    {-# OPTIONS_GHC -XNoImplicitPrelude #-}
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Foreign.Storable
    -- Copyright   :  (c) The FFI task force 2001
    -- License     :  see libraries/base/LICENSE
    --
    -- Maintainer  :  ffi@haskell.org
    -- Stability   :  provisional
    -- Portability :  portable
    --
    -- The module "Foreign.Storable" provides most elementary support for
    -- marshalling and is part of the language-independent portion of the
    -- Foreign Function Interface (FFI), and will normally be imported via
    -- the "Foreign" module.
    --
    -----------------------------------------------------------------------------

    module Foreign.Storable
            ( Storable(
                 sizeOf,         -- :: a -> Int
                 alignment,      -- :: a -> Int
                 peekElemOff,    -- :: Ptr a -> Int      -> IO a
                 pokeElemOff,    -- :: Ptr a -> Int -> a -> IO ()
                 peekByteOff,    -- :: Ptr b -> Int      -> IO a
                 pokeByteOff,    -- :: Ptr b -> Int -> a -> IO ()
                 peek,           -- :: Ptr a             -> IO a
                 poke)           -- :: Ptr a        -> a -> IO ()
            ) where

    #ifdef __NHC__
    import NHC.FFI (Storable(..),Ptr,FunPtr,StablePtr
                   ,Int8,Int16,Int32,Int64,Word8,Word16,Word32,Word64)
    #else

    import Control.Monad            ( liftM )

    #include "MachDeps.h"
    #include "HsBaseConfig.h"

    #ifdef __GLASGOW_HASKELL__
    import GHC.Storable
    import GHC.Stable       ( StablePtr )
    import GHC.IO()		-- Instance Monad IO
    import GHC.Num
    import GHC.Int
    import GHC.Word
    import GHC.Ptr
    import GHC.Err
    import GHC.Base
    #else
    import Data.Int
    import Data.Word
    import Foreign.StablePtr
    #endif

    #ifdef __HUGS__
    import Hugs.Prelude
    import Hugs.Ptr
    import Hugs.Storable
    #endif

    {- |
    The member functions of this class facilitate writing values of
    primitive types to raw memory (which may have been allocated with the
    above mentioned routines) and reading values from blocks of raw
    memory.  The class, furthermore, includes support for computing the
    storage requirements and alignment restrictions of storable types.

    Memory addresses are represented as values of type @'Ptr' a@, for some
    @a@ which is an instance of class 'Storable'.  The type argument to
    'Ptr' helps provide some valuable type safety in FFI code (you can\'t
    mix pointers of different types without an explicit cast), while
    helping the Haskell type system figure out which marshalling method is
    needed for a given pointer.

    All marshalling between Haskell and a foreign language ultimately
    boils down to translating Haskell data structures into the binary
    representation of a corresponding data structure of the foreign
    language and vice versa.  To code this marshalling in Haskell, it is
    necessary to manipulate primitive data types stored in unstructured
    memory blocks.  The class 'Storable' facilitates this manipulation on
    all types for which it is instantiated, which are the standard basic
    types of Haskell, the fixed size @Int@ types ('Int8', 'Int16',
    'Int32', 'Int64'), the fixed size @Word@ types ('Word8', 'Word16',
    'Word32', 'Word64'), 'StablePtr', all types from "Foreign.C.Types",
    as well as 'Ptr'.

    Minimal complete definition: 'sizeOf', 'alignment', one of 'peek',
    'peekElemOff' and 'peekByteOff', and one of 'poke', 'pokeElemOff' and
    'pokeByteOff'.
    -}

    class Storable a where

       sizeOf      :: a -> Int
       -- ^ Computes the storage requirements (in bytes) of the argument.
       -- The value of the argument is not used.

       alignment   :: a -> Int
       -- ^ Computes the alignment constraint of the argument.  An
       -- alignment constraint @x@ is fulfilled by any address divisible
       -- by @x@.  The value of the argument is not used.

       peekElemOff :: Ptr a -> Int      -> IO a
       -- ^       Read a value from a memory area regarded as an array
       --         of values of the same kind.  The first argument specifies
       --         the start address of the array and the second the index into
       --         the array (the first element of the array has index
       --         @0@).  The following equality holds,
       --
       -- > peekElemOff addr idx = IOExts.fixIO $ \result ->
       -- >   peek (addr `plusPtr` (idx * sizeOf result))
       --
       --         Note that this is only a specification, not
       --         necessarily the concrete implementation of the
       --         function.

       pokeElemOff :: Ptr a -> Int -> a -> IO ()
       -- ^       Write a value to a memory area regarded as an array of
       --         values of the same kind.  The following equality holds:
       --
       -- > pokeElemOff addr idx x =
       -- >   poke (addr `plusPtr` (idx * sizeOf x)) x

       peekByteOff :: Ptr b -> Int      -> IO a
       -- ^       Read a value from a memory location given by a base
       --         address and offset.  The following equality holds:
       --
       -- > peekByteOff addr off = peek (addr `plusPtr` off)

       pokeByteOff :: Ptr b -> Int -> a -> IO ()
       -- ^       Write a value to a memory location given by a base
       --         address and offset.  The following equality holds:
       --
       -- > pokeByteOff addr off x = poke (addr `plusPtr` off) x

       peek        :: Ptr a      -> IO a
       -- ^ Read a value from the given memory location.
       --
       --  Note that the peek and poke functions might require properly
       --  aligned addresses to function correctly.  This is architecture
       --  dependent; thus, portable code should ensure that when peeking or
       --  poking values of some type @a@, the alignment
       --  constraint for @a@, as given by the function
       --  'alignment' is fulfilled.

       poke        :: Ptr a -> a -> IO ()
       -- ^ Write the given value to the given memory location.  Alignment
       -- restrictions might apply; see 'peek'.

       -- circular default instances
    #ifdef __GLASGOW_HASKELL__
       peekElemOff = peekElemOff_ undefined
          where peekElemOff_ :: a -> Ptr a -> Int -> IO a
                peekElemOff_ undef ptr off = peekByteOff ptr (off * sizeOf undef)
    #else
       peekElemOff ptr off = peekByteOff ptr (off * sizeOfPtr ptr undefined)
    #endif
       pokeElemOff ptr off val = pokeByteOff ptr (off * sizeOf val) val

       peekByteOff ptr off = peek (ptr `plusPtr` off)
       pokeByteOff ptr off = poke (ptr `plusPtr` off)

       peek ptr = peekElemOff ptr 0
       poke ptr = pokeElemOff ptr 0

    #ifndef __GLASGOW_HASKELL__
    sizeOfPtr :: Storable a => Ptr a -> a -> Int
    sizeOfPtr px x = sizeOf x
    #endif

    -- System-dependent, but rather obvious instances

    instance Storable Bool where
       sizeOf _          = sizeOf (undefined::HTYPE_INT)
       alignment _       = alignment (undefined::HTYPE_INT)
       peekElemOff p i   = liftM (/= (0::HTYPE_INT)) $ peekElemOff (castPtr p) i
       pokeElemOff p i x = pokeElemOff (castPtr p) i (if x then 1 else 0::HTYPE_INT)

    #define STORABLE(T,size,align,read,write)       \
    instance Storable (T) where {                   \
        sizeOf    _ = size;                         \
        alignment _ = align;                        \
        peekElemOff = read;                         \
        pokeElemOff = write }

    #ifdef __GLASGOW_HASKELL__
    STORABLE(Char,SIZEOF_INT32,ALIGNMENT_INT32,
             readWideCharOffPtr,writeWideCharOffPtr)
    #elif defined(__HUGS__)
    STORABLE(Char,SIZEOF_HSCHAR,ALIGNMENT_HSCHAR,
             readCharOffPtr,writeCharOffPtr)
    #endif

    STORABLE(Int,SIZEOF_HSINT,ALIGNMENT_HSINT,
             readIntOffPtr,writeIntOffPtr)

    #ifndef __NHC__
    STORABLE(Word,SIZEOF_HSWORD,ALIGNMENT_HSWORD,
             readWordOffPtr,writeWordOffPtr)
    #endif

    STORABLE((Ptr a),SIZEOF_HSPTR,ALIGNMENT_HSPTR,
             readPtrOffPtr,writePtrOffPtr)

    STORABLE((FunPtr a),SIZEOF_HSFUNPTR,ALIGNMENT_HSFUNPTR,
             readFunPtrOffPtr,writeFunPtrOffPtr)

    STORABLE((StablePtr a),SIZEOF_HSSTABLEPTR,ALIGNMENT_HSSTABLEPTR,
             readStablePtrOffPtr,writeStablePtrOffPtr)

    STORABLE(Float,SIZEOF_HSFLOAT,ALIGNMENT_HSFLOAT,
             readFloatOffPtr,writeFloatOffPtr)

    STORABLE(Double,SIZEOF_HSDOUBLE,ALIGNMENT_HSDOUBLE,
             readDoubleOffPtr,writeDoubleOffPtr)

    STORABLE(Word8,SIZEOF_WORD8,ALIGNMENT_WORD8,
             readWord8OffPtr,writeWord8OffPtr)

    STORABLE(Word16,SIZEOF_WORD16,ALIGNMENT_WORD16,
             readWord16OffPtr,writeWord16OffPtr)

    STORABLE(Word32,SIZEOF_WORD32,ALIGNMENT_WORD32,
             readWord32OffPtr,writeWord32OffPtr)

    STORABLE(Word64,SIZEOF_WORD64,ALIGNMENT_WORD64,
             readWord64OffPtr,writeWord64OffPtr)

    STORABLE(Int8,SIZEOF_INT8,ALIGNMENT_INT8,
             readInt8OffPtr,writeInt8OffPtr)

    STORABLE(Int16,SIZEOF_INT16,ALIGNMENT_INT16,
             readInt16OffPtr,writeInt16OffPtr)

    STORABLE(Int32,SIZEOF_INT32,ALIGNMENT_INT32,
             readInt32OffPtr,writeInt32OffPtr)

    STORABLE(Int64,SIZEOF_INT64,ALIGNMENT_INT64,
             readInt64OffPtr,writeInt64OffPtr)

    #endif