Some pre-conditions at compile-time (reprise)

[bearophile]

Some time ago I have suggested the idea of running the 
pre-conditions at compile-time where possible:
http://d.puremagic.com/issues/show_bug.cgi?id=5906

This helps turn some run-time errors in compile-time errors, when 
the inputs are statically known. This is expecially handy for 
literals of user-defined subtypes.

But Walter answered a problem:

> The user must make an explicit choice if code is to be run at 
> compile
> time or run time.


They have just accepted the Ada 2012 standard, that among other 
things (like the "old" (prestate) for contract programming) 
introduces a "Static_Predicate" aspect (this page is not very 
readable):

http://www.ada-auth.org/standards/12rm/html/RM-3-2-4.html

Static_Predicate allows to attach a compile-time predicate that 
verifies the validity of a subtype (there is also 
"Dynamic_Predicate").


So an idea to remove the problem found by Walter is to introduce 
"static preconditions" in D (there is a "static" before the "in", 
currently this code is a syntax error):



import std.ascii: isDigit;

struct Digit {
     char d;
     alias this = d;

     this(in dchar c)
     static in {
         assert(isDigit(c));
     } body {
         // Currently this cast is necessary,
         // despite the pre-condition.
         d = cast(char)c;
     }
}

void main() {
     // Currently not allowed handy syntax:
     // Digit[] d = ['5', '6'];

     // Becomes a compile-time error:
     Digit[] d = [Digit('5'), Digit('x')];
}



Adding "static" to a precondition means the compiler will try to 
run the precondition at compile-time where possible, and at 
run-time where it's not possible. (So a "static in" does not mean 
the pre-condition will always run at compile-time).

If you don't add a "static" before "in" the precondition will be 
run only at run time, as usual. This avoids the unwanted growth 
of compilation times that Walter wants to avoid.

Bye,
bearophile