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November 06, 2010
Re: Should pure functions be prevented from reading changeable immutable
Don:

> Pure functions are allowed to read immutable global variables.
> Currently, this even includes globals which are initialized from inside 
> 'static this()'.
> Here's an example of how this can be a problem:
> 
> immutable int unstable;
> 
> pure int buggy() { return unstable; }
> 
> static this() {
>      // fails even though buggy is pure
>      assert( buggy() == ( ++unstable , buggy() ) );
> }
> 
> I suspect that such functions should be forbidden from being 'pure'.
> Note that they cannot be used in CTFE (conceptually, all other @safe 
> pure functions could be used in CTFE, even though the current 
> implementation doesn't always allow it).

I don't understand what exactly you propose to ban.
Do you want to ban any function to be pure if such function reads any global immutable/const variable? Or do you want to just ban pure functions that read global immutables if an only if such global immutables are initialized inside static this()? :-) I suspect this is a silly question.


> The motivation for wanting to ban them is to prevent the optimiser from 
> generating bad code.

Sounds like a worth goal.

Bye,
bearophile
November 06, 2010
Re: Should pure functions be prevented from reading changeable immutable
bearophile wrote:
> Don:
> 
>> Pure functions are allowed to read immutable global variables.
>> Currently, this even includes globals which are initialized from inside 
>> 'static this()'.
>> Here's an example of how this can be a problem:
>>
>> immutable int unstable;
>>
>> pure int buggy() { return unstable; }
>>
>> static this() {
>>      // fails even though buggy is pure
>>      assert( buggy() == ( ++unstable , buggy() ) );
>> }
>>
>> I suspect that such functions should be forbidden from being 'pure'.
>> Note that they cannot be used in CTFE (conceptually, all other @safe 
>> pure functions could be used in CTFE, even though the current 
>> implementation doesn't always allow it).
> 
> I don't understand what exactly you propose to ban.
> Do you want to ban any function to be pure if such function reads any global immutable/const variable? Or do you want to just ban pure functions that read global immutables if an only if such global immutables are initialized inside static this()?

The second. Consider:
immutable int x;
immutable int y = 2;

static this() { x = 3; }

y should be usable from inside pure, x should not.
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