[phobos] [D-Programming-Language/phobos] 4f28db: not really pure
Don Clugston
dclugston at googlemail.com
Tue Jun 28 08:22:08 PDT 2011
On 28 June 2011 15:47, Steve Schveighoffer <schveiguy at yahoo.com> wrote:
> By definition, a strong-pure function must have all parameters immutable or implicitly convertible to immutable. This function does not pass this test.
>
> However, the function call *could* be considered strong pure if the S instance itself is immutable (at the call site). Which means S s would have to be immutable, and the original assignment to &w does not compile. It was you who brought up this idea in the first place, if you recall :)
Yes, exactly.
> This means whether a function that takes const parameters is weak-pure or strong-pure can depend on what values you call it with.
Yes, though up to now it's only been very conservative: it is
strong-pure only if all parameters are immutable.
> But, Walter's changes to phobos seems to imply that member functions cannot even be weak-pure. I don't see any reason to disallow the member functions as weak-pure (and I think you are saying the same thing). If this is Walter's case, I think it has no merit, but I'll reserve judgment until I know that this is in fact Walter's concern.
Now my guess to what's happened here is that the 'this' parameter had
escaped checks, so functions were erroneously being marked as strongly
pure.
But this shouldn't prevent them from being marked as weakly pure. But
I'm only guessing, maybe Walter has a different concern.
> BTW, immutable member functions could definitely be strong-pure (depending on the other parameters).
They could, but such cases are exceedingly rare and not very useful. I
don't think they matter. It's weak purity that matters for member
functions, since it allows them to be used in pure (static) functions.
- Don.
>> From: Don Clugston <dclugston at googlemail.com>
>> To: Discuss the phobos library for D <phobos at puremagic.com>
>> Cc:
>> Sent: Tuesday, June 28, 2011 8:34 AM
>> Subject: Re: [phobos] [D-Programming-Language/phobos] 4f28db: not really pure
>>
>> My guess is that Walter is concerned about cases like this:
>>
>> struct S
>> {
>> int *p;
>> void foo() pure const
>> {
>> return *p;
>> }
>> }
>>
>> static int w;
>>
>> void blah()
>> {
>> S s;
>> s.p = &w;
>> s.foo();
>> }
>>
>> Although foo's only parameter is 'this', which is const, it still
>> reads from a global variable.
>> This means is that it's very difficult for a member function to be
>> strongly pure. In fact, we wouldn't lose much by assuming that member
>> functions are NEVER strongly pure.
>>
>> *But* this doesn't mean that they can't be weakly pure. If foo() is
>> called from inside a strongly pure function, there is just no way that
>> it can be impure, because there is no way that a global variable can
>> be smuggled into it. The global would have to pass through the
>> strongly pure function, and there is no way of doing that.
>>
>>
>> On 28 June 2011 13:49, Steve Schveighoffer <schveiguy at yahoo.com> wrote:
>>> I looked at the changes you made to phobos. Let me examine one of them:
>>>
>>> struct Complex(T) if (isFloatingPoint!T)
>>> {
>>> /** The real part of the number. */
>>> T re;
>>>
>>> /** The imaginary part of the number. */
>>> T im;
>>>
>>>
>>> @safe nothrow /* removed attribute: pure */ // The following functions
>> depend only on std.math.
>>> {
>>>
>>> /** Calculate the absolute value (or modulus) of the number. */
>>> @property T abs() const
>>> {
>>> return hypot(re, im);
>>> }
>>> In fact, the abs function *is* pure (since hypot is pure). Which is why
>> I'm confused. It does not modify any global or shared state.
>>>
>>> If the compiler no longer compiles this as pure, then it does *NOT*
>> implement weak-pure semantics. So I'm not sure what you mean by "I
>> agree". This is a step backwards.
>>>
>>> I'll examine it directly. Complex!T.abs takes one parameter, a ref
>> Complex!T. Given the definition of weak-pure, a function is allowed to accept
>> and access references to mutable data -- even change them -- as long as the data
>> is not typed as shared. It also does not call any impure functions or access
>> any global variables. I think you are completely wrong in thinking this cannot
>> be pure. If the compiler does not accept this, then the compiler has not
>> implemented weak-pure.
>>>
>>> Do you have another case (even a theoretical one) that would have
>> incorrectly compiled before this change? Certainly the changes I looked at were
>> not accepts-invalid cases, given the context that weak-pure was implemented.
>>>
>>> The other option is you simply don't want weak-pure. Which is a design
>> decision I don't agree with, but it's your decision. It might be hard
>> to get those worms back in the can now!
>>>
>>> -Steve
>>>
>>>
>>> ----- Original Message -----
>>>> From: Walter Bright <walter at digitalmars.com>
>>>> To: phobos at puremagic.com
>>>> Cc:
>>>> Sent: Monday, June 27, 2011 2:28 PM
>>>> Subject: Re: [phobos] [D-Programming-Language/phobos] 4f28db: not
>> really pure
>>>>
>>>> I understand and agree, but the previous code was so broken it would
>> allow
>>>> assignment through the 'this' pointer.
>>>>
>>>> On 6/27/2011 3:51 AM, Steve Schveighoffer wrote:
>>>>> ----- Original Message -----
>>>>>
>>>>>> From: Walter Bright<walter at digitalmars.com>
>>>>>>
>>>>>>
>>>>>>
>>>>>> On 6/25/2011 8:25 PM, Jonathan M Davis wrote:
>>>>>>> On 2011-06-25 20:23, Walter Bright wrote:
>>>>>>>> On 6/25/2011 8:01 PM, Jonathan M Davis wrote:
>>>>>>>>> If they're not really pure, they
>> shouldn't
>>>> compile with
>>>>>> pure.
>>>>>>>> They won't in the upcoming checkin.
>>>>>>>>
>>>>>>>>> Also, not being
>>>>>>>>>
>>>>>>>>> able to use opAssign in pure functions could be
>> rather
>>>> annoying.
>>>>>> Why
>>>>>>>>> doesn't that work?
>>>>>>>> Pure functions cannot mutate data through pointers
>> passed to
>>>> them.
>>>>>>> Weakly pure functions are supposed to be able to.
>>>>>>>
>>>>>>>
>>>>>> The compiler wasn't checking this at all. It's
>> possible we can
>>>> loosen
>>>>>> things up
>>>>>> after we understand the issues better, but we should start
>> with being
>>>> very
>>>>>> conservative about what purity means.
>>>>> Don is probably the best to explain it, since he formalized the
>> notion of
>>>> weak pure, but I wholeheartedly agree with Jonathan, member functions
>> that
>>>> access or mutate mutable members can and should be pure, as long as
>> they
>>>> don't access or mutate shared or global data.
>>>>>
>>>>> If that isn't true, then the whole notion of weak purity that
>> allows
>>>> pure functions to be useful is out the window. I.e. pure functions
>> will be
>>>> reserved to the likes of std.math.sin, and nobody will mark any major
>> functions
>>>> pure.
>>>>>
>>>>> I thought weak purity had already been scrutinized, accepted, and
>>>> purposefully added to the compiler? Were you not part of that
>> discussion (I
>>>> thought you were)? Does this reverse that decision? If so, I think it
>> is the
>>>> wrong move. Weak purity made pure functions not only useful, but
>> actually
>>>> pleasant to work with.
>>>>>
>>>>> To recap weak purity, a pure function is one that does not access
>> mutable
>>>> or const data that is shared or global, and it can only call pure
>> functions.
>>>>> A Strong-pure function is a pure function whose parameters and
>> return value
>>>> are all immutable or implicitly convertible to immutable.
>>>>>
>>>>> The compiler can only make pure-related optimizations on
>> strong-pure
>>>> functions. However, since weak-pure functions are still pure,
>> strong-pure
>>>> functions can call them.
>>>>>
>>>>> For instance, a useful idiom would be for a weak-pure function to
>> sort an
>>>> array in place. Without weak-purity, one has to re-implement sorting
>> an array
>>>> in every pure function they need it for, or write a functional-style
>> sort (not a
>>>> very easy or efficient thing).
>>>>>
>>>>> In other words, weak purity makes pure functions written in an
>> imperative
>>>> style able to be modular. Without it, you have to resort to functional
>> style,
>>>> or mixin everything into your local scope.
>>>>>
>>>>> -Steve
>>>>>
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>>>>>
>>>>>
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