[erlang-questions] Must and May convention

[Roman Galeev]

> -pure([f/1, g/0, h/3]).

So, you declare f/1 is pure, but how do you really know it is pure? Meaning
it should be a way to prove f/1 is pure, at least with some tool like
dialyzer.

On Thu, Sep 28, 2017 at 10:27 AM, zxq9 <zxq9@REDACTED> wrote:

> On 2017年09月28日 木曜日 10:01:12 you wrote:
> > > I really wish Dialyzer accepted (and checked) explicit declarations of
> > purity.
> >
> > i could not agree more
> > that would be useful feature and amazing time saver!
> >
> > I am currently working on a toll that creates DB of function properties,
> > and motivation was exactly finding non pure functions in any given
> > project.
>
> I've messed around with this a bit and, not liking syntaxtic additions
> (for the most part), I've played around a little with this idea. The one
> I've come up with that allows building a checkable graph is what I'm doing
> already:
>
> -pure([f/1, g/0, h/3]).
>
> So that works just like an -export attribute and when the compiler rolls
> over it you actually get a nice list in module_info:
>
> 1> zuuid:module_info(attributes).
> [{vsn,[161185231735429547750212483364357911358]},
>  {author,"Craig Everett <zxq9@REDACTED>"},
>  {behavior,[application]},
>  {pure,[{v3,1},
>         {v3,2},
>         {v3_hash,2},
>         {v5,1},
>         {v5,2},
>         {v5_hash,2},
>         {read_uuid,1},
>         {read_uuid_string,1},
>         {read_mac,1},
>         {read_mac_string,1},
>         {string,1},
>         {string,2},
>         {binary,1},
>         {binary,2},
>         {strhexs_to_uuid,1},
>         {strhexs_to_mac,1},
>         {strhexs_to_integers,1},
>         {bins_to_strhexs,1},
>         {binary_to_strhex,1}]}]
>
> Quite easy to build a graph around this sort of data. And it comes only at
> the cost of actually including a -pure declaration.
>
> The problem, of course, is actually making a -pure declaration and keeping
> it in sync with the module code over time -- and that this is invisible to
> Dialyzer right now.
>
> That said, if no unsafe calls or actions are taken in a function Dialyzer
> could infer which functions are pure and help generate such a list. Even
> better, of course would be if it knew the difference and examined each
> function to build the graph of pureness automatically...
>
> But I digress.
>
> Save the (maybe not easy) task of making Dialyzer able to infer purity
> (and this is impossible anyway when Dialyzer hits a wall of ambiguity
> such as a call to M:F(A) or apply(M, F, A) -- which are pretty important!),
> it would even nicer if we had a pure function spec declaration form.
>
> -pure f() -> term().
>
> And just leave the original
>
> -spec f() -> {ok, Value :: term()} | {error, Reason :: term()}.
>
> form alone.
>
> That shouldn't break any old code, and leave a safe path to updating the
> stdlib and internals of existing projects... without anything significant
> changing until people are ready for it.
>
> And no new syntax.
>
> Where a new bit of syntax may be nice is if, for example, a way to declare
> *what* side effects a function has or might have. I haven't any good idea
> how to go about that because I've never thought of it or seen a system that
> declares categories of side-effects... but its an interesting idea that
> might help make "unit testing" of modules that have side effects actually
> mean something (for once) and move it closer to the usefulness of actual
> user testing (which is amazing at finding the boneheaded, easy to fix,
> 90% of bugs that are concrete and repeatable that unit tests are for some
> reason consistently blind to).
>
> -Craig
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-- 
With best regards,
     Roman Galeev,
     +420 702 817 968
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