[DIP] In-place struct initialization

[cym13 via Digitalmars-d]

In accordance to the new DIP process you can find the full 
presentation of the change here: 
https://github.com/dlang/DIPs/pull/22

This DIP aims at providing better orthogonality and more 
importantly a way to have keyword arguments in the language at 
little cost and great benefit by extending static initialization 
to avoid having to declare a variable. It makes the following 
code legal:

    struct totalArgs {
        int tax;
        int discount;
    }

    int total(int subtotal, totalArgs args = totalArgs.init) {
        return subtotal + args.tax - args.discount;
    }

    unittest {
        assert(total(42) == 42);
        assert(total(42, totalArgs(tax: 50)) == 92);
        assert(total(42, totalArgs(discount: 20, tax: 50)) == 72);

        int defaultTotal(int subtotal) {
            immutable defaultSet = totalArgs(tax: 20);
            return total(subtotal, defaultSet);
        }
    }

Here is the rational as found in the DIP:

Static struct initialization has great properties:

- It is explicit using named attributes
- Order of declaration doesn't matter
- Not all attributes have to be specified

No function call provide those properties, and consequently no 
constructor
can benefit from it either. Authorizing such struct 
initialization makes the
language more orthogonal and opens new doors.

The most interesting is to use structs to mimic keyword arguments 
for
functions. By encapsulating possible arguments in a struct it is 
possible to
use in-place initialization to provide a clean interface very 
similar to
keyword arguments such as seen in python or ruby.

As it stands now the way to provide complex argument set to a 
function is
either to generate lots of constructors for the different cases 
which is
messy or by setting a struct up before passing it to the function 
in a C-way
fashion. This change provides ways to design better high-level 
interfaces.

Besides the change is completely retro-compatible in a nice way: 
the library
itself is just defining an argument struct and using it in its 
function
interface. Code using older compilers can setup the struct 
without in-place
initialization and modern compilers benefit from a cleaner 
interface.

This change also helps interfacing C code that uses structs.


I'm convinced such a change would be great for D but only if 
properly reviewed, please don't hesitate to comment it.