On Mon, Nov 2, 2009 at 2:03 PM, Solomon Foster <colo...@gmail.com> wrote:
> On Mon, Nov 2, 2009 at 10:21 AM, Carl Mäsak <cma...@gmail.com> wrote:
>> Solomon (>), Moritz (>>):
>>>> the current spec doesn't allow immutable containers to call .map with a
>>>> block that implicitly uses $_ as an implicit parameter.
>>>>
>>>> Here's why:
>>>>
>>>> S06 says
>>>>
>>>>> The C<$_> variable functions as a placeholder in a block without any
>>>>> other placeholders or signature. Any bare block without placeholders
>>>>> really has a parameter like this:
>>>>>
>>>>> -> $_ is rw = OUTER::<$_> { .mumble }
>>>>
>>>> So $_ is marked as rw, which is checked at binding time.
>>>>
>>>> Now lists are immutable, meaning that you can't bind a list item rw, so
>>>> even if the block doesn't try to modify $_, calling the { .say } block
>>>> fails.
>>>>
>>>> (Note that 'for' has the same problem)
>>>>
>>>> How should that be resolved? Should a temporary variable be created that
>>>> can be changed, without any effect? or should it fail() on detecting a
>>>> modification? Somehow this approach seems very backward to me...
>>>
>>> Is there a reason $_ is readonly isn't a possible solution?
>>
>> For one thing, then you couldn't do this:
>>
>> my @a = 1, 2, 3;
>> for @a {
>> ++$_;
>> }
>
> I meant, make it readonly if the container being iterated over is
> readonly. We're working hard to make the language prefer immutable
> things, it seems very odd to short circuit that in a key area like
> this...
>
> (Though personally, I would prefer it if map was readonly by default
> on all containers.)
Or Larry could just find an elegant way to do it. Yay!
--
Solomon Foster: colo...@gmail.com
HarmonyWare, Inc: http://www.harmonyware.com
