Henning Thielemann wrote:
On Sat, 22 Nov 2008, Janis Voigtlaender wrote:
Definitely. And that surfaces even in quite innocently looking programs
and statements about them. The introductory example of the following
technical report may be amusing in that respect:
http://wwwtcs.inf.tu-dresden.de/~voigt/TUD-FI08-08.pdf
In example 1, I don't see on the one hand, why 'takeWhile (null.tail)'
could fail with "tail: empty list",
Because the (GHC) semantics described in the following paper:
http://doi.acm.org/10.1145/301631.301637
says so.
You can also check it by calculating with the definitions from Section 3
and Figure 8 of the above technical report. Or see it demonstrated on
slide pages 32-38 of:
http://wwwtcs.inf.tu-dresden.de/~voigt/nwpt2008-slides.pdf
since all lists in '[[i] | i <-
[1..(div 1 0)]]' are non-empty (namely singletons).
This does not really have anything to do with the above, but I may just
as well say that all lists in '[[i] | i <- [1..(div 1 0)]]' are empty.
Or that they are all of length 17. Because there are no lists in '[[i] |
i <- [1..(div 1 0)]]'. (Note that the "supply" [1..(div 1 0)] is itself
erroneous.) The "tail: empty list" failure mentioned above really has
nothing at all to do with the concrete expression '[[i] | i <- [1..(div
1 0)]]'. Any other erroneous expression, such as just 'error "div-by-0"'
would lead to the same result.
On the other hand,
aren't those imprecise error problems not just proofs that mixing up
errors and exceptions (here treating errors as exceptions) is a bad
thing? 'error' is only a candy version of 'undefined' for simplifying
debugging. If all 'error's are replaced by 'undefined' (plain bottom)
then 'takeWhile p (map h l)' and 'map h (takeWhile (p.h) l)' behave also
visually identical, don't they?
Yes, if all 'error's are replaced by 'undefined', then the two
expressions are semantically equivalent. But I don't buy this as a
decisive argument in the "errors vs. exceptions" debate.
Ciao, Janis.
--
Dr. Janis Voigtlaender
http://wwwtcs.inf.tu-dresden.de/~voigt/
mailto:[EMAIL PROTECTED]
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