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.)
--
Solomon Foster: colo...@gmail.com
HarmonyWare, Inc: http://www.harmonyware.com
