so i'm taking a look at the current docs / definition in base (and i'm testing testing out seeing what happens if we changed the role and just build ghc and base as is)
i'm replicating the docs below:
but the important detail is the phantom role in the current code is deliberate ...
because no guarantees are made about the relationship between the choice in type parameter ( which putatively will always have the same heap rep in haskell land)
and the representation at the other side of the pointer.
a good example might be Ptr Char. Is this a single location of a 32bit / utf32 code point, or an array of utf8 code points or utf32 code points?
is it null terminated vs there being an extra sequence length? These are all valid things that can be happening. And
| -- Data pointers.
|
|
|
| -- The role of Ptr's parameter is phantom, as there is no relation between |
| -- the Haskell representation and whathever the user puts at the end of the |
| -- pointer. And phantom is useful to implement castPtr (see #9163) |
|
|
| -- redundant role annotation checks that this doesn't change |
| type role Ptr phantom |
| data Ptr a = Ptr Addr# |
| deriving ( Eq -- ^ @since 2.01 |
| , Ord -- ^ @since 2.01 |
| ) |
| -- ^ A value of type @'Ptr' a@ represents a pointer to an object, or an |
| -- array of objects, which may be marshalled to or from Haskell values |
| -- of type @a@. |
| -- |
| -- The type @a@ will often be an instance of class |
| -- 'Foreign.Storable.Storable' which provides the marshalling operations. |
| -- However this is not essential, and you can provide your own operations |
| -- to access the pointer. For example you might write small foreign |
| -- functions to get or set the fields of a C @struct@. |
unrelatedly, while i'm not familiar with how to use modern coercible bits, the folllowing subset of what i mentioned earlier seems to work fine
{-# Language PatternSynonyms, FlexibleContexts, RoleAnnotations, RankNTypes, MagicHash #-}
module Ptr where
import GHC.Exts hiding (Ptr)
import Data.Type.Coercion
import Unsafe.Coerce
data Ptr a = Ptr Addr#
type role Ptr nominal
castPtr :: Ptr a -> Ptr b
castPtr = unsafeCoerce
ptrCoercible
:: (forall a b. Coercible (Ptr a) (Ptr b) => r)
-> r
ptrCoercible r = r