Andrei Alexandrescu pisze:
downs wrote:
aarti_pl wrote:
Andrei Alexandrescu pisze:
aarti_pl wrote:
Don pisze:
There's been some interesting discussion about operator overloading
over the past six months, but to take the next step, I think we need
to ground it in reality. What are the use cases?
I think that D's existing opCmp() takes care of the plethora of
trivial cases where <, >= etc are overloaded. It's the cases where
the arithmetic and logical operations are overloaded that are
particularly interesting to me.
The following mathematical cases immediately spring to mind:
* complex numbers
* quaternions (interesting since * is anti-commutative, a*b = -b*a)
* vectors
* matrices
* tensors
* bigint operations (including bigint, bigfloat,...)
I think that all of those are easily defensible.
But I know of very few reasonable non-mathematical uses.
In C++, I've seen them used for iostreams, regexps, and some stuff
that is quite frankly bizarre.
So, please post any use cases which you consider convincing.
DSL support in mother language. As an example I can give SQL in D or
mockup tests description language (usually also in D - not as a
separate script language).
In my previous posts I already put few arguments why it is sometimes
much more handy to use "DSL in mother language" approach rather than
string mixins with DSL language itself.
I saw that, but few, if any, of the arguments you made apply to
operators vs. named methods. You argued on string mixins vs. using
named symbols. In fact one argument of yours works against you as
there's no completion for operators.
Andrei
I put my argument much earlier in this discussion:
eg. here:
http://www.digitalmars.com/webnews/newsgroups.php?art_group=digitalmars.D&article_id=81040
To sum up: using methods to emulate operators is just completely
unreadable and looks awful. It's very easy to make an error using such a
technique.
Later, I just replayed to posts convincing that solution for supporting
DSL languages in D is using string mixins. I believe I could provide a
few weighty arguments to support my opinion that it is not always best
solution for this problem. In many cases it is much better to integrate
DSL into D as a kind of API, not separate sub-language. Then we can get
some support from IDE, while using string mixins we probably would get
nothing...
Best Regards
Marcin Kuszczak
(aarti_pl)
Scrapple.Tools uses operator overloading to provide fake infix
keywords along the lines of [2, 3, 4] /map/ (int i) { return
format(i); }, with a simple and convenient syntax for defining them
(mixin(Operator!("map", "something something use lhs and rhs; ")); ).
Forcing the end user to write mixin(function()) for such keywords is
*NOT* the way to go, for several reasons: a) it's hard to extend, and
b) it's needlessly verbose.
The reason infix keywords are useful, is because they can be used as a
simple way to chain bijective operations together, i.e. a /foo/ b
/map/ c /select/ d. Without infix keywords, this would take the form
of select(map(foo(a, b), c), d), which is an atrocity because it has
to be read in two directions - middle-leftwards for the operations,
and middle-rightwards for the parameters.
(and yes, I know it's wrong to rely on operator evaluation order. So
sue me. )
Of course, a more convenient solution would be the ability to extend
the D syntax manually, but that's unlikely to appear in our lifetime.
I sometimes think of a subtoken-based approach, e.g. any function name
starting and ending with an underscore is by definition infix. It's the
kind of solution that turns Walter's nose so I never brought it up to him.
Andrei
Maybe:
R op[Infix|Postfix]_Name(A a, B b ...);
with some additional forms for special characters. These special forms
could be additionally restricted as necessary, so there would be no
Postfix '&&', but only Infix etc. etc.
Possible examples:
------------------
SqlExpression opInfix_LIKE(string a, string b);
int opInfix_&(int a, int b);
bool opInfix_&&(bool a, bool b);
SqlExpression opInfix_AND(string a, string b);
Below examples for postfix operators; I have not yet idea of syntax for
calling side:
int opPostfix_+(int a, int b); //for e.g. Reversed Polish Notation
int opPostfix_*(int a, int b);
It seems that prefix will not be necessary, as prefix notation is just a
standard function call.
BR
Marcin Kuszczak
(aarti_pl)
