Coercible questions

[Brandon Allbery]

I think there's some work going on to expose the representations, which
would enable some ability to coerce. But possibly not this much, as they're
separate RuntimeReps so you don't combine signed and unsigned numbers
inadvertently; currently that's a little magical inside ghc iirc, with the
RuntimeRep the only way to distinguish at all and that vanishing
post-typechecking.

On Sat, Oct 5, 2019 at 12:30 PM Michal Terepeta <michal.terepeta at gmail.com>
wrote:

> Adding +ghc-devs <ghc-devs at haskell.org> to continue the thread
>
> Hi Sandy,
>
> Thanks for the answer! Do you think there is some fundamental reason for
> this? Or just a matter of implementing this in GHC? It seems to me that
> this should work just fine as long as the runtime representation is the
> same.
>
> And a related question--is it safe to `unsafeCoerce` an `Int` to a `Word`?
> The only reason for why this could be problematic that comes to my mind is
> that there could be an assumption that different `data`s do not alias each
> other (although `newtype`s can due to `Coercible` functionality). But I'm
> not sure this is ever used by GHC? Are there any other reasons why this
> could be problematic?
>
> Thanks!
>
> - Michal
>
>
>
> On Sat, Oct 5, 2019 at 5:27 PM Sandy Maguire <sandy at sandymaguire.me>
> wrote:
>
>> Hi Michal,
>>
>> Datas aren't coercible, only newtypes. This is why you can't coerce Ints
>> and Words, and why Foo and Bar don't work.
>>
>> Sandy
>>
>> On Sat, Oct 5, 2019 at 4:17 PM Michal Terepeta <michal.terepeta at gmail.com>
>> wrote:
>>
>>> Hi,
>>>
>>> I've started looking into using `Data.Coerce` (and the `Coercible`
>>> type-class) for a personal project and was wondering why coercing between
>>> `Int` and `Word` is not allowed? I don't see any fundamental reason why
>>> this shouldn't work...
>>>
>>> Perhaps, it's just a matter of GHC's implementation details leaking out?
>>> IIRC internally GHC has separate `RuntimeRep`/`PrimRep` for a `Word#` and
>>> for an `Int#`. If that's the case, would it make sense to unify these?
>>> Their actual runtime representation should be the same and I'd expect most
>>> (all?) of their differences should be attached to `PrimOp`s.
>>>
>>> And that leads me to another question--what exactly goes wrong here:
>>> ```
>>> data Foo = Foo Int#
>>> data Bar = Bar Int#
>>>
>>> test :: Bar
>>> test = coerce (Foo 42#)
>>> ```
>>> Which fails with: "Couldn't match representation of type ‘Foo’ with
>>> that of ‘Bar’ arising from a use of ‘coerce’"
>>>
>>> Perhaps I'm just misunderstanding exactly how `Coercible` works?
>>>
>>> Thanks in advance!
>>>
>>> - Michal
>>>
>>> PS. The ability to coerce through things like lists is amazing :)
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>>
>>
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-- 
brandon s allbery kf8nh
allbery.b at gmail.com
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