Partially pure (Re: Fully transitive const is not necessary)

[Walter Bright]

Janice Caron wrote:
> What about this one. Is f pure?
> 
>     import std.date;
> 
>     void main()
>     {
>         invariant s = toString(getUTCtime());
> 
>         pure string f()
>         {
>             return s;
>         }
> 
>         writefln(s);
>     }
> 
> The function f relies only on the string s. which is fully invariant
> and in scope. Yet f will give a different return value for the same
> input (void), each time the program is run.
> 
> I assume the answer is no, but this one seems a bit grey to me. Is it
> impure because s is not global?

f() is pure because it relies on invariant data. Invariant data can be 
initialized at runtime, as long as it is before any pure functions 
attempt to read it.