Function that calculates in compile time when it can

[Artur Skawina]

On 08/07/12 19:43, Marco Leise wrote:
> Am Mon, 06 Aug 2012 14:21:37 +0200
> schrieb "Minas Mina" <minas_mina1990 at hotmail.co.uk>:
> 
>> I want to write a fibonacci(n) function that calculates the 
>> result.
>> a) if n is known at compile time, use a template
>> b) if not, use a normal function
>>
>> I know how to write a template version:
>> template fib(ulong n)
>> {
>> 	static if( n < 2 )
>> 		const fib = n;
>> 	else
>> 		const fib = fib!(n-1) + fib!(n-2);
>> }
>>
>> But how can I 'know' if n is known at compile time to make it use 
>> the other version? (which I won't post 'cause it is fairly easy).
> 
> That's easy: http://dpaste.dzfl.pl/521f47a0

It's not a function, but a template - meaning not only does it require
a different syntax, but also that it can only be used with literals and
symbols. Ie 'fib!(expression_not_evaluable_at_ct)' like 'fib!(variable+1)'
will fail to compile.

artur

PS. Your code does not work with older compilers (like the 2.057 based
gdc that i'm using); this version works and is also more readable:

   template isSymbol(alias S) {
      enum isSymbol = __traits(compiles, __traits(parent, S));
   }
   
   template fib(alias N) {
      static if (isSymbol!N) {
         @property typeof(N+N) fib(typeof(N) N = N) {
            return N<2 ? N : fib(N-1)+fib(N-2);
         }
      }
      else {
         static if (N<2)
            alias N fib;
         else
            enum fib = fib!(N-1)+fib!(N-2);
      }
   }