Metaprogramming in D : Some Real-world Examples

[Denis Koroskin]

On Tue, 10 Nov 2009 03:27:20 +0300, Bill Baxter <wbaxter at gmail.com> wrote:

> On Mon, Nov 9, 2009 at 4:09 PM, Walter Bright
> <newshound1 at digitalmars.com> wrote:
>> Looks like Bill Baxter is giving a presentation on D Nov. 18!
>>
>> http://www.nwcpp.org/
>
> Yep, that's right, and I'd be quite grateful to you smart folks here
> if you could share your meta-programming favorites with me!   If
> you've got a real-world example of meta-programming in D that you
> think is particularly handy, then please send it my way
>
> I'm looking for small-but-useful things that are easy to explain, and
> make something easier than it would be otherwise.  Things like places
> where static if can save your butt,  or loop unrolling,  and passing
> code snippets to functions like in std.algorithm.
>
> Things like a compile-time raytracer or regexp parser (though quite
> cool!) are not what I'm after.  Too involved for a short talk.
>
> --bb

kprintf (a printf variant) in XoMB is a nice example imo. It accepts  
format as a template parameter, doing all the checks at compile-time (and  
more).

http://github.com/xomboverlord/xomb-bare-bones/blob/6d924a9fd7cafe43aa50f38c0cd04c44187d4993/kernel/core/kprintf.d

/* This template will generate code for printing and will do
  * all parsing of the format string at compile time
  *
  * USAGE:
  *   kprintf!("format string {specifier} ... ")(args...);
  *
  * EXAMPLES:
  *   kprintf!("Integer: {}")(10);
  *   kprintf!("{!cls}Cleared the screen.")();
  *   kprintf!("{!pos:2,3}At position (2,3)")();
  *   kprintf!("{!fg:LightBlue!bg:Gray}{}")(25);
  *   kprintf!("{!fg:Red}redness")();
  *   kprintf!("{x} Hex!")(145);
  *   kprintf!("Curly Brace: {{")();
  *
  * COMMANDS:
  *   !cls - Clears the screen.
  *   !fg  - Sets the foreground color, see the Color enum
  *            in kernel/dev/console.d.
  *   !bg  - Sets the background color, same as above.
  *   !pos - Moves the cursor to the x and y given, see example above.
  *
  * SPECIFIERS:
  *   {x}  - Prints the hex value.
  *   {u}  - Treats as unsigned.
  *   {}   - Prints common form.
  *
  * WHY IS IT COOL?
  *   - Compile time parsing of format strings
  *   - Type checking at compile time as well
  *   - That means it can tell you that you are dumb before you execute.
  *   - No need to specify type information.
  *
  *   - So we can do this and not care about the
  *      output of the function:
  *
  *   auto blah = someFunction();
  *   kprintf!("Some Arbitrary Info: {}")(blah);
  *
  *      WOWWY WOW WOW!
  *
  */