[Issue 9148] 'pure' is broken

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=9148

--- Comment #18 from timon.g...@gmx.ch ---
(In reply to Kenji Hara from comment #17)
> (In reply to timon.gehr from comment #16)
> > I think this is not right. Why should impure nested functions not be able to
> > access the enclosing context corresponding to frames of pure functions?
> 
> Because the nested impure function may modify the context of other pure
> functions.
> 
> int g;
> int impureFuncCall() { return g; }
> 
> auto func(out int delegate() pure pureDg, out void delegate() impureDg) pure
> {
>     int x = 1;      // become a closure variable
> 
>     int foo() pure  // weak purity
>     {
>         return x;
>     }
> 
>     auto bar()()
>     {
>         // modify the context of pure funciton 'foo'
>         // depending on the global state,
>         if (impureFuncCall())
>             x = 2;      // !!
>     }
> 
>     pureDg = &foo;
>     impureDg = &bar!();
> }
> 
> void main()
> {
>     int delegate() pure pureDg;
>     void delegate() impureDg;
> 
>     func(pureDg, impureDg);
> 
>     assert(pureDg() == 1);
> 
>     g = 1;  // modify the global state
>     impureDg(); // modify the context of pureDg.
> 
>     assert(pureDg() == 1);  // fails~
> }

I understand. My question was why you consider this to be a _problem_.
pureDg is weakly pure. I can change the context of a weakly pure delegate
depending on global state in other ways if I want to.

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