Bug 1255 – operator overloading

Hi, I try to figure out, whether a port of boost::spirit to D would be possible. This library is heavily using C++ operator overloading. Now there are some problems I figured out: C++ allows the * operator to be overloaded in case of dereferencing. Spirit uses it as the "kleene star": *(expr) means repeat expr 0 or more times. D has opMul(_r), which overloads: "a*b" and "null*b", but not: "*b" since the latter means dereferencing. In the docs it's said "!", "&&" and "||" will likely never be overloadable. C++ allows all of them to be overloaded and spirit makes use of it. "!(expr)" means expr is optional (so 0 or 1 match). "||" and "&&" are used for sequential matching. So my feature req for D: without further arguments: *(expr) -> opDeref() !(expr) -> opNot() &(expr) -> opAddr() WITH one more argument and _r: expr1 && expr2 -> opAndAnd expr1 || expr2 -> opOrOr expr1 , expr2 -> opComma The following is not possible in C++: value = expr1 expr2; Because nothing is not overloadable as an operator. What about opNothing in D ;) ? Thanks in advance, Daniel

As Jarrett pointed out in the NG follow-up, porting Spirit line-by-line is undoutedly the wrong way to go about it. With spirit you can have the syntax: boost::spirit::rule<ScannerT> factor, term; term = factor >> *( ( '*' >> factor) | ( '/' >> factor ) ); Which admittedly looks pretty similar to BNF. But with the compile time text processing stuff that's been going into D you can just use the BNF: mixin(Rule!( "term ::= factor ( ( '*' factor ) | ( '/' factor ) )*" )); And maybe eventually the 'mixin' part won't be necessary either, but for now it is. Operator overloading abuse is cute, but it's just working around the lack of ability in C++ to do more generic compile-time processing. The biggest issue with operator overloading for this kind of thing is that you're stuck with the built-in precedence rules and operators (*prefix* kleen star?) One approach might be to build a standard API-based syntax for a parser-generator first (like C# ports of boost - http://www.codeproject.com/csharp/spart.asp). Then use the compile time string facilities in D to translate BNF strings to that API. Just a thought. But in any event, I can tell you for sure that "I want to use unary * as a Kleen star operator so D needs to have opDeref()" is not an argument that will gain any traction with Walter. He's a firm believer in operator overloads not changing the semantics of the underlying operator.

It would be really nice without the need of writing "mixin" and with more compile-time function capabilities. Having an API and then translate the EBNF string onto it sounds like a good idea. This way the EBNF can be kept 100%, so there would be no reason for having ">>" like it's necessary in spirit. Thanks for clarifying this.