[contest] Is a Cow an animal ++

[Norbert Nemec]

IMHO, the test misses the point of compile-time metaprogramming: The 
concept of "state" belongs to run-time. The D compile-time language is 
purely functional and does not know a state or even an "order of execution".

The conditions "cannot die or be eaten twice" are, at their core, issues 
of state. Any "solutions" that claim to catch one of the last three 
errors must either go beyond the purely functional nature of the 
compile-time language or rely on additional constraints (e.g. no reuse 
of previous elements)

Of course, one could devise a language with non-functional compile time 
features, but within D, this would fundamentally break the existing 
concept of meta-programming.




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