Re: how implement takeWhile

[Philippe Sigaud]

On Mon, Aug 6, 2012 at 5:32 PM, Philippe Sigaud
<philippe.sig...@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)());
        
}