Re: Proposal: give Ptr a nominal role

Pattern synonyms can be bundled with type constructors these days. On Sun, Nov 4, 2018, 7:01 AM Bertram Felgenhauer via Libraries < libraries@haskell.org wrote:

Carter Schonwald wrote:

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https://ghc.haskell.org/trac/ghc/ticket/9163 seems to talk about *why* Ptr being phantom actually has a pretty measurable impact on code. (cast ptr becomes a noop and this has a measurable impact on a number of application measures)

David's point, I believe, is that the same performance problem could have been solved by turning Ptr into a newtype, without giving up the representational that it had before. In fact he brought this up in the same trac ticket,

https://ghc.haskell.org/trac/ghc/ticket/9163#comment:38

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On Tue, Oct 30, 2018 at 5:57 PM David Feuer wrote:

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Thankfully, we have enough power to fix this now.

data Addr = Ptr_ Addr#

newtype Ptr a = Ptr_ Addr type role Ptr nominal

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

castPtr :: Ptr a -> Ptr b castPtr (Ptr a) = Ptr a

ptrCoercible :: ((forall a b. Coercible (Ptr a) (Ptr b)) => r) -> r ptrCoercible r = r

ptrCoercion :: Coercion (Ptr a) (Ptr b) ptrCoercion = Coercion

I propose that we do this.

This will break some imports, since

import GHC.Ptr (Ptr (..))

will not import the pattern synonym. This is how Ptr is imported in both bytestring and vector. But it's not a big deal, I suppose.

Cheers,

Bertram _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries