On 07/11/2012 01:10 AM, Walter Bright wrote:
On 7/10/2012 3:49 PM, Timon Gehr wrote:
I understand, but the downside of not making these functions const is it
will torpedo the use of functional style programming in D.
Making these functions const will torpedo the use of OO style
programming in D.
No, it won't, there are simple workarounds.
In @safe code there are none. There are also simple workarounds for the
functional style programming case: just do not annotate.
OO is all about data encapsulation, aliasing and
mutation.
Also consider that:
- Functional style programs use structs and delegates, not class
objects. Classes are an OO abstraction.
- The most well-known functional programming language, Haskell, mutates
existing state a great deal at runtime, since almost _everything_ is
lazily initialized. I have written some Haskell-style D code, and
could not use the 'const', 'immutable' or 'pure' qualifiers.
- what actually harms functional style programming in D the most is the
lack of tail calls. (I have some other gripes, but this is the most
important one.)
- D is multi-paradigm. A program can be functional style in many ways,
and still mutate state. (Functional programming languages have to
invent idioms and syntactic sugar to provide an abstraction for
mutation. The compiler has to detect those and optimize them into
mutation.)
I consider those points important.
A straightforward workaround is to use PIMPL to encapsulate the logical
const stuff, and then cast the reference to const to use inside the
opEquals.
I didn't get that.
PIMPL means pointer to implementation. Your const struct is just a
wrapper around a pointer, and that pointer can be cast to mutable before
calling member functions on it.
Or:
bool opEquals(const Object p) const
{
Object q = cast() p;
Object r = cast() this;
return r.non_const_equals(q);
}
Of course, you'd have to make this @trusted.
It is not safe to do that. Marking it @trusted would be a lie. We
cannot build a type system on the premise that any OO code will break
it and at the same time claim that it provides actual guarantees.
