Idea: partially pure functions

[Bruno Medeiros]

Steven Schveighoffer wrote:
> "Bruno Medeiros" wrote
>> I state that if you have 'pure' implemented (in a sensible manner), it 
>> takes zero effort to implement partial/contextual pure.
>>
>> The definition of partial pure functions requires no extra effort from the 
>> compiler to check. (The *same* check for normal pure functions is made: 
>> that no external state is accessed unless invariant)
>>
>> The usage of partial pure functions also requires no extra effort. Inside 
>> a pure function body, the compiler already has to check if no external, 
>> non-invariant variables are accessed, like this:
>>
>>   Foo globalVar;
>>
>>   pure void func(...) {
>>     localVar = globalVar + 1; // not allowed
>>
>> then just apply this same check on *any* expression, including function 
>> call expressions:
>>
>>   partiallyPureFunction(localVar, globalVar); // not allowed
>>   partiallyPureFunction(localVar, localVar2); // but this is allowed
> 
> What if localVar is a pointer to global data, or transitively contains a 
> pointer to global data?  There is no way to know whether localVar is a 
> pointer to global data or 'contained' heap data without lots of context 
> checking.  

Whatever checks you have to make for localVar, you still have to do them 
with normal pure. (And not that it matters to the point, how do you 
initialize a local var with global data?)

> If partially pure functions with mutable parameters were allowed, 
> I'd say it's a requirement that you can only call them from partially or 
> fully pure functions, as local pointers from those functions are guaranteed 
> to be contained within the function.
> 

I'm not sure I understand what you're saying. Are you saying that 
partially pure functions should not be called from *non-pure* functions? 
What's the sense in that?


-- 
Bruno Medeiros - Software Developer, MSc. in CS/E graduate
http://www.prowiki.org/wiki4d/wiki.cgi?BrunoMedeiros#D