 14:35:05 -0000
@@ -2,7 +2,7 @@
 link=static
 load=yes
 # load=static should replace use of alwayslink
-functions='Functions/Misc/* Functions/MIME/* Functions/Prompts/*'
+functions='Functions/Exceptions/* Functions/Misc/* Functions/MIME/* Functions/Prompts/*'
 
 nozshdep=1
 alwayslink=1

-- 
Peter Stephenson <pws@csr.com>                  Software Engineer
CSR PLC, Churchill House, Cambridge Business Park, Cowley Road
Cambridge, CB4 0WZ, UK                          Tel: +44 (0)1223 692070


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