It is Storable that makes Ptr operate as typed pointers. I also think Ptr on its own doesn't have that much about its argument type, so I'm +1 on the proposal.

Btw, shouldn't every type be storable? In current Haskell, Maybes or (->)s aren't Storable, yet in C++, arrays of std::optionals or std::functions are well-defined.

As an exception, for Void, I agree that they must remain not Storable since it has no values.

2018년 10월 30일 (화) 08:31, Carter Schonwald <carter.schonwald@gmail.com>님이 작성:
the parametricity isn't for when you know things, its for saying "these are possibly different or possibly the same, dont let me mix them up though, cause I dont know yet"


On Mon, Oct 29, 2018 at 7:03 PM Daniel Cartwright <chessai1996@gmail.com> wrote:
I'm not sure that argument applies at all, when talking about _incorrect_ usages of Ptr. Sure, Addr probably shouldn't be used when there is meaningful type information/value to recover, but neither should Ptr be used when there is none.

The argument being made is not to make 'better', per se, and there definitely won't be a 'mathematical statement' about this, but it certainly may be made clearer - in my opinion, the usages of 'Ptr' that i've already brought up are inherently unclear because of the bogus phantom type associated with 'Ptr'. The illustration of this begs no code that doesn't already exist in corelibs.


On Mon, Oct 29, 2018 at 6:19 PM Carter Schonwald <carter.schonwald@gmail.com> wrote:
to zoom out: what code is improved? what code is made better/clearer? No one has articulated this clearly. 

The one example of Addr being used in Vector.Storable.Mutable is not an argument in favor of using Addr. Its an argument against it existing.

i'm looking for evidence, in the form of code i can look at then say "yes, this is better code" when comparing the two. Or a mathematical statement of "what is made better"

@David Feuer , @Daniel , do you have one?

when i'm writing complicated code, MORE polymorphism helps me usually.

I can write some code like the following and even though I'm using it with Int at argument,
I *Know* that i'm not mixing up arguments/values that i write as different types. I cannot do this with Address!

{-# SPECIALIZE INLINE computeStarts :: [(Int,Int)]->Int->Int ->[(Int,Int)] #-}
computeStarts:: (Enum a, Ord a, Num b )=>[(a,b)]-> a -> a -> [(a,b)]

parametricity (even when constrained by type classes) is a powerful and foundational tool for good programming in haskell and similar languages

there has been nothing stated here that successfully articulates a good reason to forgo/discourage parametricity as an engineering tool. for thats what Addr is. 
A datatype thats never safe in isolation, and discourages using parametricity to write correct software.

a very strong case is needed to forgo parametricity. 




On Mon, Oct 29, 2018 at 5:33 PM David Feuer <david.feuer@gmail.com> wrote:
Good point! Call it nominal then.

On Mon, Oct 29, 2018, 5:24 PM Carter Schonwald <carter.schonwald@gmail.com> wrote:
absolutely false, represeentational equality of the  type a in `Ptr a` does not mean the memory representation at the corresponding address is the same.
(it sometimes is true, but memory packing/alignment details in structs in C  for otherwise equivlanet structs should rule this out)

aka, `a` being representationally equal to `b` via haskell newtypes does not mean the memory representation at `Ptr a`, and `Ptr b` are the same. a trivial example is when
host and network byte order aren't the same (eg big vs little endian memory encodings)

On Mon, Oct 29, 2018 at 12:28 PM David Feuer <david.feuer@gmail.com> wrote:
What? Of course you can dereference it. You dereference it, getting a
value of type `Void`,
and apply absurd to get whatever you want in the world. This, of
course, is utter nonsense,
unless *having* the Ptr Void means that something has already gone
wrong. It's pretty
hard for me to imagine a situation where this is actually what you
want. A Ptr () isn't nonsense.
It is not terrible to use Ptr () to represent an Addr, but I wonder if
it sends the wrong message.
By the way: there's another argument for having Addr in base for now.
We would really
*like* for Ptr's parameter to have a *representational* role, but we
*don't* want to require
unsafeCoerce to cast Ptrs. The solution to that in the current role system:

    data Addr = Addr Addr#

    newtype Ptr a = Ptr_ Addr
    type role Ptr representational

    pattern Ptr :: Addr# -> Ptr a
    pattern Ptr addr# = Ptr_ (Addr addr#)

    -- Allow users to reveal coercibility of pointer types locally
    ptrCoercion :: Coercion (Ptr a) (Ptr b)
    ptrCoercion = Coercion

    castPtr :: Ptr a -> Ptr b
    castPtr = coerceWith ptrCoercion -- (or the now-free unwrap-rewrap
definition)


So even if we don't *expose* Addr in base, we should almost certainly *define*
it there.
On Mon, Oct 29, 2018 at 12:11 PM Carter Schonwald
<carter.schonwald@gmail.com> wrote:
>
> The point , hahah, of a Ptr void is that you can’t dereference it.  But you certainly can cast it and do address arithmetic on it!!
>
>
>
> On Mon, Oct 29, 2018 at 10:10 AM David Feuer <david.feuer@gmail.com> wrote:
>>
>> On Mon, Oct 29, 2018, 10:05 AM Sven Panne <svenpanne@gmail.com> wrote:
>>>
>>> Am Mo., 29. Okt. 2018 um 14:27 Uhr schrieb Daniel Cartwright <chessai1996@gmail.com>:
>>>>
>>>> 'Ptr Void' is not a pointer to a value of type 'Void'; there are no values of type 'Void': this type is nonsensical.
>>>
>>>
>>> That's the whole point, and it actually makes sense: If you see "Ptr Void", you can't do much with it, apart from passing it around or using castPtr on it. This is exactly what should be achieved by using "Ptr Void" in an API. This is basically the same as "void *" in C/C++.
>>
>>
>> No, it does not make sense. The approximate equivalent of C's void* is Ptr Any. Ptr Void promises to give you anything you want on dereference, which is nonsense.
>>
>>>
>>> You can't store or read "()", so the same holds as for Void (which didn't exist when the FFI was created IIRC).
>>
>>
>> Sure you can. Storing () does nothing and reading it gives (). Our () is somewhat similar to C's void return type.
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