User Defined Attributes

[bearophile]

Walter Bright:

> consider the type "int". Different modules impute different 
> meanings into it all the time, and it doesn't cause terrible 
> compatibility problems between modules.

The usage of naked basic types as int and double cause troubles. 
Haskell programmers avoid some of them defining strong types like 
this (this syntax also allows you to specify exactly what 
operations you want to be allowed on this new type, with that 
"deriving"):

newtype Angle = A Double
     deriving (Eq, Ord, Num, Fractional)

Similarly you can also tell apart angles represented in degrees 
from angles in radians, and avoid some bugs. (It's possible to 
overdo this idea, and make the code too much fussy and not handy 
to write, so you have to keep some balance and not use too many 
new types).

A possible D syntax, using std.typecons.Typedef:

alias Angle = SubTypedef!(double, Eq, Ord, Num);

That equals to:
alias Angle = Typedef!(double, Subtype, Eq, Ord, Num);

Where Eq, etc, are pre-defined template mixins that implement 
different operations, like equality, opCmp, etc, that Typedef 
mixes in.

Bye,
bearophile