Re: rfc: array literals

On Fri, 12 Dec 2008, Yann LeCun wrote:

> The proper way to fix this is to make array literal create a new
> matrix every time, and return it.

Alternatively, we could make array literals immutable. The
the code below would generate an error; the user would have
to explicitly create a copy that he can then modify.

Not sure way is be better.

Ralf


>
> On Friday 12 December 2008, Ralf Juengling wrote:
>> Hello,
>>
>> Consider this code, which exhibits a malicious bug.
>>
>> (defun test0 ()
>>    (for (i 0 3)
>>      (let ((v [0 0 0 0 0]))
>>        (v 0 (+ (v 0) 1))
>>        (printf "%f " (v 0)) ))
>>    (print) )
>>
>> ? (test0)
>> 1 2 3 4
>> = ()
>>
>> The desired output is "1 1 1 1". If I remember correctly,
>> somewhere in the documentation array literals are advertised
>> as a means to simulate something like "static variables" in
>> C. But here it does not square with the semantics of let.
>>
>> In my opinion, this is behavior will be confusing to many
>> users and we should change it in Lush2. When compiled, v
>> should simply be allocated on the stack when it is not returned
>> (and dynamically otherwise).
>>
>> Ralf
>>
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>
>
>

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