how implement takeWhile

[Philippe Sigaud]

On Mon, Aug 6, 2012 at 5:32 PM, Philippe Sigaud
<philippe.sigaud at gmail.com> wrote:

> What I'd like to know and may test myself is: is there any speed
> difference in this functional-oriented D code and a more standard
> (C-ish) way to obtain the same result?

Here it is. Answer: no noticeable difference. The functional way also
works at CT, that's great.
Of course, the functional code is (to my eyes) easier to read, easier
to debug and easier to modify.

import std.stdio;
import std.algorithm;
import std.range;

void main()
{
	enum max = int.max;
	
	// C-ish
	long a,b, temp, sum;
	a = 1;
	b = 1;
	while ( b < max)
	{
		if (b % 2 == 0) sum += b; // filter and sum
		temp = b;
		b = a + b;
		a = temp;
	}
	writeln(sum);
	
	// Haskell-ish
	writeln(recurrence!((a,n) => a[n-1]+a[n-2])(1L,1L)
	        .until!(a => a >= max)()
			.filter!(a => a%2 == 0)()
			.reduce!((a,b) => a+b)());
			
	// Works at CT too!
	pragma(msg, recurrence!((a,n) => a[n-1]+a[n-2])(1L,1L)
	        .until!(a => a >= max)()
			.filter!(a => a%2 == 0)()
			.reduce!((a,b) => a+b)());
	
}