foreach thoughts

[Jakob Ovrum]

On Tuesday, 14 January 2014 at 08:23:05 UTC, Manu wrote:
> 1. A termination condition (ie, while)
>
> foreach(t; things) iterates each thing, but it's common in 
> traditional for
> loops to have an && in the second 'while' term, to add an 
> additional
> termination condition.
> for(i=0; i<things.length && things[i].someCondition; ++i)
>
> Or with foreach:
> foreach(i, t; things)
> {
>   if(t.someCondition)
>     break;
>   ...
> }

foreach(t; things.until!(t => t.someCondition))
{
}

Unfortunately foreach over a range does not automatically support 
an index loop variable. We could add something like 
std.range.enumerate to support this, but I think it's a common 
enough requirement that a language amendment is warranted (there 
are some subtleties involved in implementing it though - 
specifically when combined with automatic tuple expansion).

> 2. A filter
>
> The other thing is the ability to skip uninteresting elements. 
> This is
> typically performed with the first line of the loop testing a 
> condition,
> and then continue:
> foreach(i, t; things)
> {
>   if(!t.isInteresting)
>     continue;
>   ...
> }

foreach(t; things.filter!(t => t.isInteresting))
{
}

Ditto about the index loop variable.

> I've tried to approach the problem with std.algorithm, but I 
> find the
> std.algorithm statement to be much more noisy and usually 
> longer when the
> loops are sufficiently simple (as they usually are in my case, 
> which is why
> the trivial conditions are so distracting by contrast).

The two examples above look a *lot* cleaner and less noisy 
(declarative!) to me than the imperative approach using if-break 
or if-continue.

> I also find that the exclamation mark overload in typical 
> std.algorithm
> statements tends to visually obscure the trivial condition I'm 
> wanting to
> insert in the first place.

You'll have to get used to the exclamation mark, otherwise you'll 
never be able to fully appreciate D's generic programming. I 
quite like it - I don't think there's anything objectively ugly 
about it.

> Also, I have to import std.algorithm, which then imports the 
> universe... >_<

This is fixable. We shouldn't reach for language changes to 
compensate for library deficiencies.