Purity

[Bruno Medeiros]

On 28/01/2011 20:25, Simen kjaeraas wrote:
> Bruno Medeiros <brunodomedeiros+spam at com.gmail> wrote:
>
>> On 27/01/2011 21:05, Simen kjaeraas wrote:
>>> Bruno Medeiros <brunodomedeiros+spam at com.gmail> wrote:
>>>
>>>> string[] func(string arg) pure {
>>>> string elem2 = "blah".idup;
>>>> return [ arg, elem2 ];
>>>> }
>>>>
>>>> The compiler *cannot* know (well, looking at the signature only of
>>>> course) how to properly deepdup the result from the first return
>>>> value, so as to give the exact same result as if func was called again.
>>>
>>> Could you please elucidate, as I am unsure of your reasoning for saying
>>> the compiler cannot know how to deepdup the result.
>>>
>>
>>
>> string str = "blah";
>> string[] var1 = func(str);
>> string[] var2 = func(str);
>>
>> How can the compiler optimize the second call to func, the one that is
>> assigned to var2, such that he deepdups var1 instead of calling func
>> again? Which code would be generated?
>>
>> The compiler can't do that because of all the transitive data of var1,
>> the compiler doesn't know which of it was newly allocated by func, and
>> which of it was reused from func's parameters or some other global
>> inputs.
>
> But for immutable data (like the contents of the elements of a string[]),
> that doesn't matter, does it?
>

Maybe it won't matter for the *contents of the elements* of the string 
array, but the whole result value has to be /the same/ as if the 
optimization was not applied. Otherwise the optimization is invalid, 
even if for most uses of the result value it would not make a difference 
for the program.


-- 
Bruno Medeiros - Software Engineer