This is the full syntax for the ON phrase. Note there are slightly different options for the different phrases. Specifically, only ON ERROR has the THROW option. STOP conditions are thrown by default, so there is no need to specify TRHOW in the ON STOP syntax. ON ENDKEY and ON QUIT do not have the THROW option since they are older constructs and do not participate in the newer structured error handling model. In addition, ON QUIT does not require the UNDO option, unlike the others.

ON ERROR UNDO
  [label1]
  [     , LEAVE [ label2 ]
     |  , NEXT [ label2 ]
     |  , RETRY [ label1 ]
     |  , RETURN [ return-value |
                  ERROR [ return-value | error-object-expression ] |
                  NO-APPLY ] 
     |  , THROW 
  ]
 
ON  QUIT [ UNDO [ label1 ] ]
  [     , LEAVE [ label2 ]
     |  , NEXT [ label2 ]
     |  , RETRY [ label1 ]
     |  , RETURN [ return-value |
                  ERROR [ return-value | error-object-expression ] |
                  NO-APPLY ]
  ]
 
ON [ STOP | ENDKEY ] UNDO
  [label1]
  [     , LEAVE [ label2 ]
     |  , NEXT [ label2 ]
     |  , RETRY [ label1 ]
     |  , RETURN [ return-value |
                  ERROR [ return-value | error-object-expression ] |
                  NO-APPLY ]
  ]
label1
The name of the block whose processing you want to undo. If you do not name a block with label1, the AVM undoes the processing of the block started by the statement that contains the ON ERROR/STOP/ENDKEY phrase.
LEAVE [label2]
Indicates that after undoing the processing of a block, the AVM leaves the block labeled label2. If you do not name a block, the AVM leaves the block labeled with label1. There are restrictions. For example, you cannot undo an outer block, but leave only the inner block.
NEXT [label2]
Indicates that after undoing the processing of a block, the AVM executes the next iteration of the block you name with the label2 option. If you do not name a block with the NEXT option, the AVM executes the next iteration of the block that contains the ON phrase.
RETRY [label1]
Indicates that after undoing the processing of a block, the AVM repeats the same iteration of the block.

Because RETRY in a block without user input results in an infinite loop, the AVM automatically checks for this possibility and converts a RETRY block into a LEAVE block, or a NEXT block, if it is an iterating block. This behavior is often referred to as infinite loop protection.

RETURN ...
Returns to the calling routine, if there is one. The following table describes various RETURN options:
Option Description
return-value In procedures and VOID methods, this must be a CHARACTER string. The caller can use the RETURN-VALUE function to read the returned value. For user-defined functions, non-VOID methods and property getters, the value must match the specified return type.
ERROR Undoes the current subtransaction, and raises ERROR in the caller. You cannot specify ERROR within a user-interface trigger block or a destructor.

For user-defined functions see note below.
ERROR return-value Undoes the current subtransaction, and raises ERROR in the caller. The CHARACTER string you provide is available to the caller in the RETURN-VALUE function. The AVM also creates an AppError object and stores the return-value in the ReturnValue property.

For user-defined functions see note below.
ERROR error-object-expression Undoes the current subtransaction, and raises ERROR in the caller. The specified error object instance is thrown to the caller.

For user-defined functions see note below.
NO-APPLY In a user-interface trigger, prevents the AVM from performing the default behavior for the trigger event. Otherwise, the option is ignored.
Note: Using RETURN ERROR in a user-defined function sets the target variable of the function to the Unknown value (?) instead of raising ERROR in the caller. See Raise ERROR to the caller of a user-defined function for more detail.
THROW

Use this directive to explicitly propagate an error to the enclosing block, if there is one, otherwise to the caller. You can learn more about throwing error objects in Raise Conditions.