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

[David Feuer]

On Thu, Oct 25, 2018, 11:28 PM Carter Schonwald <carter.schonwald at gmail.com>
wrote:

> Pretending we’re talking about this with storable as our example semantics
> :
>
> Ptr Void
> Corresponds to a memory address you don’t want to read or write to.
>

This is not so easy to think about (there's not much utility in such a
type). Perhaps Ptr Void is a reasonable type for pointers known to be null
or outside the address space, but I'm not sure.


> Ptr () corresponds to a memory location that’s pretty boring to read /
> write to
>

Yes! In other words, it's *not* what people should be using for a pointer
to an arbitrary non-Haskell address. That's precisely what Addr is for.


> On Thu, Oct 25, 2018 at 11:22 PM Carter Schonwald <
> carter.schonwald at gmail.com> wrote:
>
>> ... when is a valid pointer not going to point at byte addressable memory
>> on memory architectures ghc can support or target?
>>
>> On Thu, Oct 25, 2018 at 8:27 PM Daniel Cartwright <chessai1996 at gmail.com>
>> wrote:
>>
>>> Now, one could argue that `Ptr ()` isn't a lie, it sort of reads like
>>> C's void pointer. But surely something like `Ptr Word8` is a lie, when it
>>> is not actually a Ptr to Word8 values.
>>>
>>> On Thu, Oct 25, 2018 at 8:11 PM Daniel Cartwright <chessai1996 at gmail.com>
>>> wrote:
>>>
>>>> yes, only the type and its instances should be moved as far as i'm
>>>> aware.
>>>>
>>>> Also, it's more than just base.
>>>>
>>>> this Ptr is a lie:
>>>> https://hackage.haskell.org/package/ghc-compact-0.1.0.0/docs/GHC-Compact-Serialized.html#t:SerializedCompact
>>>> these Ptrs are lies:
>>>> https://hackage.haskell.org/package/base-4.12.0.0/docs/GHC-IO-Handle.html
>>>> in GHC.Stats, the foreign import "getRTSStats" has `Ptr () -> IO ()`,
>>>> this Ptr () is also a lie
>>>>
>>>> These are just off the top of my head, there are more
>>>>
>>>>
>>>> On Thu, Oct 25, 2018 at 6:46 PM Carter Schonwald <
>>>> carter.schonwald at gmail.com> wrote:
>>>>
>>>>> hrmm, what are the pieces of base that are using Ptr when they really
>>>>> should be using Addr? This would help me understand what would be made
>>>>> better in base :)
>>>>>
>>>>> On Thu, Oct 25, 2018 at 6:19 PM David Feuer <david.feuer at gmail.com>
>>>>> wrote:
>>>>>
>>>>>> We shouldn't really need to move anything into base except Addr and
>>>>>> its base instances.
>>>>>>
>>>>>> On Oct 25, 2018 5:59 PM, "Carter Schonwald" <
>>>>>> carter.schonwald at gmail.com> wrote:
>>>>>>
>>>>>> Indeed.  The monad transformer instances for primmonad need to live
>>>>>> in primmonad OR transformers to avoid orphans.
>>>>>>
>>>>>> Either way, unless transformers moves into base (unlikely), no way
>>>>>> anything using prim monad will.
>>>>>>
>>>>>> On Thu, Oct 25, 2018 at 3:34 PM Andrew Martin <
>>>>>> andrew.thaddeus at gmail.com> wrote:
>>>>>>
>>>>>>> I like the idea of moving the type Addr into base. But we cannot
>>>>>>> move the entire module since it has functions that talk about PrimMonad,
>>>>>>> and we definitely don't want to move that into base.
>>>>>>>
>>>>>>> On Thu, Oct 25, 2018 at 11:25 AM Daniel Cartwright <
>>>>>>> chessai1996 at gmail.com> wrote:
>>>>>>>
>>>>>>>> Motivation: There are a lot of places in base where 'Ptr a' is used
>>>>>>>> in place of 'Addr', because in base there is no 'Addr', only 'Addr#'. The
>>>>>>>> problem lies in the fact that many of these uses of 'Ptr a' are lying; the
>>>>>>>> 'a' value is meaningless. Authors of functions therein have used things
>>>>>>>> like 'Ptr ()', 'Ptr Word8', 'Ptr a', but these types do not mean what they
>>>>>>>> say they mean - they're just Addr. There are probably other motivations for
>>>>>>>> this that I can't think of off the top of my head right now.
>>>>>>>> _______________________________________________
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>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> --
>>>>>>> -Andrew Thaddeus Martin
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>>>>>>>
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>>>>>>
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>>>>
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