Idea: rather than trying to change state, one could attempt a different
approach:
When "food" is "eaten", this action could define a new compile time
object "eater(food)" as some value. When eating something twice, this
would attempt to define the same object as two different values, causing
a compiler error.
Of course, this still means that the process of eating must happen at
compile time. Obviously the following can never be caught at compile time:
if(runtime_userinput) {
rabbit1.eat(carrot);
}
if(another_runtime_userinput) {
rabbit2.eat(carrot);
}
Which confirms my original position: the final three tests in the
contest are flawed by concept. If the actions of eating and slaughtering
are supposed to happen at run-time, no compiler in the world can
reliably detect them happening twice. Of course, if the whole action
happens at compile-time, it is a simple matter of finding a nice syntax
to express a regular program as compile-time code (e.g. within a CTFE
routine)
On 09/27/2010 02:26 PM, bearophile wrote:
This is a harder variant of an old and very simple OOP problem:
http://merd.sourceforge.net/pixel/language-study/various/is-a-cow-an-animal/
The problem presents a hierarchy of types, some rules, and 11 tests, each one
of them represents a bug because it contains a violation of the rules. You have
to write the program in a way that the type system refuses as many of those 11
tests as possible at compile-time. There are many acceptable ways to write the
program, for example you may use functional programming, so you are able to
return different types, that enforce different rules, so it's not a contest,
it's more like a demonstration.
A basic Python implementation catches none of the 11 tests at compile time :-)
A basic D implementation that I have written catches five of the 11 bugs at
compile time:
http://ideone.com/87q67
I will try to write a more statically typed D version.
The site shows four C++ versions, the fourth is able to catch 8 of the 11 bugs
at compile-time. A type system that supports typestates is probably able to
catch all 11 bugs (because for example the Cow type has two states, alive and
eaten, and you can't perform some operations on a eaten cow, like eating it
again), but probably it's very hard to do this with D.
The fourth C++ version contains a line that I am am not even sure how to
translate to D (maybe there is a workaround with template constraints):
(void) static_cast<My_kind *>((Kind *) 0);
Bye,
bearophile
