Proposal: Move primitive-Data.Primitive.Addr API into base

[David Feuer]

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 at 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 at gmail.com> wrote:
>>
>> On Mon, Oct 29, 2018, 10:05 AM Sven Panne <svenpanne at gmail.com> wrote:
>>>
>>> Am Mo., 29. Okt. 2018 um 14:27 Uhr schrieb Daniel Cartwright <chessai1996 at 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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