[Issue 8185] Pure functions and pointers

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http://d.puremagic.com/issues/show_bug.cgi?id=8185



--- Comment #36 from Jonathan M Davis <jmdavisProg at gmx.com> 2012-06-04 08:45:00 PDT ---
> int f(size_t) pure;

> __gshared int tmp;
> void g(size_t, ref int dummy = tmp) pure;

> void h(size_t a, size_t b) pure
> {
>    int res = f(a);
>    g(b);
>    assert(res == f(a)); // may fail, no guaranties by language!
>}

Your g(b) causes h to be impure, because it accesses tmp, which is __gshared.
Also, as far as eliding additional calls to pure functions, at present, they
only occur within the same line, and I think that may only ever occur within
the same expression (it's either expression or statement, I'm not sure which).
So, the eliding of additional pure function calls is going to be quite rare.
The _primary_ benefit of pure is how it enables you to reason about your code.
You _know_ that f doesn't mess with anything other than the argument that you
passed to it without having to look at its body at all.

Oh, and the assertion _is_ guaranteed to pass. a and res are both value types.
Neither res nor a are passed to anything or accessed in any way other than in
the the lines with the calls to f, and even if g were impure, and it screwed
with whatever argument was passed as the first argument to the h call, it
wouldn't be able to mess with the value of a, because it was already copied.

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