Foreign.Ptr

base-4.10.1.0: Basic libraries

Copyright | (c) The FFI task force 2001
License | BSD-style (see the file libraries/base/LICENSE)
Maintainer | ffi@haskell.org
Stability | provisional
Portability | portable
Safe Haskell | Trustworthy
Language | Haskell2010

Foreign.Ptr

Contents

- Data pointers
- Function pointers
- Integral types with lossless conversion to and from pointers

Description

This module provides typed pointers to foreign data. It is part of the Foreign Function Interface (FFI) and will normally be imported via the Foreign module.

Synopsis

- data Ptr a
- nullPtr :: Ptr a
- castPtr :: Ptr a -> Ptr b
- plusPtr :: Ptr a -> Int -> Ptr b
- alignPtr :: Ptr a -> Int -> Ptr a
- minusPtr :: Ptr a -> Ptr b -> Int
- data FunPtr a
- nullFunPtr :: FunPtr a
- castFunPtr :: FunPtr a -> FunPtr b
- castFunPtrToPtr :: FunPtr a -> Ptr b
- castPtrToFunPtr :: Ptr a -> FunPtr b
- freeHaskellFunPtr :: FunPtr a -> IO ()
- newtype IntPtr = IntPtr Int
- ptrToIntPtr :: Ptr a -> IntPtr
- intPtrToPtr :: IntPtr -> Ptr a
- newtype WordPtr = WordPtr Word
- ptrToWordPtr :: Ptr a -> WordPtr
- wordPtrToPtr :: WordPtr -> Ptr a

# Data pointers

data Ptr a Source #

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 `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`.

Instances

Generic1 k (URec k (Ptr ())) # |
Associated Types type Rep1 (URec k (Ptr ())) (f :: URec k (Ptr ()) -> *) :: k -> * Source # Methods from1 :: f a -> Rep1 (URec k (Ptr ())) f a Source # to1 :: Rep1 (URec k (Ptr ())) f a -> f a Source #
Eq (Ptr a) # |
Methods (==) :: Ptr a -> Ptr a -> Bool Source # (/=) :: Ptr a -> Ptr a -> Bool Source #
Data a => Data (Ptr a) # | Since: 4.8.0.0
Methods gfoldl :: (forall d b. Data d => c (d -> b) -> d -> c b) -> (forall g. g -> c g) -> Ptr a -> c (Ptr a) Source # gunfold :: (forall b r. Data b => c (b -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c (Ptr a) Source # toConstr :: Ptr a -> Constr Source # dataTypeOf :: Ptr a -> DataType Source # dataCast1 :: Typeable (* -> *) t => (forall d. Data d => c (t d)) -> Maybe (c (Ptr a)) Source # dataCast2 :: Typeable (* -> * -> *) t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c (Ptr a)) Source # gmapT :: (forall b. Data b => b -> b) -> Ptr a -> Ptr a Source # gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> Ptr a -> r Source # gmapQr :: (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> Ptr a -> r Source # gmapQ :: (forall d. Data d => d -> u) -> Ptr a -> [u] Source # gmapQi :: Int -> (forall d. Data d => d -> u) -> Ptr a -> u Source # gmapM :: Monad m => (forall d. Data d => d -> m d) -> Ptr a -> m (Ptr a) Source # gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> Ptr a -> m (Ptr a) Source # gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> Ptr a -> m (Ptr a) Source #
Ord (Ptr a) # |
Methods compare :: Ptr a -> Ptr a -> Ordering Source # (<) :: Ptr a -> Ptr a -> Bool Source # (<=) :: Ptr a -> Ptr a -> Bool Source # (>) :: Ptr a -> Ptr a -> Bool Source # (>=) :: Ptr a -> Ptr a -> Bool Source # max :: Ptr a -> Ptr a -> Ptr a Source # min :: Ptr a -> Ptr a -> Ptr a Source #
Show (Ptr a) # | Since: 2.1
Methods showsPrec :: Int -> Ptr a -> ShowS Source # show :: Ptr a -> String Source # showList :: [Ptr a] -> ShowS Source #
Storable (Ptr a) # | Since: 2.1
Methods sizeOf :: Ptr a -> Int Source # alignment :: Ptr a -> Int Source # peekElemOff :: Ptr (Ptr a) -> Int -> IO (Ptr a) Source # pokeElemOff :: Ptr (Ptr a) -> Int -> Ptr a -> IO () Source # peekByteOff :: Ptr b -> Int -> IO (Ptr a) Source # pokeByteOff :: Ptr b -> Int -> Ptr a -> IO () Source # peek :: Ptr (Ptr a) -> IO (Ptr a) Source # poke :: Ptr (Ptr a) -> Ptr a -> IO () Source #
Functor (URec * (Ptr ())) # |
Methods fmap :: (a -> b) -> URec * (Ptr ()) a -> URec * (Ptr ()) b Source # (<$) :: a -> URec * (Ptr ()) b -> URec * (Ptr ()) a Source #
Foldable (URec * (Ptr ())) # |
Methods fold :: Monoid m => URec * (Ptr ()) m -> m Source # foldMap :: Monoid m => (a -> m) -> URec * (Ptr ()) a -> m Source # foldr :: (a -> b -> b) -> b -> URec * (Ptr ()) a -> b Source # foldr' :: (a -> b -> b) -> b -> URec * (Ptr ()) a -> b Source # foldl :: (b -> a -> b) -> b -> URec * (Ptr ()) a -> b Source # foldl' :: (b -> a -> b) -> b -> URec * (Ptr ()) a -> b Source # foldr1 :: (a -> a -> a) -> URec * (Ptr ()) a -> a Source # foldl1 :: (a -> a -> a) -> URec * (Ptr ()) a -> a Source # toList :: URec * (Ptr ()) a -> [a] Source # null :: URec * (Ptr ()) a -> Bool Source # length :: URec * (Ptr ()) a -> Int Source # elem :: Eq a => a -> URec * (Ptr ()) a -> Bool Source # maximum :: Ord a => URec * (Ptr ()) a -> a Source # minimum :: Ord a => URec * (Ptr ()) a -> a Source # sum :: Num a => URec * (Ptr ()) a -> a Source # product :: Num a => URec * (Ptr ()) a -> a Source #
Traversable (URec * (Ptr ())) # |
Methods traverse :: Applicative f => (a -> f b) -> URec * (Ptr ()) a -> f (URec * (Ptr ()) b) Source # sequenceA :: Applicative f => URec * (Ptr ()) (f a) -> f (URec * (Ptr ()) a) Source # mapM :: Monad m => (a -> m b) -> URec * (Ptr ()) a -> m (URec * (Ptr ()) b) Source # sequence :: Monad m => URec * (Ptr ()) (m a) -> m (URec * (Ptr ()) a) Source #
Eq (URec k (Ptr ()) p) # |
Methods (==) :: URec k (Ptr ()) p -> URec k (Ptr ()) p -> Bool Source # (/=) :: URec k (Ptr ()) p -> URec k (Ptr ()) p -> Bool Source #
Ord (URec k (Ptr ()) p) # |
Methods compare :: URec k (Ptr ()) p -> URec k (Ptr ()) p -> Ordering Source # (<) :: URec k (Ptr ()) p -> URec k (Ptr ()) p -> Bool Source # (<=) :: URec k (Ptr ()) p -> URec k (Ptr ()) p -> Bool Source # (>) :: URec k (Ptr ()) p -> URec k (Ptr ()) p -> Bool Source # (>=) :: URec k (Ptr ()) p -> URec k (Ptr ()) p -> Bool Source # max :: URec k (Ptr ()) p -> URec k (Ptr ()) p -> URec k (Ptr ()) p Source # min :: URec k (Ptr ()) p -> URec k (Ptr ()) p -> URec k (Ptr ()) p Source #
Generic (URec k (Ptr ()) p) # |
Associated Types type Rep (URec k (Ptr ()) p) :: * -> * Source # Methods from :: URec k (Ptr ()) p -> Rep (URec k (Ptr ()) p) x Source # to :: Rep (URec k (Ptr ()) p) x -> URec k (Ptr ()) p Source #
data URec k (Ptr ()) # | Used for marking occurrences of `Addr#` Since: 4.9.0.0
data URec k (Ptr ()) = UAddr { uAddr# :: Addr# }
type Rep1 k (URec k (Ptr ())) # |
type Rep1 k (URec k (Ptr ())) = D1 k (MetaData "URec" "GHC.Generics" "base" False) (C1 k (MetaCons "UAddr" PrefixI True) (S1 k (MetaSel (Just Symbol "uAddr#") NoSourceUnpackedness NoSourceStrictness DecidedLazy) (UAddr k)))
type Rep (URec k (Ptr ()) p) # |
type Rep (URec k (Ptr ()) p) = D1 * (MetaData "URec" "GHC.Generics" "base" False) (C1 * (MetaCons "UAddr" PrefixI True) (S1 * (MetaSel (Just Symbol "uAddr#") NoSourceUnpackedness NoSourceStrictness DecidedLazy) (UAddr *)))

nullPtr :: Ptr a Source #

The constant `nullPtr` contains a distinguished value of `Ptr` that is not associated with a valid memory location.

castPtr :: Ptr a -> Ptr b Source #

The `castPtr` function casts a pointer from one type to another.

plusPtr :: Ptr a -> Int -> Ptr b Source #

Advances the given address by the given offset in bytes.

alignPtr :: Ptr a -> Int -> Ptr a Source #

Given an arbitrary address and an alignment constraint, `alignPtr` yields the next higher address that fulfills the alignment constraint. An alignment constraint `x` is fulfilled by any address divisible by `x`. This operation is idempotent.

minusPtr :: Ptr a -> Ptr b -> Int Source #

Computes the offset required to get from the second to the first argument. We have

    p2 == p1 `plusPtr` (p2 `minusPtr` p1)

# Function pointers

data FunPtr a Source #

A value of type ``FunPtr` a` is a pointer to a function callable from foreign code. The type `a` will normally be a foreign type, a function type with zero or more arguments where

- the argument types are marshallable foreign types, i.e. `Char`, `Int`, `Double`, `Float`, `Bool`, `Int8`, `Int16`, `Int32`, `Int64`, `Word8`, `Word16`, `Word32`, `Word64`, ``Ptr` a`, ``FunPtr` a`, ``StablePtr` a` or a renaming of any of these using `newtype`.
- the return type is either a marshallable foreign type or has the form ``IO` t` where `t` is a marshallable foreign type or `()`.

A value of type ``FunPtr` a` may be a pointer to a foreign function, either returned by another foreign function or imported with a a static address import like

    foreign import ccall "stdlib.h &free"
      p_free :: FunPtr (Ptr a -> IO ())

or a pointer to a Haskell function created using a wrapper stub declared to produce a `FunPtr` of the correct type. For example:

    type Compare = Int -> Int -> Bool
    foreign import ccall "wrapper"
      mkCompare :: Compare -> IO (FunPtr Compare)

Calls to wrapper stubs like `mkCompare` allocate storage, which should be released with `freeHaskellFunPtr` when no longer required.

To convert `FunPtr` values to corresponding Haskell functions, one can define a dynamic stub for the specific foreign type, e.g.

    type IntFunction = CInt -> IO ()
    foreign import ccall "dynamic"
      mkFun :: FunPtr IntFunction -> IntFunction

Instances

Eq (FunPtr a) # |
Methods (==) :: FunPtr a -> FunPtr a -> Bool Source # (/=) :: FunPtr a -> FunPtr a -> Bool Source #
Ord (FunPtr a) # |
Methods compare :: FunPtr a -> FunPtr a -> Ordering Source # (<) :: FunPtr a -> FunPtr a -> Bool Source # (<=) :: FunPtr a -> FunPtr a -> Bool Source # (>) :: FunPtr a -> FunPtr a -> Bool Source # (>=) :: FunPtr a -> FunPtr a -> Bool Source # max :: FunPtr a -> FunPtr a -> FunPtr a Source # min :: FunPtr a -> FunPtr a -> FunPtr a Source #
Show (FunPtr a) # | Since: 2.1
Methods showsPrec :: Int -> FunPtr a -> ShowS Source # show :: FunPtr a -> String Source # showList :: [FunPtr a] -> ShowS Source #
Storable (FunPtr a) # | Since: 2.1
Methods sizeOf :: FunPtr a -> Int Source # alignment :: FunPtr a -> Int Source # peekElemOff :: Ptr (FunPtr a) -> Int -> IO (FunPtr a) Source # pokeElemOff :: Ptr (FunPtr a) -> Int -> FunPtr a -> IO () Source # peekByteOff :: Ptr b -> Int -> IO (FunPtr a) Source # pokeByteOff :: Ptr b -> Int -> FunPtr a -> IO () Source # peek :: Ptr (FunPtr a) -> IO (FunPtr a) Source # poke :: Ptr (FunPtr a) -> FunPtr a -> IO () Source #

nullFunPtr :: FunPtr a Source #

The constant `nullFunPtr` contains a distinguished value of `FunPtr` that is not associated with a valid memory location.

castFunPtr :: FunPtr a -> FunPtr b Source #

Casts a `FunPtr` to a `FunPtr` of a different type.

castFunPtrToPtr :: FunPtr a -> Ptr b Source #

Casts a `FunPtr` to a `Ptr`.

Note: this is valid only on architectures where data and function pointers range over the same set of addresses, and should only be used for bindings to external libraries whose interface already relies on this assumption.

castPtrToFunPtr :: Ptr a -> FunPtr b Source #

Casts a `Ptr` to a `FunPtr`.

Note: this is valid only on architectures where data and function pointers range over the same set of addresses, and should only be used for bindings to external libraries whose interface already relies on this assumption.

freeHaskellFunPtr :: FunPtr a -> IO () Source #

Release the storage associated with the given `FunPtr`, which must have been obtained from a wrapper stub. This should be called whenever the return value from a foreign import wrapper function is no longer required; otherwise, the storage it uses will leak.

# Integral types with lossless conversion to and from pointers

newtype IntPtr Source #

A signed integral type that can be losslessly converted to and from `Ptr`. This type is also compatible with the C99 type `intptr_t`, and can be marshalled to and from that type safely.

Constructors

IntPtr Int |

Instances

Bounded IntPtr # |
Methods minBound :: IntPtr Source # maxBound :: IntPtr Source #
Enum IntPtr # |
Methods succ :: IntPtr -> IntPtr Source # pred :: IntPtr -> IntPtr Source # toEnum :: Int -> IntPtr Source # fromEnum :: IntPtr -> Int Source # enumFrom :: IntPtr -> [IntPtr] Source # enumFromThen :: IntPtr -> IntPtr -> [IntPtr] Source # enumFromTo :: IntPtr -> IntPtr -> [IntPtr] Source # enumFromThenTo :: IntPtr -> IntPtr -> IntPtr -> [IntPtr] Source #
Eq IntPtr # |
Methods (==) :: IntPtr -> IntPtr -> Bool Source # (/=) :: IntPtr -> IntPtr -> Bool Source #
Integral IntPtr # |
Methods quot :: IntPtr -> IntPtr -> IntPtr Source # rem :: IntPtr -> IntPtr -> IntPtr Source # div :: IntPtr -> IntPtr -> IntPtr Source # mod :: IntPtr -> IntPtr -> IntPtr Source # quotRem :: IntPtr -> IntPtr -> (IntPtr, IntPtr) Source # divMod :: IntPtr -> IntPtr -> (IntPtr, IntPtr) Source # toInteger :: IntPtr -> Integer Source #
Num IntPtr # |
Methods (+) :: IntPtr -> IntPtr -> IntPtr Source # (-) :: IntPtr -> IntPtr -> IntPtr Source # (*) :: IntPtr -> IntPtr -> IntPtr Source # negate :: IntPtr -> IntPtr Source # abs :: IntPtr -> IntPtr Source # signum :: IntPtr -> IntPtr Source # fromInteger :: Integer -> IntPtr Source #
Ord IntPtr # |
Methods compare :: IntPtr -> IntPtr -> Ordering Source # (<) :: IntPtr -> IntPtr -> Bool Source # (<=) :: IntPtr -> IntPtr -> Bool Source # (>) :: IntPtr -> IntPtr -> Bool Source # (>=) :: IntPtr -> IntPtr -> Bool Source # max :: IntPtr -> IntPtr -> IntPtr Source # min :: IntPtr -> IntPtr -> IntPtr Source #
Read IntPtr # |
Methods readsPrec :: Int -> ReadS IntPtr Source # readList :: ReadS [IntPtr] Source # readPrec :: ReadPrec IntPtr Source # readListPrec :: ReadPrec [IntPtr] Source #
Real IntPtr # |
Methods toRational :: IntPtr -> Rational Source #
Show IntPtr # |
Methods showsPrec :: Int -> IntPtr -> ShowS Source # show :: IntPtr -> String Source # showList :: [IntPtr] -> ShowS Source #
FiniteBits IntPtr # |
Methods finiteBitSize :: IntPtr -> Int Source # countLeadingZeros :: IntPtr -> Int Source # countTrailingZeros :: IntPtr -> Int Source #
Bits IntPtr # |
Methods (.&.) :: IntPtr -> IntPtr -> IntPtr Source # (.|.) :: IntPtr -> IntPtr -> IntPtr Source # xor :: IntPtr -> IntPtr -> IntPtr Source # complement :: IntPtr -> IntPtr Source # shift :: IntPtr -> Int -> IntPtr Source # rotate :: IntPtr -> Int -> IntPtr Source # zeroBits :: IntPtr Source # bit :: Int -> IntPtr Source # setBit :: IntPtr -> Int -> IntPtr Source # clearBit :: IntPtr -> Int -> IntPtr Source # complementBit :: IntPtr -> Int -> IntPtr Source # testBit :: IntPtr -> Int -> Bool Source # bitSizeMaybe :: IntPtr -> Maybe Int Source # bitSize :: IntPtr -> Int Source # isSigned :: IntPtr -> Bool Source # shiftL :: IntPtr -> Int -> IntPtr Source # unsafeShiftL :: IntPtr -> Int -> IntPtr Source # shiftR :: IntPtr -> Int -> IntPtr Source # unsafeShiftR :: IntPtr -> Int -> IntPtr Source # rotateL :: IntPtr -> Int -> IntPtr Source # rotateR :: IntPtr -> Int -> IntPtr Source # popCount :: IntPtr -> Int Source #
Storable IntPtr # |
Methods sizeOf :: IntPtr -> Int Source # alignment :: IntPtr -> Int Source # peekElemOff :: Ptr IntPtr -> Int -> IO IntPtr Source # pokeElemOff :: Ptr IntPtr -> Int -> IntPtr -> IO () Source # peekByteOff :: Ptr b -> Int -> IO IntPtr Source # pokeByteOff :: Ptr b -> Int -> IntPtr -> IO () Source # peek :: Ptr IntPtr -> IO IntPtr Source # poke :: Ptr IntPtr -> IntPtr -> IO () Source #

ptrToIntPtr :: Ptr a -> IntPtr Source #

casts a `Ptr` to an `IntPtr`

intPtrToPtr :: IntPtr -> Ptr a Source #

casts an `IntPtr` to a `Ptr`

newtype WordPtr Source #

An unsigned integral type that can be losslessly converted to and from `Ptr`. This type is also compatible with the C99 type `uintptr_t`, and can be marshalled to and from that type safely.

Constructors

WordPtr Word |

Instances

Bounded WordPtr # |
Methods minBound :: WordPtr Source # maxBound :: WordPtr Source #
Enum WordPtr # |
Methods succ :: WordPtr -> WordPtr Source # pred :: WordPtr -> WordPtr Source # toEnum :: Int -> WordPtr Source # fromEnum :: WordPtr -> Int Source # enumFrom :: WordPtr -> [WordPtr] Source # enumFromThen :: WordPtr -> WordPtr -> [WordPtr] Source # enumFromTo :: WordPtr -> WordPtr -> [WordPtr] Source # enumFromThenTo :: WordPtr -> WordPtr -> WordPtr -> [WordPtr] Source #
Eq WordPtr # |
Methods (==) :: WordPtr -> WordPtr -> Bool Source # (/=) :: WordPtr -> WordPtr -> Bool Source #
Integral WordPtr # |
Methods quot :: WordPtr -> WordPtr -> WordPtr Source # rem :: WordPtr -> WordPtr -> WordPtr Source # div :: WordPtr -> WordPtr -> WordPtr Source # mod :: WordPtr -> WordPtr -> WordPtr Source # quotRem :: WordPtr -> WordPtr -> (WordPtr, WordPtr) Source # divMod :: WordPtr -> WordPtr -> (WordPtr, WordPtr) Source # toInteger :: WordPtr -> Integer Source #
Num WordPtr # |
Methods (+) :: WordPtr -> WordPtr -> WordPtr Source # (-) :: WordPtr -> WordPtr -> WordPtr Source # (*) :: WordPtr -> WordPtr -> WordPtr Source # negate :: WordPtr -> WordPtr Source # abs :: WordPtr -> WordPtr Source # signum :: WordPtr -> WordPtr Source # fromInteger :: Integer -> WordPtr Source #
Ord WordPtr # |
Methods compare :: WordPtr -> WordPtr -> Ordering Source # (<) :: WordPtr -> WordPtr -> Bool Source # (<=) :: WordPtr -> WordPtr -> Bool Source # (>) :: WordPtr -> WordPtr -> Bool Source # (>=) :: WordPtr -> WordPtr -> Bool Source # max :: WordPtr -> WordPtr -> WordPtr Source # min :: WordPtr -> WordPtr -> WordPtr Source #
Read WordPtr # |
Methods readsPrec :: Int -> ReadS WordPtr Source # readList :: ReadS [WordPtr] Source # readPrec :: ReadPrec WordPtr Source # readListPrec :: ReadPrec [WordPtr] Source #
Real WordPtr # |
Methods toRational :: WordPtr -> Rational Source #
Show WordPtr # |
Methods showsPrec :: Int -> WordPtr -> ShowS Source # show :: WordPtr -> String Source # showList :: [WordPtr] -> ShowS Source #
FiniteBits WordPtr # |
Methods finiteBitSize :: WordPtr -> Int Source # countLeadingZeros :: WordPtr -> Int Source # countTrailingZeros :: WordPtr -> Int Source #
Bits WordPtr # |
Methods (.&.) :: WordPtr -> WordPtr -> WordPtr Source # (.|.) :: WordPtr -> WordPtr -> WordPtr Source # xor :: WordPtr -> WordPtr -> WordPtr Source # complement :: WordPtr -> WordPtr Source # shift :: WordPtr -> Int -> WordPtr Source # rotate :: WordPtr -> Int -> WordPtr Source # zeroBits :: WordPtr Source # bit :: Int -> WordPtr Source # setBit :: WordPtr -> Int -> WordPtr Source # clearBit :: WordPtr -> Int -> WordPtr Source # complementBit :: WordPtr -> Int -> WordPtr Source # testBit :: WordPtr -> Int -> Bool Source # bitSizeMaybe :: WordPtr -> Maybe Int Source # bitSize :: WordPtr -> Int Source # isSigned :: WordPtr -> Bool Source # shiftL :: WordPtr -> Int -> WordPtr Source # unsafeShiftL :: WordPtr -> Int -> WordPtr Source # shiftR :: WordPtr -> Int -> WordPtr Source # unsafeShiftR :: WordPtr -> Int -> WordPtr Source # rotateL :: WordPtr -> Int -> WordPtr Source # rotateR :: WordPtr -> Int -> WordPtr Source # popCount :: WordPtr -> Int Source #
Storable WordPtr # |
Methods sizeOf :: WordPtr -> Int Source # alignment :: WordPtr -> Int Source # peekElemOff :: Ptr WordPtr -> Int -> IO WordPtr Source # pokeElemOff :: Ptr WordPtr -> Int -> WordPtr -> IO () Source # peekByteOff :: Ptr b -> Int -> IO WordPtr Source # pokeByteOff :: Ptr b -> Int -> WordPtr -> IO () Source # peek :: Ptr WordPtr -> IO WordPtr Source # poke :: Ptr WordPtr -> WordPtr -> IO () Source #

ptrToWordPtr :: Ptr a -> WordPtr Source #

casts a `Ptr` to a `WordPtr`

wordPtrToPtr :: WordPtr -> Ptr a Source #

casts a `WordPtr` to a `Ptr`