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FreeM exposes three means of processing M program execution errors:
FreeM-style error processing exposes a read/write error trap in $ZTRAP. The contents of $ZTRAP must be either empty or a valid M entryref, to which FreeM will GOTO if an error occurs. Each program stack execution level can have its own $ZTRAP error handler enabled.
DSM 2.0-style error processing emulates the $ZTRAP behavior of Digital Standard MUMPS v2. It has the same behavior as FreeM-style error handling, with the exception that in DSM 2.0-style error processing, only one $ZTRAP error handler is set across all program stack execution levels.
Standard error processing uses the NEW-able $ETRAP variable to store error handler code, which may be any valid M code. The code in $ETRAP will run when an error occurs or the $ECODE ISV becomes non-empty. Stack information for standard error handling is provided by the $STACK ISV, the $STACK() intrinsic pseudo-function, and the NEW-able $ESTACK ISV.
If $ETRAP is non-empty when an error condition occurs, $ZTRAP is ignored, regardless of whether FreeM-style or DSM 2.0-style error processing is enabled at the time of the error.
For further information on switching between FreeM-style and DSM 2.0-style $ZTRAP error handling, see the documentation for the BREAK command.
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