Re: Coercible questions

Adding +ghc-devs 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 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 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 :) _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs

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