From Modula-2 Reloaded

WiP: Local Exception Handling

PROCEDURE Foobar( p1, p2, p3 : T ) : Status;
BEGIN

IF foo = NIL THEN
    BAILOUT(FooNilError); (* raise local exception *)
ELSE
     doFooStuff;
END;

DoBar(bar);
IF bar = 0 THEN
    BAILOUT(BarZeroError); (* raise local exception *)
ELSE
    doBarStuff;
END;

DoBaz(baz);
IF baz > MAXBAZ THEN
    BAILOUT(BazOverflowError); (* raise local exception *)
ELSE
    doBazStuff;
END;

RETURN Success;

(* handle local exceptions *)
ON BAILOUT
   BazOverflowError :
      UndoBarStuff;
   BarZeroError :
      UndoFooStuff;
   FooNilError :
      RETURN Failure;
END Foobar;

Restrictions:

• a procedure can only have one ON BAILOUT section
• if present, the ON BAILOUT section must be at the bottom
• BAILOUT() cannot be called from within the ON BAILOUT section
• once inside the ON BAILOUT section, program control cannot be transferred back to the procedure body

Pros:

• increases clarity by emphasising the normal (error-free) flow of a program
• helps to avoid cache misses due to large sections of inlined error handling code
• does not incur any cost in the general case (when no exception is raised)
• raising an exception incurs minimal cost (typically one machine cycle only)
• very easy to implement (a BAILOUT() call is translated into a JUMP instruction)

Cons:

• adds an additional feature