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digitalmars.D.learn - should chain be pure

reply "Daniel Davidson" <nospam spam.com> writes:
I would like to correctly annotate my functions with pure. I've 
hit a function that is calling chain which breaks purity. Is 
chain really not pure?

The relevant section of code is:

       ...
       auto sortedRage = assumeSorted!("a.when < 
b.when")(opSlice());
       auto trisection = sortedRage.trisect(needle);
       auto ccRate = Rate(0.0);

       if(trisection[0].length) {
         ccRate = trisection[0][$-1].value;
       }
       foreach(dateRate; chain(trisection[1], trisection[2])) {
       ...

Is there a reasonable work around?

Thanks
Dan
Oct 15 2013
parent reply "bearophile" <bearophileHUGS lycos.com> writes:
Daniel Davidson:


 I would like to correctly annotate my functions with pure. I've 
 hit a function that is calling chain which breaks purity. Is 
 chain really not pure?


Phobos is slowly being annotated with pure/nothrow (and  safe) 
but not all functions are already tagged.

You could fix the problem sending a Phobos patch.

Bye,
bearophile
Oct 15 2013
parent reply "Daniel Davidson" <nospam spam.com> writes:
On Tuesday, 15 October 2013 at 13:43:55 UTC, bearophile wrote:

 Daniel Davidson:


 I would like to correctly annotate my functions with pure. 
 I've hit a function that is calling chain which breaks purity. 
 Is chain really not pure?


 Phobos is slowly being annotated with pure/nothrow (and  safe) 
 but not all functions are already tagged.

 You could fix the problem sending a Phobos patch.

 Bye,
 bearophile


That is probably beyond my pay-grade at the moment :-)

If I just add pure to chain and run unittests all sorts of errors 
cascade and I don't think I'm in a position to pull it off.

If you are agreeing that chain should be pure and it is just 
following all the calls and making all of them pure, until that 
happens by the professionals - is there a casting solution so I 
can fake a pure and move on?
Oct 15 2013
parent reply "bearophile" <bearophileHUGS lycos.com> writes:
Daniel Davidson:


 If you are agreeing that chain should be pure and it is just 
 following all the calls and making all of them pure, until that 
 happens by the professionals - is there a casting solution so I 
 can fake a pure and move on?


chain is a template, and in Phobos often templates are not 
annotated with pure/nothrow, the compiler infers those attributes.

Regarding your code, perhaps you can put your call in an impure 
delegate and than cast it, but D has no direct means to "cast" 
purity, because it's highly unsafe and it's against the idea of 
having purity in the language.

So I suggest to replace the pure in your function/method tree 
with /*pure*/, and later fix the code if/when chains becomes pure.

Bye,
bearophile
Oct 15 2013
parent reply "monarch_dodra" <monarchdodra gmail.com> writes:
On Tuesday, 15 October 2013 at 14:15:15 UTC, bearophile wrote:

 Daniel Davidson:


 If you are agreeing that chain should be pure and it is just 
 following all the calls and making all of them pure, until 
 that happens by the professionals - is there a casting 
 solution so I can fake a pure and move on?


 chain is a template, and in Phobos often templates are not 
 annotated with pure/nothrow, the compiler infers those 
 attributes.

 Regarding your code, perhaps you can put your call in an impure 
 delegate and than cast it, but D has no direct means to "cast" 
 purity, because it's highly unsafe and it's against the idea of 
 having purity in the language.

 So I suggest to replace the pure in your function/method tree 
 with /*pure*/, and later fix the code if/when chains becomes 
 pure.

 Bye,
 bearophile


 bearophile: The problem is actually with voldemort. Chain is 
implemented as:

auto chain(Arg...)(Args args)
{
     static struct Result //Non template struct
     {
         auto front(); //Non template function in a non-template 
struct.
     }
}

The problem is that the whole inference things stops at this 
level: the attributes of "front" are not infered, so chain is not 
pure simply because it isn't a template.

This could be simply solved by making Result a non voldermort 
"ChainResult" outside of the body of chain.

I'd do this, but Kenji had mentioned before that he thought the 
attributes should be inferred.

So for now, I didn't personally make the effort of doing anything 
to fix it myself. But if someone else where make the effort, I'd 
review and probably pull.
Oct 16 2013
parent Jonathan M Davis <jmdavisProg gmx.com> writes:
On Wednesday, October 16, 2013 09:29:31 monarch_dodra wrote:

 The problem is that the whole inference things stops at this
 level: the attributes of "front" are not infered, so chain is not
 pure simply because it isn't a template.


http://d.puremagic.com/issues/show_bug.cgi?id=10329

And I think that there are older bug reports which are similar. Regardless, 
attribute inference is pretty poor right now. A _lot_ of Phobos doesn't have 
its attributes being properly inferred precisely because the compiler stops at 
the first level of templated stuff rather than fully inferring a template,
which 
quickly makes attribute inference nearly useless. It should definitely be fixed 
at the compiler level though. Anything else would just be patching holes in a 
sinking ship.

- Jonathan M Davis
Oct 16 2013