Templates problem

[Russel Winder via Digitalmars-d-learn]

On Wed, 2016-09-07 at 20:32 +1200, rikki cattermole via Digitalmars-d-
learn wrote:
> 
[…]
> Ok, I have it mostly compiling.
> 
> void run_mean() {}
> void run_mode() {}
> void run_stdDev() {}

For my code the functions have to be declared within the main function
rather than being at module level. But this turns out to be a non-
issue.

> void main() {
> 	import std.datetime;
> 	import std.algorithm : map;
> 	import std.conv;
> 	import std.array;

I am still not a fan of this import all symbols approach, like the map
import, I would do the same for the other modules' functions used.

> 	const results = benchmark!(run_mean, run_mode, run_stdDev)(1);
> 	const times = map!((TickDuration t) { return 
> (to!Duration(t)).total!"seconds"; })(results[]);

It turns out my problem was here. See below.

> 	import std.stdio;
> 	foreach(v; times) {
> 		writeln(v);	
> 	}
> }
> 
> However times really can't be const. Since times is an input range
> which 
> gets modified as part of the iterations.

Indeed. I keep forgetting that ranges are use once mutable entities,
and there is no option. s/const/auto/ solves the problem, but the more
likely general solution is to retain the const and reify the array. But
it depends on usage, obviously.

> So what exactly do you want times to be? If you want an array .array 
> from std.algorithm it which can be const. Either way there is
> something 
> I'm missing as to why const is so important here.

In this case it isn't as you can't use a constant range. The problem I
am having is when I cannot use const or better immutable. I'm a single
assignment sort of person, but D ranges break all that orthodoxy.

-- 
Russel.
=============================================================================
Dr Russel Winder      t: +44 20 7585 2200   voip: sip:russel.winder at ekiga.net
41 Buckmaster Road    m: +44 7770 465 077   xmpp: russel at winder.org.uk
London SW11 1EN, UK   w: www.russel.org.uk  skype: russel_winder
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