D Programming Language 2.0

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Last update Tue Jun 17 20:13:38 2008

# Rationale

Questions about the reasons for various design decisions for D often come up. This addresses many of them.

## Operator Overloading

### Why not name them operator+(), operator*(), etc.?

This is the way C++ does it, and it is appealing to be able to refer to overloading '+' with 'operator+'. The trouble is things don't quite fit. For example, there are the comparison operators <, <=, >, and >=. In C++, all four must be overloaded to get complete coverage. In D, only a cmp() function must be defined, and the comparison operations are derived from that by semantic analysis.

Overloading operator/() also provides no symmetric way, as a member function, to overload the reverse operation. For example,

    class A
    {
        int operator/(int i);		// overloads (a/i)
        static operator/(int i, A a)	// overloads (i/a)
    }

The second overload does the reverse overload, but it cannot be virtual, and so has a confusing asymmetry with the first overload.

### Why not allow globally defined operator overload functions?

1. Operator overloading can only be done with an argument as an object, so they logically belong as member functions of that object. That does leave the case of what to do when the operands are objects of different types:

       class A { }
       class B { }
       int opAdd(class A, class B);

   Should opAdd() be in class A or B? The obvious stylistic solution would be to put it in the class of the first operand,

       class A
       {
           int opAdd(class B) { }
       }

2. Operator overloads usually need access to private members of a class, and making them global breaks the object oriented encapsulation of a class.
3. (2) can be addressed by operator overloads automatically gaining "friend" access, but such unusual behavior is at odds with D being simple.

### Why not allow user definable operators?

These can be very useful for attaching new infix operations to various unicode symbols. The trouble is that in D, the tokens are supposed to be completely independent of the semantic analysis. User definable operators would break that.

### Why not allow user definable operator precedence?

The trouble is this affects the syntax analysis, and the syntax analysis is supposed to be completely independent of the semantic analysis in D.

### Why not use operator names like __add__ and __div__ instead of opAdd, opDiv, etc.?

__ keywords should indicate a proprietary language extension, not a basic part of the language.

### Why not have binary operator overloads be static members, so both arguments are specified, and there no longer is any issue with the reverse operations?

This means that the operator overload cannot be virtual, and so likely would be implemented as a shell around another virtual function to do the real work. This will wind up looking like an ugly hack. Secondly, the opCmp() function is already an operator overload in Object, it needs to be virtual for several reasons, and making it asymmetric with the way other operator overloads are done is unnecessary confusion.

## Properties

### Why does D have properties like T.infinity in the core language to give the infinity of a floating point type, rather than doing it in a library like C++: `std::numeric_limits::infinity` ?

Let's rephrase that as "if there's a way to express it in the existing language, why build it in to the core language?" In regards to T.infinity:

1. Building it in to the core language means the core language knows what a floating point infinity is. Being layered in templates, typedefs, casts, const bit patterns, etc., it doesn't know what it is, and is unlikely to give sensible error messages if misused.
2. A side effect of (1) is it is unlikely to be able to use it effectively in constant folding and other optimizations.
3. Instantiating templates, loading `#include` files, etc., all costs compile time and memory.
4. The worst, though, is the lengths gone to just to get at infinity, implying "the language and compiler don't know anything about IEEE 754 floating point - so it cannot be relied on." And in fact many otherwise excellent C++ compilers do not handle NaN's correctly in floating point comparisons. (Digital Mars C++ does do it correctly.) C++98 doesn't say anything about NaN or Infinity handling in expressions or library functions. So it must be assumed it doesn't work.

To sum up, there's a lot more to supporting NaNs and infinities than having a template that returns a bit pattern. It has to be built in to the compiler's core logic, and it has to permeate all the library code that deals with floating point. And it has to be in the Standard.

To illustrate, if either op1 or op2 or both are NaN, then:

    (op1 < op2)

does not yield the same result as:

    !(op1 >= op2)

if the NaNs are done correctly.

## Why use `static if(0)` rather than `if (0)?`

Some limitations are:

1. if (0) introduces a new scope, static if(...) does not. Why does this matter? It matters if one wants to conditionally declare a new variable:

       static if (...) int x; else long x;
       x = 3;

   whereas:

       if (...) int x; else long x;
       x = 3;    // error, x is not defined

2. False static if conditionals don't have to semantically work. For example, it may depend on a conditionally compiled declaration somewhere else:

       static if (...) int x;
       int test()
       {
           static if (...) return x;
           else return 0;
       }

3. Static if's can appear where only declarations are allowed:

       class Foo
       {
       	static if (...)
       	    int x;
       }

4. Static if's can declare new type aliases:

       static if (0 || is(int T)) T x;

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