[Issue 7456] Purity strength not documented

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https://d.puremagic.com/issues/show_bug.cgi?id=7456


Peter Alexander <peter.alexander.au at gmail.com> changed:

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--- Comment #2 from Peter Alexander <peter.alexander.au at gmail.com> 2014-01-11 16:16:52 PST ---
(In reply to comment #1)
> There is some debate as to whether they even should be. pure is defined quite
> correctly in the spec. weak vs strong purity is arguably just an implementation
> detail used for optimizations. I believe that this is Don's take on it (he's
> both the one who came up with strong vs weak purity and the one who updated the
> spec when pure was changed).
> 
> Now, weak vs strong purity inevitably comes up when people get confused by why
> the compiler allows what are weakly pure functions to be pure (particularly
> when compared with what TDPL says - though I think that many just look at it
> from a functional perspective and are confused even without having read TDPL).
> So, there is arguably merit in defining them somewhere. But I believe that
> Don's against it. He's mentioned before that he wants the terms to go away
> entirely.

Implicit conversion to immutable is only allowed with strongly pure functions,
so purity strength has semantic differences, not just implementation
differences. These must be documented.

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