On 13/03/2015 2:23 p.m., Dennis Ritchie wrote:
Is it possible to run this code in compile-time?
import std.stdio, std.functional;
ulong fact(ulong n)
{
alias mfact = memoize!fact;
return n < 2 ? 1 : n * mfact(n - 1);
}
void main() {
writeln(fact(10));
}
In CommonLisp variable *factorial-cache* available in CT, and RT.
Moreover, in the compiler environment of your *factorial-cache* and RT.
Thus we memoires as calculations in CT (constants), and RT.
(eval-always
(defparameter *factorial-cache* (make-hash-table))
(defun factorial (n)
(if (< n 1)
1
(setf (gethash n *factorial-cache*) (* n (factorial (1- n)))))))
(define-compiler-macro factorial (&whole whole n)
(if (constantp n) (factorial n) whole))
Here is an example I have in my Developing with compile time in mind
book[0]:
size_t factorial(size_t n) pure {
assert(n < 11);
if (n == 0) {
return 1;
} else {
return n * factorial(n - 1);
}
}
static assert(factorial(5) == 120);
Notice how it is in a static assert? Yeah that is at compile time.
You could assign it to e.g. an enum. Or force it over using
meta-programming.
If you haven't already, I would strongly recommend that you read my book
on the subject.
[0] https://leanpub.com/ctfe
