And option number five, conditions + restarts, http://www.lispworks.com/documentation/lw61/CLHS/Body/09_a.htm http://www.lispworks.com/documentation/lw61/CLHS/B

And option number five, conditions + restarts, http://www.lispworks.com/documentation/lw61/CLHS/Body/09_a.htm http://www.lispworks.com/documentation/lw61/CLHS/Body/09_a.h..., which is a bit like more formalised callbacks / exceptions depending on usage. Restarts are a great thing for both interactive (using a "continue" restart if e.g. something can be safely skipped) and programmatic usage (to customise an algorithm where you control from the outside whether a restart should be invoked in case of a "soft" exception/condition).