lazy thoughts

[Andrei Alexandrescu]

Sergey Gromov wrote:
> Mon, 12 Jan 2009 17:38:40 +0000 (UTC), dsimcha wrote:
> 
>> == Quote from Sergey Gromov (snake.scaly at gmail.com)'s article
>>> Mon, 12 Jan 2009 09:05:18 -0800, Andrei Alexandrescu wrote:
>>>> Also, lazy evaluation has the risk of getting confusing as there's a lot
>>>> of escaping. Consider:
>>>>
>>>> int[] arr = [ 1, 2, 3, 4 ];
>>>> auto squares = map!("a * a")(arr);
>>>> arr[] = [ 5, 6, 7, 8 ];
>>>>
>>>> Now iterating squares will see different numbers than the original ones.
>>> This part bothers me.  Though it's hard to tell how much danger it would
>>> impose in practice.
>> I don't understand how this is any worse/more confusing than any other case of
>> having multiple aliases to the same memory region.
> 
> First, it's an obscure alias.  Second, it wasn't so in the previous
> implementation of std.algorithm.  OTOH these are mitigated by the fact
> you're going to re-visit the map!() usage in your code anyway.

Correct. Besides, laziness enables programming patterns and styles that 
are very useful. For example, after the overhaul of std.algorithm, a 
plethora of generators will come forth.

> Also I've got an impression that Phobos tried to avoid dangerous
> aliasing as much as possible by using immutable argiments in aliasing
> algorithms.  map() obviously refuses to follow this rule.

Well it doesn't "refuse" as that suggests confrontation. It simply 
acknowledges that oftentimes map's functionality is useful over mutable 
inputs.

> On an unrelated note, I probably should make a post about Phobos
> overusing immutable arguments.  This is a direct consequence of
> declaring string as an alias of immutable(char)[] which is IMHO plain
> wrong.

This shapes up as a basic disagreement, but to a large extent we can 
work things out. For example, a generic string function such as 
startsWith(someString, prefix) should not require invariant inputs when 
it could work just as well for mutable arrays. Then everybody is happy.


Andrei