Thu, 8 Feb 2001 10:51:58 -0500, Peter Douglass
The first part of my question (not contained in your reply) is whether it is feasible to disable a developer's access to the "unsafe" numerical operations.
import Prelude hiding (quot, rem, (/) {- etc. -}) import YourPrelude -- which defines substitutes You can "disable" it now. You cannot disable them entirely - anyone can define present functions in terms of your functions if he really wants.
Whether or not an individual developer chooses to do so is another matter.
Why only quot? There are many other ways to write bottom: head [] (\(x:xs) -> (x,xs)) [] let x = x in x log (-1) asin 2 error "foo"
If you "know" the value is non-zero before run-time, then that is statically determined.
I know but the compiler does not know, and I have no way to convince it.
It is possible that the developer writes a function which returns a nonZeroNumeric value which actually has a value of zero. However, the value of requiring division to have a nonZeroNumeric denominator is to catch at compile time the "error" of failing to scrutinize (correctly or incorrectly) for zero.
IMHO it would be more painful than useful.
For most commercial software, the quality of run-time error messages is far less important than their absence.
It would not avoid them if the interface does not give a place to report the error: average xs = sum xs / case checkZero (length xs) of Just notZero -> notZero Nothing -> error "This should never happen" is not any more safe than average xs = sum xs / length xs and I can report bad input without trouble now: average xs = case length xs of 0 -> Nothing l -> Just (sum xs / l) -- __("< Marcin Kowalczyk * qrczak@knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK