Also, in my Penknife draft, I'm finding myself much more comfortable using failures rather than exceptions, letting the failures become exceptions when something's called without 'failcall. Failures form a natural tree of blame; this rulebook failed because all these functions failed, and this function failed because this other function failed....
The need to write natural-language error strings diminishes to just the base cases; the rest is pretty-printing and perhaps even a bit of SHRDLU-style "Why did you decide that?" interactive tree-drilling. ^_^
Arc's known for having rotten errors 'cause it shirks the error-checking code; with fail parameters, awesomely comprehensive error reports can be made in almost a single token.
